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Leakage diagnostics, sealant longevity, sizing and technology transfer in residential thermal distribution systems


Abstract and Figures

This field study concentrated on measurement of duct leakage to outside the conditioned space because this is most useful in energy calculations. For room by room load/comfort requirements, the total duct leakage (including leaks to conditioned space) is more appropriate, particularly for additional comfort considerations. The objective of this field study is to help to identify major sources of uncertainty and to quantify the trade offs between different test methods. The identification of the areas requiring significant improvement will aid in future development of duct leakage test methods. For example, during the course of this study a new method for correcting house pressure tests to account for the presence of duct leakage in measured envelope leakage was developed. Each of the measurement techniques investigated has resulted from a different set of priorities and hence compromises. Thus each one of them is measuring a different physical quantity, although they all report the same parameter; duct leakage to outside at operating conditions. Given that real houses do not meet all of the simplifying assumptions that must be made to achieve similarity, the same numbers from each test method are not expected. Potentially these differences can be quite large and one of the benefits of field measurement is that the differences in the measurements helps put a realistic bound on how different some of these leakage diagnostics may be.
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Leakage Diagnostics, Sealant Longevity, Sizing
and Technology Transfer in Residential Thermal
Distribution Systems: Part II
Residential Thermal Distribution Systems
Phase VI Final Report
I. Walker, M. Sherman, J. Siegel, D. Wang, C. Buchanan and M. Modera
Environmental Energy Technologies Division
Energy Performance of Buildings Group
Lawrence Berkeley National Laboratory
Berkeley, CA 94720
This study was sponsored by the California Institute for Energy Efficiency (CIEE), a
research unit of the University of California, (Award No. BG-90-73), through the U.S.
Department of Energy under Contract No. DE-AC03-76SF00098. Publication of research
results does not imply CIEE endorsement of or agreement with these findings, nor that of
any CIEE sponsor.
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Leakage Diagnostics, Sealant Longevity, Sizing
and Technology Transfer in Residential Thermal
Distribution Systems: Part II
Residential Thermal Distribution Systems
Phase VI Final Report
This report documents the Phase VI technical results of the Residential Thermal Distribution Systems
research done by Lawrence Berkeley National Laboratory (LBNL) for the California Institute for Energy
Efficiency (CIEE) through September 30, 1998.
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Table of Contents
TABLE OF CONTENTS .......................................................................................................................3
EXECUTIVE SUMMARY.....................................................................................................................5
INTRODUCTION.................................................................................................................................. 6
1. DUCT LEAKAGE DIAGNOSTICS.................................................................................................. 6
BOOT AND CABINET LEAKAGE ...............................................................................................................8
Figure 1. Differences in blocking location for boot leakage tests. Cross sections through boot and
register............................................................................................................................................. 9
Table 1. Pilot study of boot leakage test results ...............................................................................9
Table 2. Boot and cabinet leakage to outside with the systems as found (before sealing or addition
of leaks).......................................................................................................................................... 11
2. DUCT SEALANTS AND LONGEVITY TESTING .......................................................................11
Table 3. Summary of Duct Tape Failures........................................................................................ 13
SIMULATION OVERVIEW....................................................................................................................... 14
Table 4. List of REGCAP Simulation Cases................................................................................... 16
SIMULATION RESULTS.......................................................................................................................... 17
Figure 2. Simulation results for the base case system for a Sacramento Design day....................... 18
Table 5. Tons at the register and pulldown time simulation results ................................................ 18
SIMULATION CONCLUSIONS ................................................................................................................. 19
FIELD MEASUREMENTS......................................................................................................................... 19
Continuous Monitoring................................................................................................................... 20
Tons At the Register (TAR) .....................................................................................................................20
Table 6. Capacity at the registers ................................................................................................... 21
Delivery Effectiveness............................................................................................................................... 21
Table 7. Maximum Delivery Effectiveness...................................................................................... 22
Figure 3. Comparison of maximum delivery effectiveness between systems and the increase in
delivery efficiency for sealed systems.............................................................................................. 23
Diagnostics..................................................................................................................................... 23
House ventilation rates.............................................................................................................................24
Register Flows. .........................................................................................................................................24
Fan flow ....................................................................................................................................................25
Duct leakage..............................................................................................................................................26
System operating pressures in plenums and at register boots.................................................................27
Duct leakage by house pressure test (HPT)............................................................................................. 27
Duct location and dimensions................................................................................................................... 27
Field Measurement Summary and Conclusions............................................................................... 28
4. TECHNICAL TRANSFER AND SUPPORT ACTIVITIES .......................................................28
RATING OF DISTRIBUTION SYSTEMS - ASHRAE 152P .......................................................................... 28
CALIFORNIA ENERGY COMMISSION - TITLE 24/HERS........................................................................... 29
Implementation of proposed ASHRAE Standard 152P in T24 ACM .....................................................30
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Duct Leakage Testing...............................................................................................................................30
DUCT TAPE IN THE NEWS ...................................................................................................................... 31
OTHER TECHNOLOGY TRANSFER ACTIVITY............................................................................................ 32
5. REFERENCES................................................................................................................................. 35
6. RECENT PUBLICATIONS............................................................................................................. 36
7. APPENDICES ..................................................................................................................................37
Table A1. Summary of House Envelope Leakage Test Results1........................................................ 37
Table A2. Summary of Tracer Gas Results for Residential Buildings Summer 1998 ....................... 38
Table A3. System Flows and Register Flows (cfm)......................................................................... 39
Table A4. Summary of Fan Pressurization Leakage Flows (cfm) .................................................... 40
Table A5. Added Leaks, cfm........................................................................................................... 42
Table A6. Summary of House Pressure Test Duct Leakage Results ................................................. 42
Table A7. Duct and Air Handler Location ..................................................................................... 42
Table A8. Exterior Duct Surface Areas Including Plenums (ft2)..................................................... 42
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Executive Summary
This report builds on and extends our previous efforts as described in "Leakage
Diagnostics, Sealant Longevity, Sizing and Technology Transfer in Residential Thermal
Distribution Systems- CIEE Residential Thermal Distribution Systems Phase V Final
Report, October 1997". New developments include defining combined duct and
equipment efficiencies in a concept called "Tons At the Register" and on performance
issues related to field use of the aerosol sealant technology.
Some of the key results discussed in this report include:
Register, boot and air handler cabinet leakage can often represent a significant fraction
of the total duct leakage in new construction. Because of the large range of pressures
in duct systems an accurate characterization may require separating these components
through improved leakage testing.
Conventional duct tape failed our accelerated longevity testing and is not, therefore,
considered generally acceptable for use in sealing duct systems. Many other tapes and
sealing approaches are available and practical and have passed our longevity tests.
Simulations of summer temperature pull-down time have shown that duct system
improvements can be combined with equipment downsizing to save first cost, energy
consumption, and peak power and still provide equivalent or superior comfort.
Air conditioner name plate capacity ratings alone are a poor indicator of how much
cooling will actually be delivered to the conditioned space. Duct system efficiency can
have as large an impact on performance as variations in SEER.
Mechanical duct cleaning techniques do not have an adverse impact on the ducts
sealed with the Aerosol sealant. The material typically used in Aerosol sealing
techniques does not appear to present a health or safety hazard.
Results from this study were used by the California Energy Commission in the formation
of the current Energy Efficiency Standards for Low-Rise Residential Buildings (CEC,
(1998)), often referred to as Title 24.
Current information on ducts and thermal distribution research can be found at
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Residential thermal distribution systems have been found to have significant energy and
comfort implications due to direct losses from the distribution system in the form of
leakage and conduction, combined with poor mechanical equipment performance, and the
interactions between the distribution system and the equipment. This study aims to
quantify these effects through field testing and computer simulation of residential thermal
distribution systems. In addition, this report outlines our efforts to transfer the results of
this research to the marketplace so as to reduce energy losses and improve thermal
comfort. This study describes the results of efforts made during Phase VI of this
Residential Thermal Distribution Systems research. Results of Phase V were described in
Walker et al. (1997).
1. Duct Leakage Diagnostics
Review of current implementation of duct leakage testing
In Phase V of this work we performed field evaluations of several diagnostic techniques
for measuring duct leakage. These techniques have been used by LBNL and other
researchers (e.g., Brookhaven National Laboratory – see Andrews et al. (1998)), utilities
(Pacific Gas and Electric), code authorities (California Energy Commission (CEC)) and
private energy efficiency engineering companies (e.g., Proctor Engineering). Over the last
12 months we have had extensive discussions with these groups to determine their
requirements for duct leakage testing and the methods they currently use. In addition,
some of these users have performed similar side-by-side tests of the different diagnostic
techniques to those performed at LBNL.
In almost all cases, the duct leakage diagnostic of choice is the fan pressurization test of
total duct leakage (this means supply and return combined and the combined leakage to
inside and outside). The reasons for this are:
Robustness. The fan pressurization test has almost no restrictions on the type of
system it can be used on, or the weather conditions during the test (the House
Pressure Test is restricted to certain types of systems and requires calm wind
conditions). This is a very important factor in the commercial use of duct leakage
diagnostics, where the option to wait for good weather does not exist and all types of
duct systems need to be tested.
Repeatability. This is a key issue for any utility rebate or code compliance program
because builders and home owners do not want to pass or fail depending on the work
crew or weather conditions on the day of the test. Combining the results of our
previous report (Phase V) and the results of other users, the repeatability of the
pressurization testing was found to be very good.
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Precision. Although the duct pressurization method has uncertainties associated with
estimating leakage flows using system operating pressures, they can be relatively small
when the test is used for screening or compliance purposes. This is because utility
programs and code enforcement require systems to have little leakage and have simple
pass/fail criteria. Because the uncertainties for the pressurization test scale with the
amount of leakage, if the allowable leakage is set to a low number, then the
uncertainty in leakage flow will also be small.
Simplicity. It easy to interpret the results of fan pressurization without having to
perform many (or any – with the appropriate hardware) calculations. This allows the
work crew to evaluate the ducts during the test and also allows the work crew to
ensure that the test has been performed properly because they can see if the results
make any sense. Unlike the house pressure test, no envelope leakage test is required,
so a program that is only screening duct systems only needs to obtain a single piece of
equipment and train the work crews to perform a single test.
Familiarity. Work crews that have performed envelope leakage tests are familiar with
the test method for ducts, because envelope testing uses a similar apparatus and
calculation/interpretation methods.
The specific duct pressurization test varies from user to user, depending on how they wish
to use the results. For utilities and code compliance, the test must show that the duct
leakage is below some level. In this case the supply and return are not measured
separately and the duct leakage is not separated into its inside and outside components.
These simplifications reduce the time required for the test (it can take substantial amounts
of time to install airtight separations between supply and return parts of the dust system),
the skill required (no balancing of pressures as required for the leakage to outside test),
and the equipment needed (no blower door needs to be set up). Given that these tests
require that the ducts be relatively airtight in order to pass, the uncertainty in energy losses
due to not separating the supplies and returns is small. The measurement of total leakage
instead of leakage to outside biases the test because very few systems leak entirely to
outside. This means that the test produces an overestimate of the leakage that leads to
system losses. However, this bias is in the right direction from the point of view of code
enforcement/compliance because it tends to overestimate the leakage so that ducts need to
be tight to pass the test. Also, if a system passes the test for total leakage we can safely
assume that the leakage to outside is less than or equal to this number and so the duct
system is equal to or better than the standard. In enforcement/compliance testing it is
important to not pass poor systems so this bias towards higher leakage resulting form
measuring total leakage is a bias in the appropriate direction.
Future work on duct leakage diagnostics will bring together experts in the field to examine
alternative measurement strategies. This is much more important in the Home Energy
rating (HERS) application of the duct leakage tests than the compliance decisions
discussed above. For HERS, some houses will have leaky duct systems, in which case the
split between leaks to inside and outside and between supply and return can be important
when calculating the energy losses.
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Our review of duct leakage diagnostics includes input from members of the Project
Advisory Committee (PAC) of this research project, members of ASHRAE and ASTM,
HVAC contractors, conference attendees (at ASHRAE Meetings, AIVC Conferences,
ACEEE Summer Study, etc.). Additional input has come from staff at: Brookhaven
National Laboratory, Oak Ridge National Laboratory, Florida Solar Energy Center,
California Energy Commission, Proctor Engineering, Berkeley Solar Group, and
Boot and Cabinet Leakage
One of the key questions raised in the previous Phase of this work was the contribution of
leaks at boots and the HVAC equipment cabinet to the total duct leakage. These two parts
of the duct system were examined separately because they represent cases for duct system
leakage reduction that can be fixed by equipment changes (the cabinet leakage) and by
boot inspection and sealing (as proposed for Title 24) without great expense or effort for
the installer/builder. The cabinet leaks (note that this does not include the duct connections
to plenums) are also of interest because it should be relatively simple to eliminate these
leaks with a combination of changes to cabinet construction (tighter tolerances and
improved fan access door seals) and more attention paid to filling knockouts with
grommets. Although changing the manufacturing process for system cabinets may be
costly for manufacturers, the significant cabinet leakage means that it has a large potential
return in terms of energy savings and comfort. To answer these questions about boot and
cabinet leaks, additional pressurization tests were performed in the six houses used in this
study, plus a set of tests in a pilot study used to develop the test procedures.
In these additional tests, the registers were removed and a blockage placed into the boot.
A duct pressurization test was then performed and the boot leakage was determined by the
difference between the system leakage with the boots blocked and the standard
pressurization tests with the blockage at the face of the register (as illustrated in Figure 1).
A pilot study for this procedure was carried out in a new house in Alameda, CA. This was
a relatively large two-story house (about 3500 ft2 (325 m2)) with 19 supply registers and
two returns (one upstairs and one downstairs). All of the registers were mounted in the
ceiling. The ducts were pressurized to 25 Pa using a combined fan/flowmeter device. The
leakage to outside was determined by simultaneously pressurizing the house and the duct
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Register Cover
Blockage in Boot
Register grille
Note: Gap between register grille
and drywall not sealed
by register cover
Figure 1. Differences in blocking location for boot leakage tests. Cross
sections through boot and register.
Table 1. Pilot study of boot leakage test results
Duct configuration Leakage Flow, cfm25
Registers Covered, total 284
Registers Covered, to outside 219
Boots Blocked, total 71
Boots Blocked, to outside 53
The results in Table 1 show that boot leakage is the largest leakage source for this duct
system (75% of the total). In addition, three quarters of the boot leakage is to outside,
which indicates the boots are in good airflow communication with outside. For the
upstairs ceiling mounted boots this is because the back side of the boots are exposed
directly to the attic. For the downstairs ceiling mounted boots, this result implies that the
ceiling of the lower floor has good airflow connections to outside also - probably to the
attic via partition and wall leakage flow paths. This illustration that the majority of boot
leakage may not be benign (i.e. to inside) is crucial because it means that sealing of boots
cannot be neglected. From visual observation, the leakage path for these boots was
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between the ceiling drywall and the outside of the boot, rather than holes in the sheet
metal of the boot or a poor connection between boot and duct.
The cabinet leakage in most systems consists of two parts:
1. The leaks around the fan access panel/door.
2. Flow through knockouts in the cabinet.
In the houses tested for this study, the combined total of these leaks was estimated from
pressurization tests. The pressurization tests were performed with and without the
cabinets included and the difference between the two tests was the cabinet leakage. The
cabinets were isolated for these tests by inserting blockages between the equipment
cabinets and the rest of the duct system. Some houses had only the knockout leakage
tested. In these cases, the cabinet was pressurized with the knockouts open and then with
the knockouts taped over, and the difference between the two tests was the leakage
attributable to the knockouts.
In the pilot study house, there were two knockouts. Both knockouts (as is almost always
the case) were round. One knockout was ½ inch (12mm) in diameter and the other was 1-
½ inches (37 mm) in diameter. In the pilot house we did not perform the pressurization
tests for cabinet leakage, we just observed the size and location of the holes in the cabinet.
An estimate of leakage flow at 25 Pa can be made by assuming that the holes act like
sharp edged orifices with a discharge coefficient of 0.6 resulting in a combined leakage
flow of about 10 cfm25 for these two holes. As a verification of this estimate, at test sites
3 and 6, the cabinets were pressurized with the knockouts sealed and open. The
differences were found to be between 6 and 9 cfm for similar sized knockouts to those in
the pilot house.
The other 6 houses tested for this project used the pressurization tests with internal
blockages (described above) to determine the boot and cabinet leakage at 25 Pa and 50
Pa. Table 2 summarizes the results for the 25 Pa tests. The results show that cabinet
leakage averaged 19 cfm25 to outside, most of which could be reduced by improving
construction of cabinets and placing grommets in electrical knockouts. The boots
averaged about 66 cfm25 to outside. However, if we convert to the average of the
measured operating pressures of about 5 Pa for boots and 65 Pa for cabinets the results
are much different, with boot leakage of about 25 cfm and cabinet leakage of about 34
cfm. The standard deviations of the operating pressures were significant: ±3 Pa for boots
and ± 31 Pa for cabinets due to the large variation from system to system depending on
the specific installation. This was true even for houses of the same floor plan, register
location and equipment installation (sites 1 and 2).
These conversions from measured to operating pressures assumed a pressure exponent of
0.6. The results are not very sensitive to the selection of this pressure exponent with the
results being 30cfm for boots and 31 cfm for cabinets using a different pressure exponent
of 0.5. The measured results at 25 Pa and 50 Pa in this study were used to estimate the
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pressure exponent for 57 different duct leakage tests with an average exponent of 0.62
with a standard deviation of 0.09. This large number of tests results from testing different
parts of the duct system separately (e.g., supply, return and cabinet) and because the duct
systems were tested in different configurations (sealed, as found and with added holes).
Comparing the leakage to outside to the total leakage shows that the boots have about 2/3
of their leakage to outside, but almost all of the cabinet leakage is to outside.
The boots and cabinet averaged three-quarters of the system leakage at 25 Pa to outside.
The variation in this fraction is significant, with a standard deviation of 20%, indicating
that individual installation and system layout contribute strongly to determining this
fraction. In particular, house 6 had only 24% of duct leaks at the cabinet and boots
compared to an average of 70% for the other houses. Removing house 6 from the
calculations reduced the standard deviation to only 7%.
Table 2. Boot and cabinet leakage to outside with the systems as found
(before sealing or addition of leaks)
Site Boot
(cfm25) Cabinet
(cfm25) Fraction of all duct
leakage, %
1 21 51 73
2 29 15 61
4 63 17 63
5 95 26 76
6 21 6 24
Pilot house 166 0*76
Average with pilot house 66 19 74
Average w/o pilot house 46 23 60
* - estimate
2. Duct Sealants and Longevity Testing
Longevity of duct sealants has been examined in three separate experiments in this and
other recent studies. The three tests are:
- cycling tests performed on a small scale for EPA
- the current pressure and temperature cycling longevity tests, and
- constant temperature baking tests performed coincidentally with the current longevity
A previous report (Walker et al. (1997)) discussed the development of the longevity test
method and preliminary results. The final results and details of the experiment are given in
“Can Duct Tape Take the Heat” - LBNL report # 41434 and its companion Home Energy
Article (Home Energy, Vol. 15, No.4, pp. 14-19. The following section is a summary
of these reports.
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The longevity test method heated and cooled sample test connections (each sealed with a
different sealant) by alternately blowing hot (75°C, 170°F) and cold (-12°C, 10°F) air
through the connections. In addition to the cycling of air temperature, the pressure across
the leaks was also cycled between about 100 and 200 Pa. Eight test sections were tested
simultaneously, with continuous monitoring of air and surface temperatures, pressure
differences between inside the test sections and the room, and total system leakage.
Periodically (every few days) the system was turned off and the test samples removed for
individual leakage testing. This testing measured the leakage flows at 25 Pa. The leakage
of the samples was also measured before any sealing and immediately after sealing before
installation in the test apparatus. The “failure” of a sample was determined by looking at
its leakage flow as a fraction of the unsealed flow (so a leakage fraction of 100%
corresponds to the leakage with no sealant). The failure level was set to be 10% of the
unsealed leakage, although samples were often tested beyond this leakage level.
In addition to the temperature cycling tests, we also baked samples in an oven. In these
baking tests, the samples were held at relatively steady temperatures in the range of 60°C
to 80°C (140°F to 180°F). There was no pressure difference across the seal, or air flow
past the seal, that would blow a damaged seal off the sample.
Table 3 summarizes the results of the longevity testing for all three experimental
procedures: aging, baking and the original cycling tests of aerosol sealants. A total of 41
samples were testing in all the test procedures, with at least a single sample of each sealant
type and multiple samples of the sealants we are focusing on here - duct tapes. The most
important result of these tests was that the only sealants to fail were the rubber adhesive
vinyl/polyethylene backed duct tapes (RAVB duct tapes)- irrespective of their UL rating.
However, this results does not mean that all the RAVB duct tapes are guaranteed to fail -
there was one 181B approved RAVB duct tape that did not fail the tests.
In addition to performing the above tests, the longevity procedure has being prepared for
ASTM as a standard test procedure for duct sealant longevity. A draft of this procedure is
currently being considered by ASTM members.
The question has been raised whether flex duct itself may also degrade over time.
Anecdotal evidence supports this conjecture, but we are not aware of any systematic
studies on this subject. A survey of existing installations is required to shed some light on
this issue.
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Table 3. Summary of Duct Tape Failures
# of Tests Sealant Type Test Duration
Aging Test
85 different grades of duct tape 7 days, failed
3181B-FX - approved duct tape 10 days, failed
1181B-FX - approved duct tape 3 months
115-mil foil backed butyl tape 3 months
1Aerosol sealant 3 months
1181A-M and 181B-M - approved mastic 3 months
1181A-P - approved foil tape 3 months
1181A-P & 181B - approved foil tape 1 month
1Packing Tape 3 months
1181B-FX - approved Packing Tape 1 month
Baking Test
53 different grades of duct tape 34 days, failed
1181B-FX - approved duct tape 60 days, failed
2Duct Tape 4 months
3181B-FX - approved duct tape 4 months
1Packing Tape 4 Months
1181A-P - approved foil tape 4 months
1Aerosol sealant 4 months
Cycling test
4Aerosol sealant under pressure cycling
only 2 years
4Aerosol sealant with pressure and heat
cycling 2 years
3. Duct System Interactions with System Sizing and
In addition to saving energy through improving duct systems, this study examines the
possibility of downsizing of cooling equipment without sacrificing comfort conditions.
Two concepts were used to study this issue:
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1. Tons at the Register (TAR). Tons at the register is the actual cooling delivered to the
occupants, i.e. what comes out of the registers. The combined distribution system
losses and system capacity at operating conditions act to determine the TAR of a
2. Pulldown. Comfort, and hence occupant acceptability, is determined not only by
steady-state temperatures, but by how long it takes to pull down the temperature
during cooling start-up, such as when the occupants come home on a hot summer
afternoon. In addition, the delivered tons of cooling at the register during start-up
conditions are critical to customer acceptance of equipment downsizing strategies.
In this study we have used two approaches to examine this issue:
1. Computer simulations.
2. Field measurements of case study houses.
Simulation Overview
The following discussion is based on Walker, Brown, Siegel and Sherman (1998) that
covers the simulations in more detail.
For this study, a computer based simulation tool (called REGCAP) was used to calculate
cooling system performance. The thermal, moisture and ventilation parts of REGCAP are
from existing models that were specifically developed to examine attic performance. These
models of ventilation and heat transfer, excluding the ducts, have been verified with
extensive field measurements. The air flow modeling for REGCAP combines the existing
ventilation models for the house and attic with duct register and leakage flows using mass
balance of air flowing in and out of the house, attic and duct system. The thermal
modeling uses a lumped heat capacity approach so that transient effects are included. The
ventilation and thermal models interact because the house and attic ventilation rates
depend on house and attic air temperatures, air flow through duct leaks, and the energy
transferred by the duct system depends on the attic and house temperatures.
The equipment model for REGCAP uses manufacturers’ performance data that shows
how capacity changes with outside weather conditions, flow rate across the evaporator
coil, and the return air conditions. Some simple regressions have been used to interpolate
information regarding air conditioner performance changes due to incorrect system charge
and system air flow have been adapted from laboratory data (Rodriguez et al. 1995).
Using these correlations, the output from the airflow and thermal models are used
together with weather data to determine the air conditioner performance. The
temperature change across the cooling coil is determined from the mass flow rate through
the coil (the system fan flow) and the calculated capacity of the equipment.
The general data requirements for REGCAP are:
DUCTS: size, location, leakage, insulation
EQUIPMENT: manufacturers’ performance data, refrigerant charge, evaporator airflow
ENVELOPE: leakage, thermal properties
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CLIMATE: Temperature, windspeed and direction, humidity, solar radiation
The REGCAP output includes:
DUCTS: air and energy flows at the registers, losses to unconditioned space
EQUIPMENT: operating condition capacity and efficiency
ENVELOPE: Thermal losses and air flows
In order to focus on the pulldown performance of the systems, the air-conditioner is off
from midnight to 3:00 p.m.. Then at 3:00 p.m., the air handler is turned on. The
simulation model was used to calculate the system performance in 15 minute time steps
for a whole day (including times when the system is off).
The following list examines key input parameters and gives the limited range of values that
we used for the simulations in this study:
Weather. Two weather data sets (from the TMY database, NCDC 1980 for Sacramento,
CA.) were used: a design day and “hot day” (highest peak temperature). The design day
has a peak temperature of 36°C (97°F) and corresponding relative humidities in the range
of 10% to 30%. The hot day has a peak temperature of 41°C (106°F) with similar
relative humidity. The solar radiation (direct normal) is about the same for both days with
peaks of about 900W/m2 (3.3kBtu/hour/ft2).
Refrigerant Charge. Three levels were used: proper charge, typical charge 85% as found in
recent field studies (Proctor 1997 and Blasnik et al. 1996) and 70% charge (worst case found
in field tests by Proctor 1998)
Airflow across coil. Two flows were tested: 425 cfm/ton (manufacturers design specification
for the unit used in the simulations) and 345 cfm/ton (about 20% less than the design
specification and typical of that found in field studies).
Duct Leakage. Four cases:
1. Poor - 30% of fan flow leakage for both supplies and 30% for returns - this is from the
average of the worst 25% of houses surveyed in California by Jump et al. 1996.
2. Typical - 11% of fan flow leakage for both supplies and returns from field surveys by
Modera and Wilcox 1995 and Walker et al. 1997, for new construction in California (This
is also the default used in California T24 Energy Code (CEC 1998)).
3. Good - 3% of fan flow leakage for both supplies and returns. This is a leakage level that
can be achieved using current duct sealing technology if the ducts and equipment cabinet
are in unconditioned space.
4. Best - zero leakage. To realistically achieve this using existing duct systems requires
bringing ducts and equipment inside the conditioned space.
Air handler and duct location. Two cases: 1. Ducts and air handler in attic, and 2. Ducts and
air handler all inside the conditioned space.
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Equipment Capacity. The rated capacity was calculated for the simulated house using
ACCA Manual J (ACCA 1986). These calculations indicated that a rated capacity of three
tons would be required. It was assumed that a correctly design system would have this
capacity (this corresponds to the RESIZED and IDEAL systems simulated). However, this is
not typical of residential installations, so the rated capacity was also estimated based on surveys
of HVAC contractors (Vieira et al. 1996) with one ton for each 46 m2 (500 ft2) of floor area
giving a total of four tons rated capacity.
Other input parameters were fixed for every simulation
Table 4 summarizes the simulations performed for this study. Each case in Table 4 was run
twice - for both design day and hottest day conditions.
Table 4. List of REGCAP Simulation Cases
Charge Air Handler
Flow Duct
Duct and
Location Rated
[%] [CFM/Ton] [%] [Tons]
BASE 85 345 11 Attic 4
POOR 70 345 30 Attic 4
BEST 100 425 3 Attic 4
BEST RESIZED 100 425 3 Attic 3
INTERIOR DUCTS 85 345 0 House 4
IDEAL 100 425 0 House 3
IDEAL OVERSIZED 100 425 0 House 4
The BASE case is typical of new construction in California. The POOR system represents
what is often found at the worst end of the spectrum in existing homes. The BEST system
is what could reasonably be installed in new California houses using existing technologies
and careful duct and equipment installation to manufacturers’ specifications. The BEST
RESIZED system looks at the possibility of reducing the equipment capacity using the best
duct system. INTERIOR DUCTS examines the gains to be had if duct systems are moved
out of the attic. The INTERIOR DUCTS RESIZED determines if a smaller piece of
equipment can deliver the same capacity as a bigger piece of equipment with a poor duct
system. Lastly, the IDEAL system is an interior duct system that has been installed as well
as possible. The IDEAL OVERSIZED simulations were included to examine the
difference in pulldown if the IDEAL system were sized using current sizing methods (i.e.,
still 4 tons).
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Simulation Results
Three key results are examined:
1. Initial delivered capacity at the registers (TAR) with hot house, attic and duct system.
2. Pulldown time for interior to reach 24°C (75°F).
3. Final delivered capacity (TAR) at the registers with cool house and duct system. The
final capacity is determined when the system has cooled the house to 24°C (75°F).
An example illustration of the simulation results for the BASE case is given in Figure 2
(Note that Figure 2 data are truncated at the point where the indoor temperature reaches
the setpoint). The ducts are in the attic and so the supply temperature is close to the attic
temperature until the system turns on. The supply temperature is about 7°C (13°F) below
room temperature. In addition, the duct losses to the attic tend to cool the attic. This is
seen at the end of the simulation, where the attic temperature is about 5°C (9°F) cooler
than the no cooling system case.
Table 5 summarizes the tons at the register and pulldown time results for all the
simulations. The increased loads of the hottest day reduce the delivered capacities and
increase the pulldown time. As expected, the interior systems and the non-leaky attic
system (BEST) have the fastest pulldown and greatest tons at the register. For the hottest
day simulations, the POOR system heats the house when it is first turned on because the
supply air temperature is hotter than the house air - hence the negative initial delivered
capacity. This result is mainly due to the low system capacity (caused by poor charge, low
air flow and high outside temperatures) and the return leaks heating up the return air.
The simulation results in Table 5 show that the BEST RESIZED system with a smaller A/C
unit (rated at three tons rather than four tons) can provide almost the same performance as
the BASE system. The pulldown time is longer by a single simulation time period (15
minutes) for the BEST RESIZED system. Moving the ducts inside allows a smaller
capacity system (IDEAL) to have faster pulldown and more tons at the register than the
BASE system. In addition, the IDEAL system has greater initial tons at the register than
the BEST system in the attic that has 25% more nameplate capacity.
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Figure 2. Simulation results for the base case system for a Sacramento
Design day.
Table 5. Tons at the register and pulldown time simulation results
Design Day Hot Day
Initial Tons
at the
Final Tons
at the
Tons at
Tons at
BASE 40.83 270 1.55 0.58 315 1.51
POOR 40.09 390 0.68 -0.05 435 0.68
BEST 41.56 195 2.33 1.24 240 2.30
BEST RESIZED 30.80 285 1.59 0.58 330 1.59
INTERIOR DUCTS 41.68 195 2.08 1.66 240 2.02
INTERIOR RESIZED 31.20 285 1.35 1.18 345 1.33
IDEAL 31.41 240 1.72 1.41 300 1.66
IDEAL OVERSIZED 42.06 135 2.73 2.07 180 2.62
The results in Table 5 also show that the output of air conditioning equipment does not
match the nameplate rating. The final tons at the register for the ideal system (that has
correct charge, system air flow, no duct leakage, and all ducts inside) is much lower than
the rated capacity. Note that this low rated capacity is calculated directly from
manufacturers’ performance data.
Base Case
0 4 8 12 16 20 24
Time of day (hours)
Temperature (C)
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Simulation Conclusions
The simulation results show that improved ducts and system installation can allow the use
of a smaller nameplate capacity air conditioner (almost one ton less in our case, and at
least one ton in more demanding situations) without reducing the cooling delivered to the
house (tons at the register) or the pulldown time. If system nameplate capacity is
unchanged, either improving duct systems (to have little leakage) and correctly installing
the equipment, or moving the ducts inside results in significant pulldown performance
improvements. In these cases pulldown times were reduced by more than an hour and
initial tons at the register were approximately doubled.
The results also show that without knowing about the quality of installation or location of
the air conditioning system, the nominal capacity of the A/C unit is not a good indicator of
system performance. Proper sizing of cooling equipment to meet peak loads cannot be
done using nameplate ratings, but requires using manufacturers’ performance data at more
realistic conditions and an understanding of distribution system effectiveness.
Field measurements
Field measurements were made on air-conditioning systems in six houses. These
measurements included diagnostics to determine building and system characteristics and
continuous monitoring over several days to determine pulldown system performance. Six
houses were monitored for this project: 2 houses in Palm Springs, CA. (sites 1 and 2), one
house in Mountain View, CA. (site 3), two houses in Sacramento, CA. (sites 4 and 5), and
a single house in Cedar Park, TX (site 6). All of the houses were new and unoccupied,
except for the Mountain View house that had been occupied for less than a month at the
beginning of our tests. The Mountain View house was the only one occupied during the
All the houses were cooled with split system air conditioners and heated with natural gas
(using the same air handler/cabinet and ducts). The exception was the Texas house that
had a heat pump and electric resistance strip heat. The two Palm Springs and two
Sacramento houses had all of the system in the attic - air handler, equipment cabinet and
ducts. The Mountain View house had the air handler/cabinet in the garage, with the ducts
running inside the structure in soffits, floors and wall spaces. The Texas house had the air
handler/cabinet in a closet in the house, with only the supply plenum and supply ducts in
the attic.
The houses were tested in their “as found” configuration, then with the duct systems
sealed. Houses that did not have very much "as found" duct leakage had holes added.
These added holes had their flow measured using a vane anemometer during normal
system operation so that the added leaks were very well known and did not have to be
inferred from indirect measurements.
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In two houses, the cooling equipment was replaced with Energy Star equipment (greater
than SEER 13.0). The original cooling equipment in each house was rated at the federal
minimum SEER 10. In Sacramento (site 4), just the outside compressor unit and the
control system were changed. In the Texas house (site 6) the indoor coil, fan and cabinet
(and electric heating system) were also replaced.
The continuous monitoring was performed for several days in each system configuration.
During this monitoring the system was set to stay off until about 3:00 p.m., allowing the
house to warm up during the day (just like in the simulations). Then the system was
turned on and the house temperature “pulled down” to 74°F (24°C), or lower, at the
thermostat. Because temperatures were monitored in each room we were able to
determine if the temperature at the thermostat was representative of other temperatures in
the house. In most cases, the thermostat was not the warmest place in the house (it was
usually centrally mounted in a hallway away from any direct solar gains) and so the
pulldown was changed to reduce the temperature at the thermostat below the original
74°F (24°C) setpoint. Also, pulldown times were calculated for different parts of the
Continuous Monitoring
The continuous monitoring used computer based data acquisition systems to store data
approximately every 10 seconds. The monitored parameters were:
- Temperatures: at each register, in each room, outdoors, attic, garage, return plenum
and supply plenum. The supply plenum temperatures were measured at four points in
the plenum to account for spatial variation in plenum temperatures.
- Weather: wind speed, wind direction, total solar radiation and diffuse solar radiation.
- Humidity: outside, supply air, return air and attic (or garage if system located in
- Energy Consumption: Compressor unit (including fan) and distribution fan power.
The measured system temperatures were used to calculate the energy flow for each
register (and therefore the total for the system) and the energy change of the air stream at
the heat exchanger at each time step.
Tons At the Register (TAR)
Having data every few seconds allowed the calculation of TAR at different times during
the pulldown test. The TAR for each of the sites and test conditions appears in Table 6,
where the results have been concatenated from the several tests performed at each
condition. Care should be taken when interpreting these numbers because they are
sensible capacities only. Humidity data will be used in future analyses to include latent
capacity. The latent capacity is expected to be a small part of the system load at these
sites, particularly in sites 1 and 2 (located in the desert climate of Palm Springs, CA). The
nominal capacity is taken from the manufacturers nameplate - this is the traditional number
used to size cooling systems.
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The results in Table 6 show that tons at the register often starts out as being negative
because the air in the ducts, the ducts themselves, the fan and the heat exchanger coils are
hot. After this initial transient it then increases to a maximum value that occurs several
minutes later. It then usually declines slightly as the air in the house cools slowly and
changes the heat transfer at the cooling coil, and so the final value at the end of the cycle is
lower than the maximum. As with the simulations discussed earlier, the cooling delivered
to the conditioned space is always much less than the nameplate rating.
Comparing the “as found” to “sealed” results does not clearly show any benefit to sealing
the leaks. This is due to the limited number of tests and the variable weather during the
tests. Future work will examine the possibility of normalizing by weather conditions for
each test. At sites 1 and 3, the performance during the initial five minutes is improved by
sealing the leaks (at site 2 there is little change), and this transient improvement would
give better occupant comfort (initial pulldown is faster) even if the final TAR is
Table 6. Capacity at the registers
Nominal Tons at the Register
Site Condition Capacity
[Tons] First
Minute First 5
Minutes Max.
Value Final
1As Found 5-0.1 2.1 3.6 2.3
1Sealed 52.0 2.7 3.0 2.4
2As Found 50.9 2.0 2.7 2.9
2Correct Charge 51.3 2.1 2.6 2.6
2Leaks Added 51.0 2.1 2.6 2.5
2Sealed 50.9 2.2 2.9 2.8
3As Found 31.1 1.8 2.5 2.2
3Correct Charge 30.8 1.8 2.6 2.3
3Sealed 31.0 1.7 2.7 2.5
4As Found 2-1.1 0.5 1.5 1.4
4New Unit 2-0.6 0.7 1.6 1.4
4Leaks Added 2-0.8 0.4 1.3 1.1
5As Found 2.5 -0.8 0.6 1.7 1.4
5Sealed 2.5 -1.1 0.5 1.7 1.5
6As Found 30.6 0.6 1.8 1.6
6New unit 31.0 1.0 1.5 1.5
6Added Leaks 30.3 1.1 1.7 1.4
Delivery Effectiveness
Although TAR is useful from the perspective of comfort, it does not only depend on the
distribution system. For this reason, the delivery effectiveness of the distribution system
was also calculated. The delivery effectiveness is the total capacity at the registers divided
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by the capacity at the at the air handler and does not include any regain of losses or the
energy and comfort implications of duct leakage to inside. Because delivery effectiveness
is usually thought of as a steady-state value, the maximum delivery (over the course of the
pulldown test) effectiveness was used here. Note that these data are based on the sensible
delivery efficiency. Future work will evaluate delivery effectiveness including latent
effects. Table 7 and Figure 3 show that the results of sealing the duct system are much
more apparent when looking at delivery effectiveness rather than the TAR uncorrected for
weather changes as given in Table 6.
Table 7. Maximum Delivery Effectiveness
Site Condition Delivery
1As Found 86%
1Sealed 87%
2As Found 76%
2Correct Charge 72%
2Added Leaks 72%
2Sealed 84%
3As Found 99%
3Correct Charge 100%
3Sealed 99%
4As Found 94%
4New Unit 96%
4Added Leaks 87%
5As Found 77%
5Sealed 87%
6As Found 75%
6New Unit 68%
6Added Leaks 82%
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Figure 3. Comparison of maximum delivery effectiveness between systems
and the increase in delivery efficiency for sealed systems
The following diagnostic results were used to characterize the house and duct system,
determine changes in system performance (e.g., leak sealing) and to prepare the required
input parameters for future work comparing simulations to measured field data.
Envelope Leakage.
The envelope leakage was measured using a blower door test with the registers
uncovered. Therefore it includes leakage to outside via the duct system. The envelope
leakage for each of the six test sites is given in Table A1 in Appendix A. The leakage is
expressed in several ways:
- in terms of the blower test results, i.e., the flow coefficient and exponent,
- Specific Leakage Area (SLA),
- Flow at 50 Pa divided by floor area (Q50/FA).
The SLA and Q50/FA are methods of scaling leakage by house size so that comparisons
of envelope air tightness can be made between houses. In addition, the calculation of SLA
allows the comparison of these houses to a “standard” house that would meet T24 energy
0% 20% 40% 60% 80% 100% 120%
1, As Found
1, Sealed
2, As Found
2, Correct Charge
2, Added Leaks
2, Sealed
3, As Found
3, Correct Charge
3, Sealed
4, As Found
4, New Unit
4, Added Leaks
5, As Found
5, Sealed
6, As found
6, New Unit
6, Added Leaks
Maximum Delivery Effectiveness
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code. Both these methods normalize the leakage by house floor area. In addition, SLA
uses the effective leakage area of the house calculated from the flow coefficient and
exponent. These houses had an average SLA of 5.0 corresponding to 1.4 cfm/ft2 floor
area at 50 Pa. For comparison California Energy Code - Title 24 uses a default SLA of
4.9 for houses with ducted forced air systems which is very close to the measured results
in these six test houses. There was a large variation of a factor of four from house to
house indicating a large variability in construction. However, house 5 had open vents
because all the appliances were not in place and removing this house reduces the
variability considerably.
House ventilation rates.
Ventilation rates were measured using tracer decay with the system fan off and the system
fan on to determine any changes in ventilation rate due to duct leakage. The results of
these tests are summarized in Appendix A, Table A2. Because of the large variation in
ventilation rates with weather conditions, the only significant result is the change in
ventilation rate due to system operation. The fractional increases (compared to when the
system was off) in ventilation due to system operation are only valid for the particular
instances of these tests. For example, the large fractional increase at site 6 after holes
were added is because the ventilation rate was very low with the system off.
At site 1 the change was 0.17 ACH (and increase of 30% compared to the test with the
system off) with the system as found. After sealing, four tests were performed at site 1
with an average of 0.22 ACH increase in ventilation rate. This counter-intuitive result (we
expect less change in ventilation rate if we have sealed system leaks) indicates the large
uncertainty in using these tracer decay measurements. Much of this uncertainty may be
due to having different weather conditions on different days. The change in weather can
have a similar magnitude effect to the change in ventilation rate due to system operation
and future analyses will attempt to account for changes in weather between tests.
The remaining sites all showed significant increases in ventilation rate (between 0.18 and
0.63 ACH) with the system operating. The increase in ventilation for systems operating
“as found” ranged from 0.12 ACH to 0.18 ACH. Adding holes to the systems at sites 2
and 6 was an even greater effect (as expected) with increases in ventilation due to HVAC
system operation of 0.60 ACH and 0.37 ACH.
Register Flows.
The supply register flows were measured using a fan assisted flowhood. The return
register flows were measured either using a flowhood or a vane anemometer traverse.
The mean anemometer velocities were combined with an estimate of the open area of the
return grille to obtain return flows. The individual register flows were used together with
the individual register temperatures to calculate the energy flow out of each register (tons
at each register). The supplies were then combined to find the total energy flow (TAR)
for the system. The sums of the register flows are summarized in Appendix A, Table A3.
The sum of the supply register flows as found averaged over all systems was 13 % less
than the fan flow. A similar comparison could not be made for return flows due to the
large uncertainty in measuring the return register flows measured using the vane
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anemometer. In four of the six sites this uncertainty resulted in return register flows that
were greater than the fan flows. Based on our field experience, the most likely reason for
overpredicting these return flows was the difficulty in estimating the open area of the
return grille.
Fan flow
Fan flowmeter:
The fan flowmeter test methods used here are based on those in proposed ASHRAE
Standard 152P and proposed CEC ACM, Appendix F. This test uses the supply ducts as
a flowmeter. The pressure difference between the supply plenum and the conditioned
space is measured at operating conditions - Psp. The return duct is then blocked off from
the rest of the equipment and a fan flowmeter attached at the air handler access. The fan
flowmeter is turned on and adjusted so until the pressure difference between the supply
plenum and the conditioned space is the same as at normal operating conditions. The flow
through the flowmeter is the system fan flow at operating conditions. However, the fan
flowmeter does not usually produce enough flow to match the supply plenum to
conditioned space operating pressure. In these cases the fan flowmeter is operated at
maximum and the flow (Qmax) and supply plenum to conditioned space pressure (Pmax)
are recorded. Assuming a pressure exponent of 0.6 for the duct system, the measured
flow at maximum is extrapolated to the flow at the operating condition pressure difference
An alternative is to operate the fan flowmeter over a range of flows, recording the flows
and supply plenum to conditioned space pressure differences. A least squares fit can then
be used to determine Cs and ns for a power law relationship:
Then system flow at operating conditions is given by:
spsfan PCQ=
Other tests were performed with the system fan also operating in an attempt to reach the
measured system operating pressure and eliminate the need to extrapolate from lower
pressures. As Table A3 shows, there were significant differences in some cases between
the pressure flow relationship (and therefore the extrapolated fan flow) for the duct system
with the fan flowmeter alone and the fan flowmeter combined with the system fan. We
believe that this is due to changing flow patterns through the fan flowmeter (and
particularly over the flowmeter part) that are induced by having the second fan in series.
We will investigate these anomalies further in the future. However, the current results
seem to indicate that using the fan flowmeter alone (and in some cases using only a single
QQ sp
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point test with an assumed pressure exponent of 0.6) gives better results than attempting
to match the operating condition flows and pressures by using the system fan. In
addition, the results show that simultaneously operating the system air handler and the fan
flowmeter fan tends to produce a lower estimate of fan flow. In several cases the
combined fans produce a fan flow that is lower than the sum of supply flows – an unlikely
result, indicating that the fan flowmeter operating alone is the preferable test method.
Note that proposed ASHRAE standard 152 and the CEC ACM both recommend
simultaneous operation of the fan flowmeter and the system fan and these approaches may
have to be reevaluated in light of the above results.
Tracer gas:
The tracer gas fan flow measurements are shown in Table A3 together with the fan
flowmeter measurements. There is reasonable agreement between the tracer gas and fan
flowmeter (without system fan) methods. The major differences are at site 5, where the
fan flowmeter method seems to greatly overestimate the fan flow.
Duct leakage
Duct leakage was separated into: supply, return, supply boot, return boot and cabinet by
connecting the fan flowmeter at different parts of the duct system and inserting blocking in
the ducts to isolate the individual components. The separation of total leakage from
leakage to outside was accomplished by simultaneously pressurizing both the ducts and
the house. The boot and cabinet leakage was discussed previously in Section 1. These
and other test results are given in Table A4, Appendix A. In the “as found” condition,
some systems were less leaky than expected. This gave little scope for changing leakage
by sealing, so sites 2, 4 and 6 had leakage added. Although Site 3 had almost all of its
duct system (except for the cabinet, supply plenum and return platform) inside the
conditioned space, it had the greatest supply leakage to outside. This shows that the
spaces containing the ducts were not sealed with respect to outside. This result illustrates
the need for testing all duct systems for leakage, even though they may appear to be inside
conditioned spaces. Another interesting result was at Site 6, where there was significant
return leakage to outside. Although there were no return ducts as such, the closet
containing the equipment and the platform return leaked to the attic through holes around
the ceiling penetration for the supply plenum. Again, this illustrates the requirement for
system testing, where simple observation would have implied that the return leaks were to
the closet (essentially the conditioned space).
Expressing the combined total supply and return duct leakage at 25 Pa as a fraction of fan
flow, the average "as found" condition had 16% leakage with a range from 6% (site 2) to
31% (site 3). Of the four sites that were sealed, the "as found" average was 16%, and this
was reduced to 8% after sealing. The opinion of the field test personnel is that it would
be possible to do a much better job of sealing these ducts if more time were spent and if
the sealing was done before houses were in a finished state. This is because we did not
want to damage the finished surfaces in the test houses and access to duct connections is
much better before the house is complete.
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Only site 1 was below the 6% threshold required by the proposed Title 24 ACM for
obtaining credit for tight ducts in the "as found" condition. About two thirds of this
leakage was to outside, with the combined supply and return leakage to outside at 25 Pa
being 10% of fan flow. After sealing, site 2 was also at the 6% threshold and site 5 was
close - at 7%. This result shows that sealing existing systems can be effective in meeting
the proposed Title 24 duct leakage requirements for tight duct credit.
At sites 2, 4 and 6 the pulldown tests were performed with added leaks to simulate the
performance of poor duct systems. The added leaks were cut into the supply and return
plenums and resealed after the experiments were finished. At sites 2 and 6 the leakage
flows at operating conditions were measured directly using a vane anemometer. At site 4
the extra leaks were calibrated in a laboratory and the measured system operating
pressures were used to estimate the leakage flows at operating conditions. These added
leaks are summarized in Table A5.
System operating pressures in plenums and at register boots.
The register boot pressures were measured by inserting a static pressure probe to a point
inside the register grille upstream of a lip at the edge of the grille and in a location out of
the main flow stream. The measured pressures are highly dependent on the selection of
the measurement location. Moving the probe by a few millimeters into the flow can
completely change the measured pressure, including the possibility of indicating a negative
pressure. However, extensive field experimentation has shown that it is possible to obtain
repeatable results (different people getting the same result) if the procedure for the
measurements is consistent. Typical plenum operating pressures were about 65 Pa and
typical boot operating pressures were about 5 Pa. These differences in pressures across
leaks at different locations is one of the most difficult parts of extrapolating from
measured leakage at a fixed test pressure (25 Pa or 50 Pa) to the actual leakage at
operating conditions. As shown in Section 1, boots have almost three times the leakage of
the cabinets at the same measurement pressure. However, using the measured operating
pressures significantly alters their contribution to the system leakage, such that the cabinet
and boot leaks are of similar magnitude.
Duct leakage by house pressure test (HPT).
The HPT results are summarized in Table A6 in Appendix A. The results showed some
anomalies, with the leakage going in the wrong direction in three of the five leakage
change configurations measured (i.e. in many cases the HPT predicted less leakage when
holes were add to the system, and increased leakage when the systems were sealed.).
Expressing the HPT leakage as fractions of fan flow, the mean supply leakage was 7% and
the mean return leakage was 3% in the as found condition.
Duct location and dimensions.
The duct location and dimension information is summarized in Tables A7 and A8. For all
but site 3, all the supply ducts were in the attic, with the air handler and equipment also in
the attic at sites 1, 2, 4 and 5. At site 3 the ducts were in between the first and second
floors, running in the joist bays and in drop ceiling soffits. In addition, the air handler,
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plenums and equipment were all in the garage at site 3. At site 6, the air handler and
equipment were in a closet inside the thermal envelope of the house. The exposed supply
and return duct surface areas averaged 26% and 6% of floor area respectively (not
including site 3). This is almost the same as the current proposal in ASHRAE Standard
152P of 27% for supplies and 5% for single return systems.
Field Measurement Summary and Conclusions
The total duct leakage of 16% of fan flow for the test houses was less than in previous
studies (typically in the range of 20% to 30% for new construction). This average lies
between the default value and minimum requirement for duct leakage credit in T24
(22% and 6% respectively). In fact, we added leakage to three of the duct systems to
simulate more typically leaky systems.
Testing at a fixed pressure, boots have about three times the leakage of cabinets (66
cfm25 compared to 25 cfm25 respectively). However, converting to measured
operating pressures of about 5 Pa for boots and 65 Pa for cabinets makes the leakage
from boots and cabinets much closer (25 cfm25 and 34 cfm25 respectively). About
three quarters of total system leakage was found to be at the boots and cabinets.
Some simple sealing techniques can be used on existing systems to meet the 6%
leakage criteria required for T24 duct efficiency credit.
The measurement procedure for air hander flow using a fan flowmeter should NOT
simultaneously use the air handler fan to reach operating pressure conditions and it is
better to extrapolate from the pressure obtained using the fan flowmeter only to the
measured operating system pressure using a simple calculation procedure.
The duct leaks added an average of 0.15 ACH to the ventilation rates of the tested
The average measured envelope leakage for these test houses was within 2% of the
T24 default indicating that the T24 default is a reasonable value.
The measured duct surface areas corresponded very well to those defaults in the
proposed ASHRAE Standard 152P showing that these defaults are reasonable values.
4. Technical Transfer and Support Activities
Rating of Distribution Systems - ASHRAE 152P
Over the last 12 months, this proposed standard has undergone many detailed changes.
Many of the changes were developed in order to make the standard easier to use and to
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make it more of a standard rating method. The following is a list of some of the most
important changes:
Item 1. Require input data sheets as part of the standard. The use of standardized data
sheets showing the source of all the input data will make reporting of use of the standard
more uniform.
Item 2. Add temperature tables to give design and seasonal conditions instead of
referring to the ASHRAE handbook of Fundamentals. This removes the need for the user
to look up weather data and perform the calculations required to convert from design to
seasonal conditions. Because these tables also include the design and seasonal humidity
parameters, many tedious psychrometric calculations are no longer required to be
performed by the user of the standard.
Item 3. Add specific indoor dry bulb and indoor wetbulb temperatures. Previous drafts
of the standard allowed the user to specify indoor conditions. Because the indoor
conditions have a very powerful influence on the calculated efficiencies, this led to
distribution system efficiencies that depended as much on the user selected indoor
conditions than on the system performance. This flexibility was removed in order that the
standard produces efficiencies that can be directly compared. In other words, it makes the
standard more of a rating system than a calculation method for individual conditions. It
was considered by the committee that this standard’s purpose is to be a rating system
rather than just a calculation method (for use in codes and standards).
Item 4. Move all defaults to a (non-required) appendix. The exception is to retain the
default for exposed duct surface area. This change was made so that a system cannot be
rated based on defaults only. In most applications of this standard the user (e.g., code
authorities and Home Energy Raters) will want to provide their own defaults or fixed
values for things such as duct leakage. As explained in more detail in the following
section, this is the path that CEC has taken with the Title 24 Alternative Calculation
Manual (T24 ACM).
The draft standard has been submitted to ASHRAE and should be out for public review in
the near future.
California Energy Commission - Title 24/HERS
One of the most significant technology transfer activities we have made has been the
inclusion of credits for good (i.e. not leaky) ducts in Title 24. LBNL has continued to
work with CEC staff (mostly through conference calls and attending ACM workshops) to
incorporate draft ASHRAE Standard 152P calculation procedures into Title 24 and
HERS. This procedure has been a two way process, with the decisions made in order to
incorporate the standard calculations into the T24 ACM, being reflected in changes in the
draft standard itself. An example change to proposed ASHRAE 152P stemming from the
work with CEC is the inclusion of explicit indoor conditions and design and seasonal
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weather condition data (in T24 this was represented by the 16 California Climate zones
common to all T24 calculations).
Implementation of proposed ASHRAE Standard 152P in T24 ACM
The T24 ACM uses a simplified version of the draft ASHRAE standard 152P. The key
simplifications are:
1. Using sensible calculations only. This is possible because almost all California climates
are relatively dry in the summer.
2. Reducing the possible duct locations to: Attic, Basement, Crawlspace, Other.
3. Including specific duct location temperatures for the 16 California climate zones used
in the T24 ACM.
4. Removal of the cyclic loss factor.
5. Simplified equipment interaction calculations.
6. Having defaults and diagnostics for fan flow, duct leakage and duct surface area. The
default fan flow is based on the floor area of the building (0.7 cfm/ft2 [13 m3/hour/m2]
floor area for climate zones 8-15 and 0.50 cfm/ft2 [9 m3/hour/m2] floor area for climate
zones 1-6 & 16). The equipment capacity is determined by fixing the temperature
difference across the heating or cooling heat exchangers to be 55°F (30.5°C) for
heating and 20°F (11°C) for cooling. The fan flow and capacity defaults area needed
because T24 energy plan checks do not include equipment specifications.
Duct Leakage Testing
The implementation of field testing of the installed duct system required much debate and
discussion between LBNL staff, CEC and other interested parties. Note that the
requirement for field testing of duct systems to obtain an energy credit is much more
onerous than the requirements for any other efficiency credit, e.g., high thermal efficiency
windows or placement of insulation in wall cavities are not field tested. The method used
for the ACM is as follows:
In order to get credit for duct leakage reduction, the duct system must be tested. For
houses built before 1999 the default duct leakage is 28% (all the duct leakage numbers are
split evenly between supply and return ducts and are expressed as a fraction of fan flow).
For houses built after January 1st, 1999 the default duct leakage is 22%. The available
credit for sealing ducts is restricted to a single leakage number of 8%. This credit (using
8% instead of 22% or 28%) was selected as a balance between being large enough to be
worth the effort of testing a system without the possibility of too much free credit being
given for duct sealing. In order to be able to use the reduced leakage value of 8%, the
ducts must be leakage tested. The allowable leakage for a completed system is 6% and for
a system tested at rough-in it is 4%. The difference between these measured values and
those used in the calculation is to allow for measurement uncertainties so that systems that
really have more leakage at operating conditions do not get to take the reduced leakage
credit. Although the leakage testing may also overpredict the actual duct leakage, it was
thought to be better to eliminate the possibility of houses passing that should not.
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The leakage testing method is to have all systems certified by the installer with verification
sampling by a third party (HERS raters). The verification sampling occurs for every seven
houses, with retesting and repair requirements for failure to meet the leakage
specifications. If systems are certified at rough-in, there is a requirement for inspection of
the sealing of duct system connections at final inspection and the sealing method used
between register boots and wallboard cannot be cloth backed rubber adhesive duct tape.
This last requirement is the result of the duct sealant longevity testing carried performed
for this project. If the rough-in leakage test was performed without the air handler
installed, than this connection must also be inspected at final and it is not to be sealed with
cloth backed duct rubber adhesive tape.
The fixed values that can be assigned to duct leakage limits the change in distribution
efficiency to about 10% (based on a sensitivity analysis performed by CEC). Note that
this is not a fraction of the distribution system efficiency but is the change in distribution
system performance, e.g., an increase from 70% to 80% (not to 77%). Including other
changes - such as duct location- increases the available change in distribution system
efficiency to about 25%, going form all ducts in the attic with default leakage to all ducts
inside conditioned space.
Duct tape in the news
The result of our longevity testing: “the only seal that doesn’t work on ducts is duct tape”
has been of considerable interest to the media, manufacturers and testing facilities.
Because duct tape is something familiar to most people, and it has been used in many
“humorous” non-duct applications, the media have a soft-spot for duct tape stories and
have been keen to discuss and publish our research results. Articles have appeared in
more than 60 news locations (many of them based on two articles in the Sacramento Bee
and the San Jose Mercury News that went out on wire services) and Max Sherman and
Iain Walker were interviewed on radio by NPR (National Public Radio), A.P., CBS and
CBC (Canadian Broadcasting Corporation). Surprisingly, manufacturers have not shown
a great deal of interest in our tests results. However, one manufacturer has made an
effort to examine our findings in more detail, and attended an informal session at the
ACEEE 1998 Summer Study. According to this manufacturer, they have not heard of
duct tape failures on duct systems and they were concerned that so many researchers
present at the ACEEE Study had the same anecdotal evidence to recount: “that duct tape
doesn’t stay on ducts”. We are currently working with this manufacturer to determine the
differences between the tests that they have performed (and the similar tests performed for
UL listing) and those that we have done in our laboratory.
The fact that the manufacturer did not know about this problem is probably not unique to
this manufacturer, rather, it is an indication that people who find duct tape failures have
not reported them to manufacturers. In addition, the people that purchase duct tape and
use it on duct systems do not return to houses to inspect systems unless a homeowner
complains, and therefore they have not seen enough duct tape failures to warrant
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complaining to manufacturers. The last way in which the link between duct tape failure
and manufacturers is incomplete is that there is little or no reason for HVAC contractors
to expect duct tape to perform better and they find the current performance of duct tapes
to be acceptable. We are currently working with the interested manufacturer by showing
them exactly what tests we performed and assisting them to possibly perform similar
We have attended ASTM meetings and corresponded with ASTM to discuss the
implementation of an ASTM standard for longevity testing of duct sealants. A draft of the
standard is being prepared and will be discussed at the next ASTM meeting in October
1998. More information has been put together at our duct tape web page:
Longevity Testing Interviews and Demos:
Sounds Like Science (National Public Radio) - Max Sherman Interview
As It Happens (Canadian Broadcasting Corporation) - Iain Walker Interview
Associated Press
CBS Radio News - Max Sherman Interview
WCCO Radio - Max Sherman Interview
Glennda Chui (San Jose Mercury News)
Carrie Payton (Sacramento Bee)
Dawn Storer- (Popular Science)
Other Longevity Testing Contacts:
Annals of Improbable Research
John Russel (Akron Ohio Beacon Journal)
Anchorage Daily News
This Old House (PBS Television Show)
The news of our duct tape research even made it as far as the (comical) car repair show on
NPR - “Cartalk”, and the 1998 Ig Nobel Awards.
Other technology transfer activity
We have worked closely with EPA and other LBNL staff on including duct system
efficiency calculations in the “ENERGY STAR BUILDINGS” program. This work
included providing ASHRAE 152P based calculation methods and giving input on suitable
default values and acceptable ranges of input parameters so that the calculations could be
included with the existing energy star buildings calculation methods.
The results from work done for this phase of the Thermal Distribution Efficiency research
program have been presented (and published) mostly at ASHRAE meetings and at the
ACEEE 1998 Summer Study. The following presentations have been given in the last 12
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months - some of which were based on work performed for the previous phase of this
work. Section 6 lists recent publications associated with this research program.
Walker, “Saving Tons at the Register”, ACEEE Summer Study, August 1998.
Sherman and Walker: “Can Duct Tape Take the Heat?
Summer Study, August 1998.
Walker, “Technical Background for Default Values used for Forced Air Systems in
Proposed ASHRAE standard 152P”, ASHRAE TC 6.3 Symposium., January 1998
Walker, “Field Measurements of the Interactions between Furnaces and Forced Air
Distribution Systems”, ASHRAE TC 6.3 Symposium, January 1998.
Walker, “Implementing Duct System Efficiency in Energy Codes”, informal Session at
ACEEE Summer Study, August 1998.
During this year we have been developing the Thermal Energy Distribution Web page -
This is intended to be a central reference point for getting the word out about thermal
distribution systems in buildings, and the papers resulting from the work done for the
current project will be “published” on this website.
Other Thermal Distribution System Efficiency Support Activities
Several other tasks were performed under the scope of this study that relate directly to
thermal distribution systems. The following is a summary of these activities:
1. Field Testing of Energy Star equipment (EPA). This field testing was performed
in conjunction with the field testing for Phase VI. In one of the Sacramento houses and
the Texas house the air conditioning equipment was swapped for higher efficiency Energy
Star equipment. In both cases the swap was over a standard SEER 10 unit for one rated
at SEER 13. These additional tests funded by EPA added an additional three “systems”
(the Sacramento house with SEER 13 plus the Texas house in two systems
configurations) to the database for the Phase VI work.
2. Evaluation of duct innovative connections (DOE STTR). In this work we provided
technical advice and measurements for Proctor Engineering Group (PEG) who are
developing a snap together duct fitting system. This system is designed so that a rubber
ring around the end of the duct acts as both a seal and a mechanism to hold the dust
together. This work is intended to assist PEG in developing this product to the point
where it can be released in the market place. It is hoped that the use of innovative fittings
like these will improve the installation quality of duct systems, and reduce the energy
LBNL - 42691
losses associated with forced air thermal distribution. This project was awarded Phase II
funding and we are continuing our technical support to PEG.
3. Assessment of duct sealant health and safety hazards (EPA). As part of our
ongoing work to develop the Aeroseal duct sealing system, this project looked at the
potential health and safety hazards associated with the Aeroseal product. This work is a
necessary part of the product development, particularly for new and substantially different
methods from current practice. As with the duct fittings project, this is another piece of
potential market transformation work that should lead to improvements in thermal
distribution systems. This report concluded that the materials used in the Aeroseal
system are not a significant health or safety hazard.
4. Duct Cleaning effect on aerosol sealant (EPA). An issue that has been raised
regarding the aerosol sealant is its resistance to duct cleaning. In order to examine this
effect, a sheet metal duct system was sealed with the aerosol sealant and then cleaned by
professional duct cleaners. The duct cleaners were local HVAC contractors who cleaned
the ducts as they would for a regular service call. No attempt was made to change the
tasks they performed in order to make the duct cleaning more effective. The cleaning was
about four years after the initial aerosol sealing. The system was cleaned four times in
total. Before the system was cleaned the measured leakage was 6.2 cfm25 supply and 9.3
cfm25 return.
The first contractor vacuumed duct system supply four times: supply system twice and
return system once and both systems the same time once. For the first vacuuming, all of
the registers were taped and furnace was isolated. An 8” diameter vacuum hose was
connected to the supply plenum. The system was vacuumed for about 15 minutes. The
pressure difference across the duct (measured at the furthest part of the duct system from
where the vacuum hose was connected) was 35 Pa. The relatively low vacuum pressure
was because the cardboard used to isolate the furnace was misplaced and resulted in a big
leak. After this leak was fixed, the pressure difference across the duct increased to 254
Pa. The return was vacuumed separately. The two return registers were taped and the
furnace was isolated. The return duct was also vacuumed for about 15 minutes. The
pressure difference across the return duct was 980 Pa falling to 540 Pa at the end of the
vacuum cleaning. A duct leakage measurement was performed after the above cleaning
procedures, and the supply and return leakage were unchanged - within 0.1 cfm25 of the
pre cleaning leakage.
For the next cleaning an 8” diameter vacuum hose was connected to the furnace burner
access panel. Only a rudimentary attempt was made to seal around this connection using
a rag wrapped around the hose. All of the registers were simply covered with pieces of
paper rather than being completely sealed. The pressure difference across the ducts was
about 220 Pa and stayed at the same level during the vacuuming process. After these two
vacuumings, the duct leakage was measured to be 6.6 cfm25 for the supply and 9.0 cfm25
for the return. This result shows that the leakage was again unchanged (within the
experimental uncertainty) by the vacuuming process.
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A second HVAC contractor then vacuumed the duct system with a combination of
inserting a spinning brush into the ducts at the registers and brushing all the way to plenum
if there was no damper to block it. This was a much more severe test of the aerosol sealant
because it could have been abraded by the brushes. The supply and return duct systems
were isolated and each grille was removed before inserting the spinning brush and then the
register was plugged with a piece of foam. The supply duct was depressurized to about
330Pa. After this brushing and vacuuming, the measured leakage was 7.9 cfm25 for the
supply and 9.8 cfm25 for the return. This shows a small increase was possibly caused by a
change leakage at boot to wall seal where aerosol deposition was disturbed when the grille
was removed, however this amount of extra leakage (less than 2 cfm) is not significant
given the measurement uncertainty (±1 cfm???).
It should be noted that the vacuuming only did not appear to remove much dust from the
system. The combination of vacuuming and brushing was much more effective and
included the removal of a paper cup and a half roll of duct tape!
The conclusion drawn from these tests is that duct cleaning of a system sealed using the
aerosol sealant does not remove the sealant.
5. References
ACCA, (1986), Manual J - Load Calculation for Residential Winter and Summer Air Conditioning -
Seventh Edition., Air Conditioning Contractors of America (ACCA), Washington, D.C.
Andrews, J.W., Pierce, B.L., Hedrick, R., Lubliner, M., Reid, B., and Saum, D., (1998), “Repeatability of
ASHRAE Standard 152P: results of a round robin test”, ASHRAE Transactions, Vol. No.
CEC, (1998), Low-Rise Residential Alternative Calculation Method Approval Manual for 1998 Energy
Efficiency Standards for Low-Rise Residential Buildings, California Energy Commission, Sacramento,
NCDC. 1980. Climatography of U.S., #81, Monthly Normals of Temperature, Precipitation and Heating
and Cooling Degree days 1951-1980. National Climatic Data Center, Federal Building, Asheville, NC.
Proctor, J. 1998. “Monitored In-Situ Performance of residential Air-Conditioning Systems”, ASHRAE
Trans. Vol. 104. Part 1. ASHRAE, Atlanta, GA.
Proctor, J. 1997. “Field Measurements of new residential air conditioners in Phoenix, Arizona.” ASHRAE
Trans. Vol. 103 Part 1. ASHRAE, Atlanta, GA.
Modera, M.P., and Wilcox, B., (1995), “Treatment of residential duct leakage in Title 24 energy
efficiency Standards”, California Energy Commission Contract Report.
Rodriguez, A.G., O’Neal, D.L., Bain, J.A., and Davis, M.A.(1995) “The Effect of Refrigerant Charge,
Duct Leakage, and Evaporator Air Flow on the High Temperature Performance of Air Conditioners and
Heat Pumps”, Energy Systems Laboratory report for EPRI, Texas A&M University.
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Vieira, R.K., D.S. Parker, J. F. Klongerbo, J.K. Stone, J. Cummings. 1995. “How Contractors Really Size
Air Conditioning Systems”. Proc. 1996 ACEEE Summer Study on Energy Efficiency in Buildings.
Washington DC.: American Council for an Energy-Efficient Economy). FSEC-PF-289-95. Florida Solar
Energy Center.
Walker, I.S., (1998), “Technical Background for Default Values used for Forced Air Systems in Proposed
ASHRAE Trans., (presented at ASHRAE TC 6.3 Symposium, January 1998
also as LBNL Report 40588)
Walker, I.S., Brown, K., Siegel, J. and Sherman, M.H., (1998), “Saving Tons at the Register”, Proc.
ACEEE Summer Study, Vol. 1, pp. 367-383. (LBNL # 41957).
Walker, I, Sherman, M., Modera, M., Siegel, J.Dickerhoff, D., (1997), “Leakage Diagnostics, Sealant
Longevity, Sizing and Technology Transfer in Residential Thermal Distribution Systems, CIEE
Residential Thermal Distribution Systems Phase V Final Report, October 1997, LBNL Report 41118.
6. Recent Publications
Sherman, M.H. and Walker, I.S. (1998), “Can Duct Tape Take the Heat?
Vol.15, No.4 , pp., Berkeley, CA.
Walker, I.S., (1998), “Technical Background for Default Values used for Forced Air Systems in Proposed
ASHRAE standard 152P”, ASHRAE Trans., (presented at ASHRAE TC 6.3 Symposium, January 1998
also as LBNL Report 40588)
Walker, I.S. and Modera, M.P., (1998), “Field Measurements of the Interactions between Furnaces and
Forced Air Distribution Systems”, ASHRAE Trans., (presented at ASHRAE TC 6.3 Symposium, January
1998 also as LBNL Report 40587)
Walker, I.S., “Distribution System Leakage Impacts on Apartment Building Ventilation Rates”, ASHRAE
TC 4.1 Symposium. (to be presented at the ASHRAE Winter Meeting 1999, published in ASHRAE
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7. Appendices
Appendix A. Summary of diagnostic testing of thermal
distribution systems
Table A1. Summary of House Envelope Leakage Test Results1
Site Leakage
Coefficient, C
SLA Q50/floor area (cfm/ft2)
1 134 0.61 3.7 1.01
2 96 0.57 2.3 0.62
3 192 0.61 4.9 1.31
4 110 0.64 61.61
5 247 0.57 8.3 2.25
6 243 0.59 5.2 1.40
1 - Includes duct leakage
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Table A2. Summary of Tracer Gas Results for Residential Buildings Summer
Test Date Start
Time End
Time Fan
(fan ON – OFF)
Site 1 Decay 6/2 16.21 17.23 ON 0.73 0.17
17.23 18.09 OFF 0.56
Decay 6/5 10.39 15.43 OFF 0.22 0.16
15.43 16.27 ON 0.38
Decay 6/6 12.30 15.43 OFF 0.43 0.23
15.43 16.24 ON 0.66
Decay 6/7 14.47 15.42 OFF 0.62 0.32
15.42 15.59 ON 0.74
Decay 6/9 12.54 15.18 OFF 0.38 0.15
15.18 16.09 ON 0.53
Site 2 Decay 6/8 13.08 15.52 OFF 0.47 0.63
15.52 16.24 ON 1.10
6/10 11.28 15.49 OFF 0.41 0.60
15.49 16.24 ON 1.01
Site3 Decay 7/14 16.12 16.51 ON 0.52
Decay 7/21 11.51 12.19 ON 0.35
Decay 7/21 12.19 13.12 OFF 0.16 0.19
Site 4 Decay 8/20 16.42 19.57 ON 0.41 0.18
19.57 20.31 OFF 0.23
Site6 Decay 9/4 15.48 18.00 ON 0.25 0.12
18.00 19.51 OFF 0.13
Decay 9/16 11.50 14.37 OFF 0.08 0.37
14.37 17.12 ON 0.45
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Table A3. System Flows and Register Flows (cfm)
Fan flow Measurement Method Sum of register flows
Site Configuration Tracer
Gas Using Flow meter Sum of Supply
Register Flows
Plus Supply Duct
Supply Return (Traverse or
Flow Hood)
Site 1 - as found none 1837 (F Both)*
1861 (S Both)** 1905 1752 2177 (T)
Site 1 - after sealing 2250 2033 (S Flow meter)
1817 (S HVAC)
1731 (F Both)
1772 (S Both)
1848 1778 none
Site 2 -as found none 2286 (S Flow meter)
1962 (S HVAC)
1854 (F Both)
1927 (S Both)
1873 1740 2635 (T)
Site 2 - holes added none 1554 (F Both)
1774 (S Both) 1851 1643 none
Site 2 – after sealing 1970 none 1964 1857 none
Site 3 - as found 1460 1515 (F Both)
1504 (S Both) 1692 1339 1782 (T)
Site 3 - after sealing 1600 1440 (S Flow meter)
1453 (S HVAC)
1440 (F Both)
1451 (S Both)
1746 1556 1991 (T)
Site 4 - as found 1205 1210 (F Flow meter)
1361 (S flow meter)
1099 (F Both)
1069 (S Both)
1104 1004 none
Site 4 - added holes 1240 1021 (F flow meter)
1231 (S flow meter) 1082 928 828 (H)
Site 5 - as found 1260 none 1166 1024 1023 (H)
Site 5 - after sealing 1240 1704 (F Flow meter)
2115 (S flow meter)
2286 (S Both)
1171 1118 1007 (H)
Site 6 -as found none 1408 (F flow meter)
1614 (S flow meter)
1377 (F both, HVAC on first)
1381 (S both, HVAC on first)
1415 (S HVAC)
1448 1336 1492 (T)
Site 6 - after Energy Star none 1488 (F flow meter)
1664 (S flow meter)
1199 (F both, HVAC on first)
1258 (S both, HVAC on first)
1180 (F both, flow meter on first)
1263 (S both, flow meter on first)
1444 (S HVAC on )
1499 1430 1543 (T)
Site 6 - added holes
(after Energy Star) none 1697 (F Flow meter)***
1474 (S both, HVAC on first)***
1441 (S both, flow meter on
1559 1458 1415 (T)
* - S tests use highest measured flow and pressure and extrapolate to operating conditions (n=0.6)
** - F tests are fits to data points with a forced intercept of 0 flow at 0 pressure.
*** Flow meter mounted at the return grille, flows were corrected for estimated return and cabinet leakage.
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Table A4. Summary of Fan Pressurization Leakage Flows (cfm)
Site 1.
pressures Leakage
condition Supply Supply
Boots Return Return
Boots Cabinet
As found Total 21 70 22 11 51
25 Pa To Outside 19 14 8 7 51
Sealed Total 7 36 22 0 51
To Outside 3 25 8 0 51
As found Total 32 113 33 17 71
50 Pa To Outside 29 17 13 9 71
Sealed Total 11 57 33 0 71
To Outside 4 40 13 0 71
Site 2.
pressures Leakage
condition Supply Supply
Boots Return +
Boots Cabinet
As found Total 52 55 20 n/a 15
To Outside 28 29 0 n/a 15
25 Pa Additional Total 227 55 130 n/a 15
To Outside 239 29 110 n/a 15
Sealed Total 44 50 20 n/a 15
To Outside 27 13 0 n/a 15
As found Total 84 87 30 n/a 24
To Outside 55 33 0 n/a 24
50 Pa Additional Total 323 87 179 n/a 24
To Outside 354 33 149 n/a 24
Sealed Total 71 74 30 n/a 24
To Outside 43 18 0 n/a 24
Site 3.
pressures Leakage
condition Supply Supply
Boots Return Return
Boots Cabinet
As found Total 230 220 6
25 Pa To Outside 151 190 6
Sealed Total 124 90 6
To Outside 66 78 6
As found Total 340 326 9
50 Pa To Outside 228 296 9
Sealed Total 183 128 8
To Outside 101 108 8
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Table A4 (continued). Summary of Fan Pressurization Leakage Flows (cfm)
Site 4.
pressures Leakage
condition Supply Supply
Boots Return
Boots Cabinet
As found Total 26 92 38 n/a 26
25 Pa To Outside 24 63 24 n/a 26
Holes Total 145 92 134 n/a 26
To Outside 118 63 96 n/a 26
As found Total 42 145 58 n/a 39
50 Pa To Outside 39 95 38 n/a 39
Holes Total 221 145 196 n/a 39
To Outside 179 95 138 n/a 39
Site 5.
pressures Leakage
condition Supply Supply
Boots Return Return
Boots Cabinet
As found Total 23 107 21 15 29
25 Pa To Outside 20 87 18 8 29
Sealed Total 23 22 21 0 29
To Outside 20 3 18 0 29
As found Total 36 157 34 22 41
50 Pa To Outside 31 122 27 14 41
Sealed Total 36 35 34 0 41
To Outside 31 5 27 0 41
Site 6.
pressures Leakage
condition Supply Supply
Boots Return +
Boots Cabinet
As found Total 40 33 163 n/a 22
25 Pa To Outside 36 21 51 n/a 5
Estar unit Total 40 33 163 n/a 22
To Outside 36 21 51 n/a 42
Holes Total 101 33 322 n/a 22
To Outside n/a 42
As found Total 60 52 256 n/a 34
50 Pa To Outside 54 33 79 n/a 8
Unit Total 60 52 256 33
To Outside 54 33 79 64
Holes Total 158 52 506 33
To Outside 64
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Table A5. Added Leaks, cfm
Site Supply Return
2 97 115
4 75 32
6 32 73
Table A6. Summary of House Pressure Test Duct Leakage Results
Site Condition Supply Leakage (cfm) Return Leakage (cfm)
1As found 119 19
1Added Leaks 165 63
2As found 156 78
3As found 22 0
3Sealed 6 106
4As Found 58 16
4Added Leaks 147 127
5As found 169 96
5Sealed 35 42
6As found 115 72
6Energy Star and
added leaks 90 124
Table A7. Duct and Air Handler Location
Site Supply Duct Location Return Duct Location Air Handler Location
1Attic Attic Attic
2Attic Attic Attic
3Conditioned Space Plenum in garage Garage
4Attic Attic Attic
5Attic Attic Attic
6Attic Air handler stand in closet Closet
Table A8. Exterior Duct Surface Areas Including Plenums (ft2)
Site Supply Duct Return Duct
1 364 102
2 364 102
3 10 86
4 243 76
5 291 76
6 387 0
... Analysis of many DeltaQ and DeltaQfit tests has shown that fitting to the measured data is more robust if the duct leakage pressure exponents (ns and nr) are fixed. Experiments to characterize the pressure exponent in a wide range of duct configurations have shown that a value of 0.6 is suitable for most duct systems (Walker et al. 1998 and Siegel et al. 2002). However if it is known that the leakage is in the nature of an orifice or a disconnected duct then a pressure exponent of 0.5 is preferred. ...
... The first round of repeatability testing evaluated the same duct system 20 times over several days. These results showed that the repeatability errors were similar to values obtained in repeatability studies for direct duct pressurization (Walker et al. 1998).Table 1 summarizes these repeatability testing results for DeltaQ. Rather than analyze the repeatability results in absolute terms (cfm), it can be instructive to look at them in terms of the fraction of fan flow. ...
Full-text available
The DeltaQ test has been developed in order to provide better estimates of forced air system air leakage for use in energy efficiency calculations and for compliance testing of duct systems. The DeltaQ test combines a model of the house and duct system with the results of house pressurization tests with the air handler on and off to determine the duct leakage air flows to outside conditioned space at operating conditions. The key advantage of the DeltaQ test over other methods is that it determines the air leakage flows directly, rather than requiring interpretation of indirect measurements. The results from over 200 field and laboratory tests are presented. The laboratory tests have shown that the DeltaQ repeatability uncertainties are typically 1% or less of system fan flow and that the accuracy of the test is between 1.3% and 2.5% of fan flow (or 13 cfm to 25 cfm (6 to 12 l/s) for this system).
... This study describes the results of efforts made during the Transitional Phase of this Residential Thermal Distribution Systems research. Results of earlier Phases were described in Walker et al. (1997 and. ...
... The development of the longevity test method and preliminary results have been discussed in previous phases (Walker et al. 1997 and. The final results and details of the experiment were given in "Can Duct Tape Take the Heat" -LBNL report # 41434 and its companion Home Energy Article (Home Energy,Vol. ...
Full-text available
This report was prepared as a result of work sponsored by the California Energy Commission (Commission), through a contract with the Regents of the University of California, California Institute for Energy Efficiency (CIEE). It does not necessarily represent the views of the Commission, its employees, the State of California, The Regents, or CIEE. The Commission, the Regents, the State of California, CIEE, their employees, contractors, and subcontractors, make no warranty, express or implied, and assume no legal liability for the information in this report; nor does any party represent that the use of this information will not infringe upon privately owned rights. This report has not been approved or disapproved by the Commission or CIEE, nor has the Commission or CIEE passed upon the accuracy or adequacy of the information in this report.
... A field study was made on 11 handlers. It temporarily replaces the filter in the air handler distribution system during the airflow measurement process. ...
... Concern about residential HVAC component leakage has been an issue for more than 10 years. Walker et al. (1998) measured six furnaces in a field study and found an average of 11 L/s (23 cfm) at 25 Pa (0.1 in. water) or 16 L/s (34 cfm) at operating pressures measured in the furnace outlet and blower compartment. ...
Full-text available
In recent years, great strides have been made in reducing air leakage in residential and to a lesser extent small commercial forced air duct systems. Several authorities have introduced low leakage limits for thermal distribution systems; for example, the State of California Energy Code for Buildings gives credit for systems that leak less than 6% of the total air flow at 25 Pa. Practitioners have found that a significant barrier to meeting specifications like this is the air leakage of the furnace or air handler itself. Anecdotal evidence exists for the magnitude of the air leakage of furnaces and air handlers. The states of California and Florida include air leakage limits for the furnaces in their State Building Energy Codes. However, there is currently no standard test method for measuring this air leakage that could be used for uniform and reliable ratings. This paper presents the results of laboratory measurements air leakage testing of furnaces and air handlers. The results indicate that average air leakage is significant -confirming existing anecdotal evidence. Also, the air leakage has a wide range from furnace to furnace indicating that low levels of air leakage are already attainable with existing equipment and the rating for air leakage will be able to distinguish between good and poorly performing equipment. This paper will also discuss the development of a standard test procedure (ASHRAE Standard 193 "Method of Test for Determining the Airtightness of HVAC Equipment") that will be used by Federal, State and Local code authorities and efficiency programs as well as appliance standards, utility programs and a range of other applications.
... Data on air flow rates in central forced-air residential heating and cooling systems are also sparse. No relevant data bases were identified, however, several published papers from measurements in only a few states (Davis et al., 1998, Jump et al., 1996Olson et al., 1993;Walker et al., 1996Walker et al., , 1999 provide both measured flow rates and the floor area or volume of the residence. The average flow rate per unit floor area in the 84 HVAC systems within these papers is 0.3 L s -1 m -2 (0.7 cfm ft -2 ). ...
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The process of characterizing human exposure to particulate matter requires information on both particle concentrations in microenvironments and the time-specific activity budgets of individuals among these microenvironments. Because the average amount of time spent indoors by individuals in the US is estimated to be greater than 75%, accurate characterization of particle concentrations indoors is critical to exposure assessments for the US population. In addition, it is estimated that indoor particle concentrations depend strongly on outdoor concentrations. The spatial and temporal variations of indoor particle concentrations as well as the factors that affect these variations are important to health scientists. For them, knowledge of the factors that control the relationship of indoor particle concentrations to outdoor levels is particularly important. In this report, we identify and evaluate sources of data for those factors that affect the transport to and concentration of outdoor particles in the indoor environment. Concentrations of particles indoors depend upon the fraction of outdoor particles that penetrate through the building shell or are transported via the air handling (HVAC) system, the generation of particles by indoor sources, and the loss mechanisms that occur indoors, such as deposition. To address these issues, we (i) identify and assemble relevant information including the behavior of particles during air leakage, HVAC operations, and particle filtration; (ii) review and evaluate the assembled information to distinguish data that are directly relevant to specific estimates of particle transport from those that are only indirectly useful and (iii) provide a synthesis of the currently available information on building air-leakage parameters and their effect on indoor particle matter concentrations.
... • Duct leakage using fan pressurization tests (using the procedures in proposed ASHRAE 152P) • Measurement of duct surface area, and observation of duct type, location, and insulation More details about these tests are given in Walker et al. (1998aWalker et al. ( , 1998bWalker et al. ( and 1999. ...
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In residential and light commercial construction in the United States, heating and cooling ducts are often located outside the thermal or pressure boundary of the conditioned space. This location is selected for aesthetic and space requirement reasons. Typical duct locations include attics, above dropped ceilings, crawlspaces, and attached garages. A wide body of literature has found that distribution system conduction and air leakage can cause 30-40% energy losses before cooling and heating air reaches the conditioned space. Recent innovative attempts at locating ducts in the conditioned space have had mixed results in terms of improving duct efficiency. Some of these strategies include cathedralizing attics (sealing and insulating at the attic roofline) and locating ducts in interstitial spaces. This paper reviews modeling studies that suggest substantial savings could be realized from these strategies and presents field measurements which reveal that construction planning and execution errors can prevent these strategies from being widely applied or from being effective when they are applied. These types of problems will need to be overcome for effective integration of ducts into the conditioned space.
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Commissioning California's houses can result in better performing systems and houses. In turn, this will result in more efficient use of energy, carbon emission reductions, and improved occupant comfort. In particular, commissioning houses can save a significant amount of HVAC-related energy (15 to 30% in existing houses, 10 to 20% in new conventional houses, and up to 8% in advanced energy efficiency houses). The process that we considered includes corrective measures that could be implemented together during construction or during a single site visit (e.g., air tightening, duct sealing, and refrigerant and air handler airflow corrections in a new or existing house). Taking advantage of additional, more complex opportunities (e.g., installing new windows in an existing house, replacing the heating and air conditioning system in a new or existing house) can result in additional HVAC-related energy savings (60 to 75% in existing houses, and 50 to 60% in new conventional houses). The commissioning-related system and house performance improvements and energy savings translate to additional benefits throughout California and beyond. By applying commissioning principles to their work, the building community (builders and contractors) benefit from reduced callbacks and lower warranty costs. HERS raters and inspectors will have access to an expanded market sector. As the commissioning process rectifies construction defects and code problems, building code officials benefit from better compliance with codes. The utilities benefit from reduced peak demand, which can translate into lower energy acquisition costs. As houses perform closer to expectations, governmental bodies (e.g., the California Energy Commission and the Air Resources Board) benefit from greater assurance that actual energy consumption and carbon emissions are closer to the levels mandated in codes and standards, resulting in better achievement of state energy conservation and environmental goals. California residents' quality of life is improved through better indoor environmental comfort and lower energy bills. Lower energy bills free up money for residents to spend on other needs or goals, such as additional education and health and welfare. With an expansion of existing industries and the development of new commissioning-related industries, related jobs and tax revenues will increase, further increasing the quality of life for California.
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Duct leakage is a major source of energy loss in residential buildings. Most duct leakage occurs at the connections to registers, plenums, or branches in the duct system. At each of these connections, a method of sealing the duct system is required. Typical sealing methods include tapes or mastics applied around the joints in the system. Field examinations of duct systems have shown that taped seals tend to fail over extended periods of time. The Lawrence Berkeley National Laboratory (LBNL) has been testing sealant durability for several years using accelerated test methods and found that typical duct tape (i.e., cloth-backed tapes with natural rubber adhesives) fails more rapidly than other duct sealants. This report summarizes the results of duct sealant durability testing over two years for four UL 181B-FX listed duct tapes (two cloth tapes, a foil tape and an Oriented Polypropylene (OPP) tape). One of the cloth tapes was specifically developed in collaboration with a tape manufacturer to perform better in our durability testing. The tests involved the aging of common ''core-to-collar joints'' of flexible duct to sheet metal collars. Periodic air leakage tests and visual inspection were used to document changes in sealant performance. After two years of testing, the flex-to-collar connections showed little change in air leakage, but substantial visual degradation from some products. A surprising experimental result was failure of most of the clamps used to mechanically fasten the connections. This indicates that the durability of clamps also need to be addressed ensure longevity of the duct connection. An accelerated test method developed during this study has been used as the basis for an ASTM standard (E2342-03).
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Measurements on three gas and two electric furnaces have been made to examine the field performance of these furnaces and their interactions with their forced-air distribution systems. The distribution systems were retrofitted as part of this study and the impact of retrofitting on furnace performance is discussed. In addition to field measurements, this paper will discuss how forced-air furnace systems are treated in proposed ASHRAE Standard 152P, and applies the resulting equations to the systems tested in the field. The distribution system calculations in Standard 152P are compared to the current methods employed in the "Furnaces" chapter of ASHRAE's HVAC Systems and Equipment Handbook, showing how the distribution system efficiencies calculated using Standard 152P can be incorporated into the handbook.
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ASHRAE Standard 152P (Method of Test for Determining the Design and Seasonal Efficiencies of Residential Thermal Distribution Systems) includes default values for many of the input parameters required to calculate delivery system efficiencies. These default values have several sources: measured field data in houses, laboratory testing, simple heat transfer analyses, etc. This paper will document and discuss these default values and their sources for forced air systems.
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Duct losses have a significant effect on the efficiency of delivering space cooling to U.S. homes. This effect is especially dramatic during peak demand periods where half of the cooling equipment's output can be wasted. Improving the efficiency of a duct system can save energy, but can also allow for downsizing of cooling equipment without sacrificing comfort conditions. Comfort, and hence occupant acceptability, is determined not only by steady-state temperatures, but by how long it takes to pull down the temperature during cooling start-up, such as when the occupants come home on a hot summer afternoon. Thus the delivered tons of cooling at the register during start-up conditions are critical to customer acceptance of equipment downsizing strategies. We have developed a simulation technique which takes into account such things as weather, heat-transfer (including hot attic conditions), airflow, duct tightness, duct location and insulation, and cooling equipment performance to determine the net tons of cooling delivered to occupied space. Capacity at the register has been developed as an improvement over equipment tonnage as a system sizing measure. We use this concept to demonstrate that improved ducts and better system installation is as important as equipment size, with analysis of pull-down capability as a proxy for comfort. The simulations indicate that an improved system installation including tight ducts can eliminate the need for almost a ton of rated equipment capacity in a typical new 2,000 square foot house in Sacramento, California. Our results have also shown that a good duct system can reduce capacity requirements and still provide equivalent cooling at start-up and at peak conditions.
Forced air distribution systems in residential buildings are often located outside the conditioned space, for example, in attics, crawl spaces, garages, and basements. Leaks from the ducts to these unconditioned spaces or outside can change flows through the registers and change the ventilation rates of the conditioned spaces. In this study, duct leakage flows were measured in several low-rise apartment buildings. The leakage flow measurements and other data about the apartments were used to develop a prototype apartment building. The multizone airflow model COMIS was then used on this prototype building to determine internal flows within the building, airflows through the building envelope, and the impacts of the duct leakage on the ventilation rates. The effects of sealing the duct leaks were also examined in order to determine changes in infiltration rates resulting from duct retrofits. The simulation results showed that for the prototype tested here, the excess return leakage tended to decrease envelope infiltration flows by about 20%, but the total infiltration load, including return duct leaks, more than doubled during system operation.
An experimental study was conducted to quantify the effect of several installation items on the high outdoor ambient temperature performance of air conditioners. These installation items were: improper amount of refrigerant charge, reduced evaporator airflow, and return air leakage from hot attic spaces. There were five sets of tests used for this research: two of them for the charging tests, two for the reduced evaporator airflow, and one for the return air leakage tests. For the charging tests, the indoor room conditions were 80°F (27.8°C) dry-bulb and 50% relative humidity. The outdoor conditions ranged from 95°F (35°C) all the way up to 120°F (48.9°C). Charge levels ranged from 30% undercharged to 40% overcharged for the short-tube orifice unit. For the thermal expansion valve (TXV) unit, charge levels ranged from -36% charging to +27% charging. Performance was quantified with the following variables: total capacity, energy efficiency ratio (EER), and power. The performance of the orifice unit was more sensitive to charge than it was for the TXV unit. For the TXV unit on the -27% to +27% charging range, the capacity and EER changed little with charge. A TXV unit and a short-tube orifice unit were also tested for reduced evaporator airflow. As evaporator airflow decreased, the capacity and EER both decreased as expected. However, the drop was not as significant as with the charging tests. For the extreme case of 50% reduced evaporator airflow, neither unit's capacity or EER dropped more than 25%. Return air leakage from hot attic spaces was simulated by assuming adiabatic mixing of the indoor air at normal conditions with the attic air at high temperatures. Effective capacity and EER both decreased with increased return air leakage. However, power consumption was relatively constant for all variables except outdoor temperature, which meant that for the same power consumption, the unit delivered much lower performance when there was return air leakage. The increase in sensible heat ratio (SHR) with increasing leakage showed perhaps the most detrimental effect of return air leakage on performance, which was the inability of the unit to absorb moisture from the environment.
Manual J -Load Calculation for Residential Winter and Summer Air Conditioning -Seventh Edition
ACCA, (1986), Manual J -Load Calculation for Residential Winter and Summer Air Conditioning -Seventh Edition., Air Conditioning Contractors of America (ACCA), Washington, D.C.
Low-Rise Residential Alternative Calculation Method Approval Manual for 1998 Energy Efficiency Standards for Low-Rise Residential Buildings
CEC, (1998), Low-Rise Residential Alternative Calculation Method Approval Manual for 1998 Energy Efficiency Standards for Low-Rise Residential Buildings, California Energy Commission, Sacramento, California.
Monitored In-Situ Performance of residential Air-Conditioning Systems
  • J Proctor
Proctor, J. 1998. "Monitored In-Situ Performance of residential Air-Conditioning Systems", ASHRAE Trans. Vol. 104. Part 1. ASHRAE, Atlanta, GA.
  • J W Andrews
  • B L Pierce
  • R Hedrick
  • M Lubliner
  • B Reid
  • D Saum
Andrews, J.W., Pierce, B.L., Hedrick, R., Lubliner, M., Reid, B., and Saum, D., (1998), "Repeatability of ASHRAE Standard 152P: results of a round robin test", ASHRAE Transactions, Vol. No.
Can Duct Tape Take the Heat?
  • M H Recent Publications Sherman
  • I S Walker
Recent Publications Sherman, M.H. and Walker, I.S. (1998), "Can Duct Tape Take the Heat? Vol.15, No.4, pp., Berkeley, CA.