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Field Measurements of Efficiency and Duct Retrofit Effectiveness
in Residential Forced Air Distribution Systems
David A. Jump, Iain S. Walker, and Mark P. Modera, Lawrence Berkeley National Laboratory
Forced air distribution systems can have a significant impact on the energy consumed in residences. It is
common practice in U.S. residential buildings to place such duct systems outside the conditioned space.
This results in the loss of energy by leakage and conduction to the surroundings. In order to estimate the
magnitudes of these losses, 24 houses in the Sacramento, California, area were tested before and after duct
retrofitting. The systems in these houses included conventional air conditioning, gas furnaces, electric
furnaces and heat pumps. The retrofits consisted of sealing and insulating the duct systems.
The field testing consisted of the following measurements: leakage of the house envelopes and their ductwork,
flow through individual registers, duct air temperatures, ambient temperatures, surface areas of ducts, and
HVAC equipment energy consumption. These data were used to calculate distribution system delivery
efficiency as well as the overall efficiency of the distribution system including all interactions with building
load and HVAC equipment. Analysis of the test results indicate an average increase in delivery efficiency
from 64% to 76% and a corresponding average decrease in HVAC energy use of 18%. This paper summarizes
the pre and post retrofit efficiency measurements to evaluate the retrofit effectiveness, and includes cost
estimates for the duct retrofits. The impacts of leak sealing and insulating will be examined separately.
loss by heat conduction through duct walls and loss of condi-
INTRODUCTION
tioned air through holes in the ducts. The objective of this
study was to evaluate the retrofit effectiveness of each of
It is common practice in many locations to put forced-air- these measures (including any interaction between the retro-
system ductwork outside the conditioned envelope. Typical fit measures) and to determine possible energy savings due
duct locations are attics, crawl spaces and garages. Putting to retrofitting. Also, by determining the cost of the retrofits
ducts in these non-conditioned areas increases the potential an economic evaluation of the retrofit procedures could be
for energy losses from the duct system because the ducts performed. The results of this study also provided additional
are exposed to a harsher environment and any energy lost base line information on the magnitude of duct losses.
from the ducts is outside the conditioned envelope of the
building. Previous studies (e.g., Cummings et al. 1990; Mod- Twenty four houses in Sacramento, California were used in
era 1993; Palmiter and Francisco 1994; Parker et al. 1993 this study. The houses were of varying energy use and had
and Proctor 1991) have shown losses on the order of 35% floor areas of 78 m
2
(840 ft
2
) to 372 m
2
(4000 ft
2
). There
are typical in residential construction. This contributes to was a variety of equipment tested, with 13 air conditioners,
large energy bills for home owners and to large peak eight heat pumps (one house had three heat pumps), three
demands for utilities. gas furnaces and two electric furnaces. Almost all the ducts
were located in attics and the majority were made of flexible
Some previous studies have examined the impact of duct plastic duct.
retrofits; for example,
These houses were monitored during pre- and post-retrofit
●Palmiter and Francisco made pre and post duct system periods to determine distribution system and equipment per-
retrofit measurements in six houses and found a 70% formance. Retrofits consisted of adding extra insulation to
reduction in duct leakage post retrofit and a 16% reduc- the exterior of the ducts (added insulation was foil backed
tion in heating energy consumption. 50 mm (2 in) thick, nominally RSI 1 (R-6)) and using metal
foil backed butyl tape and mastic to seal duct leaks.
●Cummings et. al. performed pre and post duct retrofit
measurements in 24 houses. They found an average By examining a wide range of houses, this study revealed
energy reduction of 18% at a retrofit cost of about $200 a wide range of potential savings from retrofitting. The diag-
per house. nostic procedures performed for this study could be used
to select houses with the greatest potential benefit fromIn the current study, duct systems were retrofitted by sealing
leaks and adding insulation. These measures reduced energy retrofitting the duct system.
Field Measurements of Efficiency and Duct Retrofit Effectiveness - 1.147
Preliminary results from a subset of the test few houses in (5) Equipment characteristics such as type of equipment
(A/C, gas furnace, heat pump), heating/cooling capac-this study were presented previously by Jump and Modera
1994. Jump and Modera focused on duct leakage and con- ity, air handler rated flow, and location within the
building.duction losses. The current paper expands on this previous
study to concentrate on delivery and overall system effi-
ciency in all the houses, and also provides estimates of the (6) House characteristics such as floor area, number of
fraction of losses attributed to leakage or conduction. stories, floor plan.
Two week measurements
FIELD TESTING
Measurements were made over a two week period so as to
Tests performed in the field consisted of two types: capture changing weather conditions and system cycling
effects in both pre- and post-retrofit periods. These measure-
(1) Diagnostic tests of building and duct system character- ments were made by installing monitoring equipment in
istics. the test houses and using a computer based automatic data
acquisition system to store the data. Air temperatures were
(2) Approximatelytwoweeksofmonitoringofcharacteris- measured with fast-response thermistors, while electric
tic temperatures, weather and HVAC power consump- power was measured with clamp-on current meters. The
tion in both pre- and post-retrofit periods. measurements included:
More detailed descriptions of the field tests can be found in (1) Register temperatures: Used together with the mea-
Jump and Modera 1994. In this paper we will present an sured register air flows to calculate energy supplied to
overview of the test procedures in order to provide a context the conditioned space during fan operation.
for the experimental results.
(2) Plenum temperatures (and relative humidity for air
Diagnostic Tests
conditioners): Used together with measured fan air
flows to calculate energy output by the equipment and
input to the ducts.
The diagnostic tests were performed to determine building
andductsystemparametersofinterestinenergycalculations. (3) Ambient temperatures: These include outside air tem-
These measured parameters were considered invariant dur- perature, and the temperature of air surrounding the
ing system operation. The following measurements were ducts (for most houses in this study this is the attic air).
performed for the diagnostic testing:
(4) Energy consumed by equipment: This was electrical
(1) House pressurization test to determine exterior enve- power consumed by the air handlers, and either electri-
lope leakage. This test was a slightly modified version cal power for air conditioning, heat pumps and electric
of ASTM 779 1991. furnaces, or natural gas consumed by gas furnaces.
(2) Register air flows. These flows were measured with
a modified flow capture hood and calibrated fan as
RESULTS
described in Jump and Modera 1994. The overall system performance is characterized by the nor-
malized power consumption given by:
(3) Fan flow. The air flow through the fan was measured
using a constant injection tracer gas technique based
on ASTM E741 1994 (see Jump and Modera 1994). P
(T
in
1T
out
)(1)
Some fan flows were measured with a fan assisted flow
capture hood method. System air leakage flow during
operation was determined directly from the measured where P is the energy consumed during a system cycle, T
in
fan and register air flows. is the indoor temperature and T
out
is the ambient temperature.
The normalized power consumption is calculated for each
system cycle, using values of P integrated over the cycle.
(4) Duct system characteristics, including the number and
location of registers, duct location, duct shape (round,
rectangular), duct material (flex duct, sheet metal or The following relationships are used to describe duct system
performance (for convenience only relationshipsfor heatingduct board), diameter and length of ducts and air han-
dler location. systems are given).
1.148 - Jump, Walker and Modera
Delivery efficiency (h
del
) is the ratio of energy supplied to analysis used monitored temperatures to calculate F
sl
once
assuming all supply leaks lost air at the supply plenumthe conditioned space through the registers (E
del
)tothe
energy input to the duct system from the equipment (E
in
) temperature, and calculated F
sl
again assuming supply leaks
lost air at an average duct temperature. The options forwhile the fan is operating. Note that the energy supplied to
the conditioned space is the net energy and includes energy estimating the appropriate temperature difference for calcu-
lating F
sl
were:removed by the return side of the system (i.e., it is not just
the energy in the air coming out of the supply registers). Option 1: All leaks were at the plenum. F
sl
is calculated
These definitions are the same as in the proposed ASHRAE using (T
sp
-T
in
) in Equation 6.
standard 152P.
Option 2: Leaks distributed over duct system. F
sl
is calcu-
h
del
4E
del
E
in
(2) lated using T
sreg
`T
sp
21T
in
in Equation 6.
where Table 1 summarizes the calculated fractional supply leakage
and conduction losses calculated using the above two
E
in
4M
e
Cp(T
sp
1T
rp
) (3) options. In Table 1, the fractional change in F
sl
(DF
sl
) and
F
sc
(DF
sc
) are given by:
E
del
4(M
e
1M
s
)Cp(T
sreg
1T
in
) (4)
1(M
e
1M
r
)Cp(T
rreg
1T
in
)DF
sl
4100(F
sl
(Option1) 1F
sl
(Option2))
F
sl
(Option1) (8)
and M
e
is the measured flow through the system fan, M
s
is
the supply leakage, M
r
is the return leakage, T
sp
is the supply DF
sc
4100(F
sc
(Option1) 1F
sc
(Option2))
F
sc
(Option1) (9)
plenum temperature, T
rp
is the return plenum temperature,
T
sreg
is the mass flow weighted supply register temperature, Because the temperature changes from the plenum to the
T
rreg
is the mass flow weighted return register temperature, registers are not very large in these systems (typically 3° C),
and Cp is the specific heat of air. the assumption of duct leak location did not have a large
impact on the test results. Averaged over all the systems the
Equipment efficiency (h
equip
) is the ratio of energy supplied supply leakage loss was changed by 9% both pre and post
to the duct system (E
in
) to the energy consumed by the retrofit. In Table 1 it was assumed that all the change in
equipment (E
equip
). Both E
in
and E
equip
include fan power. leakage losses would appear as conduction losses in order
to estimate the effect of the leak location on conduction
h
equip
4E
in
E
equip
(5) losses. Averaged over all the duct systems, the supply con-
duction loss was increased by 9% pre retrofit and by only
4% post retrofit. The changes between pre and post retrofit
The fraction of energy lost due to supply leaks (F
sl
) was were unchanged (55%) for leakage but for conduction the
estimated by assuming that all the leaks are at the plenum, option 1 change due to the retrofit was less than 1%, but
and is given by: changed to 5% using option 2.
F
sl
4M
s
Cp(T
sp
1T
in
)
E
in
(6) Because the temperature of air leaking into the return ducts
was generally unknown, the return leakage and conduction
losses are combined into a single term for fractional return
The fraction of energy lost due to supply conduction was losses (F
rloss
), such that the total losses plus the energy deliv-
given by: ered to the conditioned space by the duct system add up to
the energy supplied to the duct system:
F
sc
4(M
e
1M
s
)Cp(T
sp
1T
sreg
)
E
in
(7)
F
rloss
4M
e
C
p
(T
rp
1T
in
)
E
in
(10)
In order to isolate leakage losses from conduction losses it
was assumed that all the leaks were at the plenum. This 1(M
e
1M
r
)C
p
(T
rreg
1T
in
)
E
in
assumption has the potential for overestimating leakage
losses at the expense of conduction losses. However, the
followinganalysisshowedthatthe assumption that all supply where T
rreg
is the return register temperature and M
r
is the
return leakage mass flow. As a check, F
rloss
should be equalleaks are at the plenum did not have a large impact on the
split between supply leakage and conduction losses. This to 1 1h
del
1F
sl
1F
sc
.
Field Measurements of Efficiency and Duct Retrofit Effectiveness - 1.149
Table 1. Comparison of Methods for Calculating Supply Leakage Losses
PRE retrofit POST retrofit
Temperature difference options Option 1 Option 2 Option 1 Option 2
Temperature Difference for Leakage Losses 17.1° C 15.5° C 17.9° C 16.4° C
F
sl
, Fractional Supply leakage loss 17.6 % 16 % 7.9 % 7.2 %
DF
sl
, Fractional Change in F
sl
using options 1 and 2 9 % 9 %
F
sc
, Fractional Supply conduction loss 15.8 % 17.3 % 15.7 % 16.4 %
DF
sc
, Fractional Change in F
sc
using options 1 and 2 9 % 4 %
In this paper a binning procedure is introduced to improve
Data Binning Procedure
the comparisons. The parameters monitored for each two
week period were averaged over the cycle time of the equip-
In order to be able to compare systems between houses and ment. The cycle time was defined as the period of time
between pre- and post-retrofit periods a binning procedure from when the equipment switched on to the next time the
was used. The data was binned by indoor to outdoor tempera- equipment switched on. Data for extremely long cycles (over
ture difference to minimize the effects of changes in operat- two hours) was ignored in order to eliminate AC systems
ingconditions(ductambienttemperatures)andsystemloads. shuttingdownatnight,heatingsystemsshuttingdownduring
This binning method does not account for differences in the day or interactions with the occupants of the houses.
solar gain or the effects of the thermal mass of the building. Also, houses with undersized equipment with very long on
times were not analyzed with this binning procedure.
In the original procedure proposed by Jump and Modera
(1994) the energy consumption for each day was summed For each cycle, the power consumed by the equipment was
and the average indoor-outdoor temperature was calculated. integrated to obtain the system energy consumption. In addi-
Each day would then generate a value of energy consumed tion, the measured air flow rates through the fan and registers
at a given load. Using regression analysis to estimate energy were combined with the measured temperatures to determine
use at any temperature would allow comparisons between the energy delivered by the equipment (by convention, this
the pre- and post-retrofit periods even when the weather was was negative for cooling) and the energy delivered by the
not the same for both cases. Unfortunately, this system was registers to the rooms. With this information the equipment
found to be inadequate because: and delivery efficiencies were determined for each cycle.
The calculated delivery efficiencies and normalized power
●Two weeks of testing in each period did not provide consumption were then sorted into bins using the measured
enough data. temperatures. The bins are 2° C wide and are represented
by their middle temperature, i.e., the 20° C bin represents
●The weather range for each two week period was nar- all temperatures between 19° and 21° C. The results given
row, but weather changes between pre- and post-retrofit later are the average of all the cycles falling into each bin.
were often large. This generated large extrapolation Using this cyclic averaging results in data bins covering a
uncertainties. wide range of temperatures and allows variations with
weather conditions or due to retrofits to be observed more
●Small changes in average daily temperature were found easily.
over the two week test periods even when temperature
changes through the day were large. This meant that it
was difficult to determine correlations between energy
Diagnostic Results
use and system load (indoor-outdoor temperature). Taken as a group, these houses are representative of typical
California housing with floor areas of about 164 m
2
(1765The above limitations did not allow for comparisons between
houses, nor for calculating the overall effects of the retrofits ft
2
) and system fans producing about 1700 m
3
/hour (1000
cfm). The diagnostic results are summarized in Table 2. Noteon the duct systems performance.
1.150 - Jump, Walker and Modera
Table 2. Diagnostic Test Results and House Specifications
Fan Flow Supply Leakage % Return Leakage
m
3
/hour of Fan Flow % of Fan Flow
Floor Area System
House # Stories m
2
Type* Pre Post Pre Post Pre Post
11 135 AC 1807 1654 22 11 22 15
21 158 AC 2756 2639 24 14 24 19
32 127 AC 1408 24 17 44 15
41 155 AC 1825 1780 24 15 13 11
51 78 AC 1754
61 200 HP 2153 16 10 22 6
71 93 EF 1129 13330
81 135 HP 1413 12858
9
1HP 1623 6 0 24 16
921 372 HP 1943 15 13 34 27
93HP 1766 33 10 13 17
10 3 223 HP 1484 1468 26 10 26 1
11 2 186 HP 2354 2324 23 9 11 1
12 1 130 GF 841 817 19 20 0 6
13 2 132 HP 1972 1774 33 3 35 4
14 2 214 GF 2028 1966 21501
15 1 155 GF 1207 1415 19 30 35 27
16 1 139 EF 1713 1666 11544
17 2 177 AC 1793 1716 5260
18 2 167 AC 1471 1458 29 0 15 6
19 2 156 AC 1466 1583 13 2 7 16
20 1 242 AC 1635 1213 38 6 26 24
21 1 158 AC 1293 1171 14 8 16 16
22 1 125 AC 1849 1582 7 0 22 14
23 1 114 AC 1691 1619 2053
24 1 153 AC 1847 1813 6332
Mean - 164 - 1701** 1648 18 8 17 10
St. Dev - 59 - 391 379 10 8 13 9
* - AC: air conditioning, HP: heat pump, GF: gas furnace, EF: electric furnace
** - 1724 for fans that were also tested post-retrofit
that the supply and return flows are expressed as a fraction
Binned Test Results
of the fan flow. These results show that the system fan flows
are reduced by about four percent on average due to the The binned data for each system was examined in order to
find the bin that had the most cycles in both pre- and post-added flow resistance caused by sealing the ducts. The aver-
age pre retrofit leakage flows were 18% of fan flow for retrofit periods. For example, the data used for House 14
has T
in
422° C, T
out
410° C, DT412° C both pre andsupplies and 17% of fan flow for returns. The reduction in
leakage due to the retrofits was about 10% of fan flow for post retrofit. These particular temperatures had 31 cycles
pre retrofit and 33 cycles post retrofit. Other temperaturesupply ducts and 7% of fan flow for return ducts. These
correspond to significant reductions in leakage flows: 55% bins had a lower number of cycles pre or post retrofit. This
means that only a single bin for both pre and post retrofitof pre retrofit supply leakage and 40% of pre retrofit return
leakage was sealed. There was a large range of fractional in each house provided the results shown here. This allowed
an estimation of the effect of the retrofits to be made withoutleakage flows from system to system, as shown in the table
and by the standard deviations being a large fraction of the biases dueto changingweather conditions. Onaverage, there
were 28 cycles in each bin used in this analysis.mean value.
Field Measurements of Efficiency and Duct Retrofit Effectiveness - 1.151
Table 3 summarizes the measured delivery efficiency, nor- they do not appear in Table 3. This reduces the number
of houses compared pre and post retrofit to 17 out of themalized power consumption and what fraction of the losses
are attributed to supply leakage, supply conduction and original 24.
return losses. Some of the return losses are negative because
the return leakage and conduction energy flows acted to heat The following results are based upon averages over all systems:
the air in the ducts (for heating) or cool the air in the ducts
(for cooling) and therefore were a net benefit to the system ●Pre-retrofit, the delivery efficiency was 64% and this
increased to a post retrofit value of 76% (an increaseenergy balance. Some houses did not have complete mea-
surements and are neglected in this analysis, and therefore of 19% of the pre retrofit value). There was a wide
Table 3. Pre and Post Retrofit Two Week System Test Results
Fractional Normalized
Delivery Fractional supply power
Equipment Efficiency, h
del
, supply leak loss, conduction loss, Fractional consumption,
Efficiency, h
equip
% % % return loss, % P/DT (W/K)
House Pre Post Pre Post Pre Post Pre Post Pre Post Pre Post
22.55 2.45 68 78 24 17 7 4 2 1 453 387
31.98 1.94 63 68 31 25 12 12 1614 145 129
62.26 2.42 63 77 16 10 10 10 10 4 206 175
70.75 0.73 71 88 16 1 16 11 11 0 367 200
82.2 2.05 62 72 14 9 18 14 7 5 99 89
10 2.76 1.69 57 74 34 15 12 15 9 13 217 164
11 1.93 1.61 53 69 25 10 12 17 13 5 228 98
12 0.64 0.63 66 64 23 24 11 12 1 0 503 445
13 1.62 1.49 53 90 32468 911 120 108
14 0.96 0.94 46 57 23 6 34 40 1212 700 634
17 1.87 1.94 85 8712915 515 263 247
18 1.53 1.49 46 68 26 1 23 30 4 1 283 178
19 1.35 1.46 67 69 11 2 20 26 2 2 287 268
21 1 0.95 74 85 13 8 15 10 13 0 348 326
22 20.6 1.53 68 85 6 0 17 19 9 13 165 145
23 0.93 0.99 75 95 0 0 23 6 1 0 151 127
24 2.46 2.34 64 71 5 3 23 20 8 7 310 294
Mean 1.7 1.6 64 76 18 8 16 16 4 0 285 236
1.152 - Jump, Walker and Modera
variability from house to house from a minimum change ●The normalized power consumption (P/DT) was 285
W/K pre-retrofit and 236 W/K post-retrofit, a reductionof 3% of pre retrofit value to a maximum change of 68%. of 18%.
This indicates that diagnostic tests would be valuable in
selecting houses that would receive maximum benefit The 19% increase in delivery efficiency, combined with a
from duct system retrofitting. slightreduction in equipment efficiency and the same indoor-
outdoor temperature difference leads to an 18% reduction
●The fraction of energy lost from supply leaks was 18% in normalized power consumption. This result indicates that
in the pre-retrofit period and decreased to 8% in the the change in delivery efficiency is a good indicator of
post-retrofit period. This corresponds to the reduction system energy savings. This is illustrated in Figure 1, where
in supply leakage flows from 18% of fan flow to 8% the change in delivery efficiency is compared to change in
of fan flow. Note that despite precise agreement of the normalized power consumption. There is a strong correlation
numbers, the correlation between leakage flow and between these parameters which shows that the change in
energy losses is not exact because the retrofits changed energy consumption is mostly due to increased delivery
air temperature within the ducts between pre and post efficiency. The increased delivery efficiency is due to the
retrofit periods, as well as leakage flows. duct sealing and added insulation.
The range of normalized power consumption reduction was
●The fraction of energy lost due to supply conduction from 5% (of pre-retrofit normalized power consumption) to
was unchanged at about 16% of delivered energy for 57%, with a large variability from house to house, which is
both periods. This was because the decrease in tempera- a similar result to the delivery efficiency. This shows that
turedifference between supply plenum and supply regis- diagnostic tests will be important in selecting suitable houses
ters (indicating less energy loss) is balanced by an for duct retrofits because some systems have the potential
increase in supply duct flows because the supply leaks for large improvements and some systems do not. The selec-
have been sealed. Note that if ducts were sealed only, tion of houses with large potential savings is important for
without adding extra insulation, the conduction losses both utilities and home owners in order to maximize the
would have increased. cost effectiveness of retrofits.
●The return side losses were reduced from 4% to about There was a trend of increasing improvement in delivery
0% by the retrofit changes. Some systems had positive efficiency and reduction in normalized energy consumption
return losses and others had negative return losses. Note with systems with larger pre-retrofit supply leakage. There-
that the return losses combine both leakage and conduc- fore, supply duct leakage measurements can be used as a
tion effects. In general, the temperature differences rough guide for selecting houses that will have the greatest
between the air in the ducts and their surroundings are benefit from duct retrofits. This does not imply that return
less for return ducts than supply ducts, and the impact leakage may be neglected. In other houses there may be
of conduction losses and duct leakage duct leakage is
reduced. The exception is return duct leakage in hot Figure 1. Use of Delivery Efficiency to Predict Reduction
attics in the summer, in which case the return leakage in Energy Consumption.
has a large effect on air conditioning performance. For
example, an attic in a hot humid climate will have air
with about twice the enthalpy of indoor air. Therefore
a 10% of fan flow return leak will increase the enthalpy
of the air in the return plenum by 20%, with a corres-
ponding increase in energy use for the A/C equipment.
●The COP’s for heat pumps and air conditioners changed
from 1.89 to 1.74 and the electric and gas furnace effi-
ciencies changed from 0.78 to 0.77 before and after the
retrofit. This result shows that the retrofits can have a
slightly negative impact on the equipment operation.
●The average indoor-outdoor temperature difference was
10° C (18° F) both pre and post retrofit for the results
discussed here.
Field Measurements of Efficiency and Duct Retrofit Effectiveness - 1.153
greater return leakage than the houses used in this study. In savings for a house before retrofitting. Diagnostics should
include a determination of the ease of access to the duct
addition, climate and duct location can both have significant system because this is important in estimating the cost of
impacts on the effects of return duct leakage. Return duct the retrofits.
leakage interacts with supply duct leakage in system imbal-
ance effects on building air infiltration and in systems with
more supply duct leakage than return duct leakage, and there
CONCLUSIONS
are also safety issues concerned with backdrafting of natural The reduction in energy consumption due to sealing and
combustion appliances. insulating the duct systems was about 18% on average. Most
of this reduction in energy consumption was due to a 19%
Retrofit Costs
increase in delivery efficiency (i.e., the retrofit impact on
building loads and on equipment performance was negligi-
The cost of the retrofits is shown in Table 4 for each house. ble). There was a large variation (5% to 57%) in energy
The mean cost was $635 with a minimum of $335 for house consumption reduction and in retrofit costs from house to
15 and a maximum of $1069 for house 3. These costs do house, indicating that the energy savings and cost effective-
notincludefixedcosts per house for travel time, which would ness of duct retrofits is highly system dependent, and that
tend to reduce this variation. The costs were normalized with it would be prudent to have some simple diagnostic tests
respect to the size of the duct system (surface area) and it and inspections performed on duct systems before investing
was found that there remained a large variation from house in duct retrofits. The most significant diagnostic would be
to house. The standard deviation of these normalized costs to estimate supply duct leakage because sealing supply duct
was about 75% of the average cost. The costs were broken leaks tended to be the dominant factor in increasing delivery
down into materials and labor for both sealing and insulating. efficiency. A simple visual inspection could be used to deter-
Details of this breakdown are shown in Table 4. mine ease of access and to look for large potential duct
problems such as missing insulation or disconnected ducts.
The labor costs dominate over the materials cost (labor costs
averaged 77% of the total retrofit cost), which is why there For the houses tested in this study, the return losses were
is a large range of costs from house to house that is indepen- negligible. However, return losses cannot generally be
dent of the size of the duct system. The labor costs reflect ignored because the impact of return losses is highly depen-
the time required to seal or insulate the system, which is dent on climate and duct location.
due to ease of access to the duct system and of finding duct
system leaks. Theaverage cost of the retrofits was $635 and wasdominated
by labor costs (77% of the total). The range of costs was
Given the mean cost of $635 and energy use reduction of $335 to $1069 (a factor of three) and did not correlate with
18%, an estimate of the cost effectiveness can be made. system size, showing that ease of access to the duct system
Assuming a simple payback of five years for the break even was as important as system size in determining the cost
criteria, and the same energy use each year, a house with of retrofits.
an annual energy bill of $572 would break even. For a house
with a greater amount of energy use the retrofit will be cost
ACKNOWLEDGMENTS
effective. However, because there is a large variation in
energy reduction and retrofit cost it would be prudent to This work was supported by the Assistant Secretary for
Conservation and Renewable Energy of the U.S. Departmentperform diagnostic tests to determine the potential for energy
Table 4. Cost of Retrofitting Duct Systems (dollars)
Sealing Insulating
Materials Labor per Materials Labor per
Materials per m
2
duct Labor m
2
duct Materials per m
2
duct Labor m
2
duct Total
Mean 41 1.58 252 4.85 103 3.44 239 9.08 635
Standard
deviation 36 70 43 196 59 55 51 75 34
(% of mean)
1.154 - Jump, Walker and Modera
of Energy,Office of Building Technologies, Existing Build- Cummings, J.B., J.J. Tooley Jr., M. Moyer, and R. Dunsm-
ore. 1990. ‘‘Impacts of Duct Leakage on Infiltration Rates,ings Research Program, under Contract No. DE-AC03-
76SF00098. Space Conditioning Energy Use, and Peak Electrical
Demand in Florida Homes’’. Proc. ACEEE Summer Study
The research reported here was funded in part by the Califor- 1994. 9:65–76.
nia Institute for Energy Efficiency (CIEE), a research unit
of the University of California. Publication of research Jump, D.A., and M.P. Modera. 1994. ‘‘Impacts of Attic
results does not imply CIEE endorsement of or agreement Duct Retrofits in Sacramento Houses’’. Proc. ACEEE Sum-
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This work was also supported by the Sacramento Municipal Modera, M.P. 1993. ‘‘Characterizing the Performance of
Utility District, which identified all of the houses, interacted Residential Air Distribution Systems’’. Energy and Build-
with the homeowners, acquired the retrofit contractors, and ings. 20:65–75.
paid for the retrofits.
Palmiter, L., and P.W. Francisco. 1994. ‘‘Measured Effi-
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