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Volume 6, Number 4 77
A REVIEW OF THE SUSTAINABILITY
OF RESIDENTIAL HOT WATER INFRASTRUCTURE:
PUBLIC HEALTH, ENVIRONMENTAL IMPACTS,
AND CONSUMER DRIVERS
Randi H. Brazeaua and Marc A. Edwardsb
aCorresponding Author: Virginia Tech, 418 Durham Hall, Blacksburg, VA, USA 24060, 00-1-540-808-7878 (p),
00-1-540- 231-7916 (f), randihl@vt.edu.
bVirginia Tech, 418 Durham Hall, Blacksburg, VA, USA 24060, edwardsm@vt.edu.
ABSTRACT
Residential water heating is linked to the primary source of waterborne disease
outbreaks in the United States, and accounts for greater energy demand than the
combined water/wastewater utility sector. Furthermore, home water heating is the
second largest energy consumer in the home and thus represents an integral part of the
water-energy nexus. To date, there has been little practical research that can guide
decision-making by consumers, public health ocials and regulators with regards to
water heater selection and operation to minimize energy costs and the likelihood of
waterborne disease. Scientic uncertainties associated with existing “green” advice have
potentially created misguided policy with long-term negative repercussions. is review
is aimed at dening the current state of knowledge related to hot water infrastructure
and in highlighting current gaps in the research. While there are many sustainability
claims of certain water heater types (i.e., hot water recirculation systems and
instantaneous water heaters) these claims have not been substantiated in head-to-head
testing of the interplay between water temperature, energy, microbial growth, and
scaling, all measures that need to be better dened.
KEYWORDS
water heaters, energy, water-energy nexus, sustainable design, green energy,
premise plumbing, pathogens
RESEARCH
1. BACKGROUND
Residential water heating infrastructure is tied to the primary source of waterborne disease
outbreaks in the U.S. [1] and has a total energy demand exceeding that of the water and
wastewater utility sector combined (Table 1) [2]. Considering the high stakes, it is unfor-
tunate that there has been little practical research that can guide rational decision-making
by consumers, public health ocials, regulators and legislators. In fact, the numerous scien-
tic uncertainties associated with existing “green” advice has the potential to create misguided
78 Journal of Green Building
policy with long-term repercussions for energy consumption and public health. is research
is aimed at reducing that liability by conducting the rst practical assessment of residential
water heating infrastructure performance in terms of public health, environmental impacts,
and consumer drivers (Figure 1).
To elaborate, selection of an “optimal” new or retrot water heater system from amongst
the myriad options available is a complex decision that often begins and ends at the consumer
level by considering capital costs, comfort, reliability, maintenance, and occasionally genetic/
immuno-susceptibility to waterborne disease (Figure 1). By outlining the various factors that
should be considered with respect to water heater selection, the potential scale of complexity
becomes apparent. While water heater selection is probably most driven by consumer drivers
(i.e., costs, availability, and consumer comfort reports), environmental impacts, local factors,
and public health (Figure 1) could play a larger role if more reliable, practical assessment were
readily available.
Although some information regarding environmental impacts including water conserva-
tion, greenhouse gas emissions, and operating costs (Table 2) are available through EPA web
sources and EPA Energy Star ratings, such recommendations are based on extrapolation of
very limited new system performance data. Home owners can upt existing systems follow-
ing specic Energy Star guidelines to be eligible for up to $1500 dollars in tax incentives
for choosing certain water heaters; other systems are eligible for a 30% tax rebate with no
upper limit [3]. EPA’s WaterSense program that has been developed to “help consumers iden-
tify water ecient products and programs” specically does not include water heaters [4].
e USGBC LEED certication program rates certain models as “green” for LEED building
certication; however, some of the qualied models do not coincide with the Energy Star
FIGUR E 1. Water Heater Selection. A consumer’s selection of water heater infrastructure should
consider public health and environmental impacts.
Volume 6, Number 4 79
tax eligibility criteria. Furthermore, some cities, where water conservation has become a top
priority, have adopted ordinances which mandate new construction to have specic, water
saving, “green” plumbing designs. e Marina Coast Water District (MCWD) in California,
for example, requires that any hot water xture more than 10 linear feet from the hot water
heater has a hot water recirculation system or point-of-use demand heater [5]. Although both
nationwide and globally, these sustainable designs are being implemented to the supposed
benet of the environment and consumer and, in some cases by government mandate, there
has been very limited research assessing the water quality, health factors, and comparative
energy eciency associated with these initiatives [6, 7].
e existing recommendations can be misleading and unfounded under actual eld con-
ditions due to scaling, corrosion, and climate impacts (Table 2). Moreover, it is believed that
the type of hot water system and the quality of the water supply (i.e. nutrients and secondary
disinfectant residual level) can control the occurrence of pathogens (Table 2). But research
on this important emerging subject is only beginning, and existing data covers just a few
water heater systems and water supplies. e interdisciplinary nature of the research involv-
ing plumbing, water chemistry, microbiology, and human pathogen exposure has also been a
barrier.
is paper will review various types of water heating systems and will highlight specic
gaps in the literature while focusing on the mechanics, chemistry, microbiology, environ-
mental impacts, and consumer considerations (Figure 1) with respect to residential hot water
systems. In the sections that immediately follow, a summary of the dierent types of water
heating infrastructure that are available, what is known about their likely environmental and
public health impacts, and consideration of how local factors might dramatically alter perfor-
mance are provided.
2. WATER HEATING SYSTEMS
2.1. Energy and Public Health Implications of Residential and Commercial
Water Heater Infrastructure
Water heating in the United States has the
largest energy consumption of any water
related use. Additionally, water heating repre-
sents the second largest residential energy use
(second only to heating and cooling) and uses
more energy than all other home appliances
combined (Figure 2) [8]. e actual portion
of the energy consumption of water heating
in the home varies depending on the given
year and reporting source, [9-14] but the most
recent data from Energy Star [8] suggests water
heating accounts for 14% of residential home
energy consumption (Figure 2). Over the
past decade, between 3.3–5.5% of total U.S.
energy demand is used in residential water
heating, which slightly exceeds the estimated
3–4% combined energy demand of the water
FIGUR E 2. 2010 Data for Residential Energy
Consumption.
80 Journal of Green Building
and wastewater utility sectors (Table 1) [2, 9]. e costs of residential water heating are high
with 100 billion kWh used for electric water heating alone in 2001 at a cost of $9 billion
dollars assuming average electric rates of 9 cents per kWh, which more than doubles the $4
billion estimated energy costs for the entire water and wastewater utility sector (Table 1) [2,
9-11, 14-16]. is cost does not even include the 58 million homes that use natural gas or the
remaining 8 million homes that use an alternative source of energy for water heating. With
well over 100 million households in the United States using some type of water heating sys-
tem, research needs to address not only the relative energy consumption, but also the poten-
tial public health risks with regards to scalding and microbial growth, the relative economic
constraints, and the water saving potentials of dierent choices widely available for use.
In terms of public health, growth of opportunistic pathogens in premise plumbing was
identied as a “high priority” for research by National Research Council in 2006 [17]. “Prem-
ise plumbing” refers to the portion of potable water distribution systems beyond the prop-
erty line in buildings. is portion of the water distribution system water infrastructure poses
unique challenges for public health and has a net present value that probably exceeds that
of the main distribution system operated by water utilities [17, 18]. e ability of premise
plumbing pathogens to amplify is controlled by water temperature, residual disinfectant con-
centrations, water nutrient levels, and water age: factors directly inuenced by water heating
infrastructure type, design and operation. Hence, there will be inextricable direct linkages
between goals of reducing energy demand and maintaining public health (Figure 1), both
antagonistic and synergistic, which are only beginning to be appreciated and studied.
2.2. Overview of Water Heater Systems
is section provides an overview of the ongoing consumer dilemmas of choosing appropriate
water heating strategies for individual residences. Water heating infrastructure can be charac-
terized into four broad categories (Figure 3) including: 1) tank storage with no hot water recir-
culation, 2) tank storage with hot water recirculation, 3) centralized demand with no storage
and no hot water recirculation, and 4) point-of-use demand with no storage and no hot water
recirculation. Each of these further needs to be assessed considering key areas of local factors,
energy, and public health, and consumer drivers (Figure 1). While there is a high degree of
uncertainty in relation to dening performance for some variables, each type of infrastructure
and energy source will have characteristic impacts and susceptibility to problems (Table 2).
e subsequent sections highlight some important inter-dependencies that are emerging
in relation to design and operation of specic types of water heating infrastructure relevant
to public health, energy, water conservation, and consumer considerations. Illustrative areas
emphasized include storage vs. on-demand systems, concerns about scaling and scalding, elec-
tric vs. gas tanks, hot water recirculation, and “green” high eciency heaters.
TABLE 1. Impacts of Residential Water Heating.
Total Energy Costs
% of US Energy
Demand
Funded Research
in Progress?
Residential Water Heating $9 Billion1 [10] 3–5% [9] Very little
Water and Waste Water
Utility Sector $4 Billion [2] 3–4% [2] Numerous projects
1Electric Water Heating Only
Volume 6, Number 4 81
2.2.1. Residential Storage Water Heaters with No Hot Water Recirculation (STAND)
Residential storage water heaters are the most widely used system to heat domestic water sup-
ply. While there are many dierent sizes and types of water heaters depending on use, a stan-
dard residential water heater storage tank as dened by this paper (Figure 4) consists of a steel
cylindrical tank that may have a porcelain (or vitreous) enamel glass lining to limit corrosion.
Ambient temperature water ows to the bottom of tank from the main water line and heating
elements or gas combustion raise the water temperature to a range of 48 to 77 °C which ows
out the top of the tank to the pipe system and ultimately the destination faucet. To minimize
corrosion within the tank, a sacricial anode rod made of either aluminum or magnesium
alloy is placed within the tank. Other elements that comprise the hot water storage tank are a
drainage tap to remove accumulated sediment at the bottom of the tank and insulation (ber-
glass or urethane) to control environmental heat loss [21, 30].
Energy eciency of water heaters must include considerations of energy input to heat the
water, energy output in terms of heated product water, and losses of heat to the ambient envi-
ronment and along the pipe system [31]. Standby heat losses are dened as the energy input
required for maintaining hot temperatures in the storage tank when the system is not in use.
On-demand water heaters virtually eliminate standby losses. Any water heater with a storage
tank will have standby losses that depend on the type and quantity of insulation, surface area
of the tank and hot water distribution system, and dierential temperature between the hot
water tank and the environment [32]. More complex energy equations might account for the
potential benets of the heat loss in a cold climate, in terms of reduced costs associated with
heating the dwelling, or increased cost from cooling a dwelling in a hot climate.
2.2.1.1. Electric Versus Gas Tanks. ere are certain key dierences with regard to electric
heating and gas heating that are critical in dierentiating their performance with regards to
energy eciency, public health, environmental air quality, and cost (Figures 4 and 5, respec-
tively). First, electric water heaters typically heat the inow water through either one or two
FIGUR E 3. Common
Residential Hot Water Heaters.
A single family residence using
standard energy resources
(i.e., electricity, natural gas,
propane, or oil) could have
several different water heater
systems: A) storage tank with
no recirculation, B) storage tank
with hot water recirculation,
C) point-of-use demand with
no storage, and D) centralized
demand with no storage.
82 Journal of Green Building
TABLE 2. Key Characteristics of Representative Residential Water Heater Systems.
Energy Efficiency and
Relative Energy Demand
Standby
Energy
Loss Scaling Potential
Pathogen Growth
Potential
Installation and
Maintenance Costs
Scalding and
Consumer Concerns
Electric Heater
with Storage and
No Recirculation
(STAND)
Energy Efficiency:
HIGH 90 – 95% [19]
Energy Demand: BASELINE
FOR COMPARISON
MEDIUM –
HIGH
(Figure 6)
MEDIUM
(Scaling can reduce
energy efficiency
but has moderate
recovery after
maintenance) [20]
MEDIUM – HIGH
(Stratification
in tank, AOC2
generation, sediment
accumulation; see
Figure 4) [21]
LOW Scalding Risk: LOW –
MEDIUM (Dependent
on temperature setting)
Temperature Stability
during Shower: HIGH
Gas Heater with
Storage and No
Recirculation
Energy Efficiency:
MEDIUM 60-65% [19]
Energy Demand: LOWER
than STAND due to more
efficient production and
transportation of source
energy [19]
MEDIUM –
HIGH
(Same as
STAND)
MEDIUM – HIGH
(Scaling can increase
energy demand and
be irreversible) [20]
LOW – MEDIUM
(Water heated from
bottom eliminating
stratification,
AOC generation
and sediment
accumulation still
probable)
LOW- MEDIUM
(Permanent scaling
effects may increase fuel
costs)
Scalding Risk: LOW –
MEDIUM (Dependent
on temperature setting)
Temperature Stability
during Shower: HIGH
Electric Heater
with Storage and
Recirculation
Energy Demand:
HIGHER than STAND
due to increased energy
consumption from pump
and increased energy
losses [22]
HIGH
(Figure 6)
MEDIUM
(Same as STAND)
HIGH
(Higher Legionella
incidence in and
lower disinfectant
residual?) [22, 23]
MEDIUM
(Pipe costs doubled due
to return line; initial
pump costs; increased
fuel costs to run pump)
Scalding Risk: MEDIUM
(Hot water arrives
immediately at tap)
Temperature Stability
during Shower:
LOW [22]
On-demand
(Electric) with
No Storage and
No Recirculation
Energy Efficiency:
HIGH 95-100% [22]
Energy Demand: 8 – 50%
LOWER energy demand
than STAND [24]; High
energy demand during
short use that may lead to
grid failure [25]
NONE HIGH
(Scaling has
shown to render
on-demand systems
inoperable with no
recovery in as little
as 4 months) [26]
LOW
(No tank for
microbial growth,
AOC generation
or sediment
accumulation)
MEDIUM – HIGH
(Electrical upgrades
may be necessary;
scaling can render
systems inoperable
needing replacement;
high energy draw
during use; if
point-of-use: high
capital costs) [25-27]
Scalding Risk:
HIGH (Temperature
dependent on flow rate
and incoming water
temperature – not
setting) [24, 27]
Temperature Stability
during Shower: LOW
(Consumer issues
related to inconsistent
temperature and flow
rate) [24, 27]
Volume 6, Number 4 83
TABLE 2. (continued)
Energy Efficiency and
Relative Energy Demand
Standby
Energy
Loss Scaling Potential
Pathogen Growth
Potential
Installation and
Maintenance Costs
Scalding and
Consumer Concerns
On-demand
(Gas) With No
Storage and No
Recirculation
Energy Demand:
LOWER than electric
demand due to more
efficient production and
transportation of source
energy [19]
NONE HIGH
(Same as above)
LOW
(Same as above)
MEDIUM (Same as
electrical demand;
if point-of-use: high
capital costs to extend
gas lines throughout
home; Energy Star tax
rebate eligible) [3]
Scalding Risk: HIGH
(Same as above)
Temperature Stability
during Shower: LOW
(Same as above)
Heat Pump
(Exchanger)
Water Heater
with Storage
Energy Efficiency: HIGH
2 – 3 times more energy
efficient than STAND [28]
Energy Demand: LOWER
than STAND in temperate
climate; may be increased
demand in cold winter
months
MEDIUM –
HIGH
(Same as
STAND)
MEDIUM
(Same as STAND)
MEDIUM – HIGH
(same as STAND)
LOW- MEDIUM (Same
as STAND but with
increased energy costs
in cold winter months;
Energy Star tax rebate
eligible) [3]
Scalding Risk:
LOW – MEDIUM
(Same as STAND)
Temperature Stability
during Shower: HIGH
Solar Panels with
Electric Storage
Backup and No
Recirculation
Energy Demand: LOWER
than STAND (dependent
on regional/climate factors
and type of solar heater)
[29]
MEDIUM –
HIGH
(Same as
STAND)
MEDIUM
(Same as STAND)
MEDIUM – HIGH
(same as STAND)
HIGH (High capital
installation costs;
payback potential
depends on climate/
region; Energy Star tax
rebate eligible) [3]
Scalding Risk:
LOW – MEDIUM
(Same as STAND)
Temperature Stability
during Shower: HIGH
2AOC = Assimilable Organic Carbon
84 Journal of Green Building
electric components located at the top or middle of the tank whereas gas-red storage tanks
heat from the bottom (Figure 5). e placement of the electrical components causes vertical
thermal stratication within the tank because denser, cooler water will sink to the bottom
of the tank and is not directly heated by the components (Figure 4) [21, 30, 33]. ermal
stratication can be benecial when considering a solar collector system with electric backup
to improve performance and eciency. In fact, studies have concluded that in certain systems,
the greater the temperature dierence, the larger the eciency [21, 33]. However, when con-
sidering a traditional water heater with tank storage, water stratication and relatively cool
water at the bottom can lead to increased microbial con-
tamination (Table 2). In fact, Legionella pneumophila, a
known opportunistic pathogen that is discussed in more
detail later in this review, is believed to occur in electric
storage tank heaters in high numbers due to this stratica-
tion [21, 33].
In contrast, natural gas heating in non-scaling waters
tends to break up stratication typical of electric heaters
due to heating from the bottom of the tank; however, in
scaling waters, the internal insulating properties of thick
scale may induce stratication in gas heaters by reducing
heat transfer to the tank (Figure 5). In non-scaling waters,
occurrence of L. pneumophila was dramatically higher in
electric tanks versus natural gas as a result of stratication
(Table 2), but with high scaling, this benet might not be
signicant [21, 34] Additionally, gas-red systems typi-
cally cost less to operate than electric storage tanks if oper-
ating at full eciency.
e California Energy Commission (CEC) estimates
that a water heater using natural gas or propane as an
FIGUR E 4. Electric Storage
Heater. A cross-section of a
typical electrical hot water tank
with no hot water recirculation.
FIGUR E 5. Scaling and Gas
Water Heaters. Gas water
heaters will lose efciency as
scale builds up at the bottom
of the tank in hard water areas.
This may cause temperature
stratication and heat loss
through the vent.
Volume 6, Number 4 85
energy source in deference to electricity will save the consumer 25–65% in energy costs [35].
Calculations of lifecycle emissions and energy consumption of natural gas heaters predicts
improvements of about 40–50% versus electric [35, 36]. However, the natural gas heaters
modeled were assumed to have a high eciency of 85%. Due to build-up of scale and other
deposits (Figure 5), actual gas heater eciencies can drop 27–30% in a few months and cal-
culations based on theoretical reductions in heat transfer coecients suggest possible reduc-
tions in heat transfer eciency by up to 95% with an associated increase in operation costs
(Table 2) [20, 26, 37]. Electric tanks tend to be less susceptible to energy loss due to scaling
and can be more easily maintained because the heating element is located inside the tank and
might be subject to some self-cleaning with contraction/expansion [20].
2.2.2 Residential Storage Water Heaters with Hot Water Recirculation
Traditionally found in multi-family homes and hotels, but gaining increasing attention for
single-family residential use, a recirculation system will continuously circulate hot water from
a central water heater tank so hot water is “instantaneous” at various point-of-uses through-
out the buildings [38]. Hot water tanks with recirculation lines eliminate the “waiting” time
for hot water to reach the tap by rapidly circulating hot water via an electric pump from the
water heater to each faucet that utilizes hot water (Figure 3). e theory behind the water
saving advantages of a hot water recirculation system depends largely on behavioral patterns;
a person taking a shower, for instance, no longer needs to allow water to run or “waste” until
the water is at a comfortable temperature. As fresh hot water is pumped from the tank, water
not utilized in the hot water line is cooled as it is returned the tank to be re-heated and re-
circulated. Hot water recirculation tanks are dependent on the electric pump forcing ow
(sometimes at high velocity) from the heater to multiple point-of-use faucets [39]. In addition
to the added energy of using a pump, recirculation systems may increase other energy losses
due to increased surface area and higher temperatures, and resultant energy losses to ambient
air from the hot water distribution system. (Figure 6). Even without operation of a pump,
addition of a return line increases heat loss due to natural convection (passive recirculation) in
the system via a thermosiphon.
Optimization of pump operation is key to limit heat loss and maximizing efficiency.
Instead of running a pump continuously, the pump should be turned o during periods of low
demand, and turned on via a sensor located at the point-of-use or at a specic time to meet
demand. ere are four system conditions (Figure 6) that need to be identied and analyzed
for energy considerations: 1) standard systems with no recirculation (STAND), 2) continuous
recirculation via a pump (return line, pump always on, RECIRC-C), 3) recirculation via ther-
mosiphon eect (return line, no pump, RECIRC-T), and 4) an optimized recirculation system
(return line, pump not on continuously, RECIRC-O). RECIRC-O can be thought of as a
combination system since it will act as a RECIRC-C during periods when the pump is operat-
ing and a RECIRC-T when the pump is o (Figure 6).
e Oak Ridge National Laboratory (ORNL) in conjunction with the City of Paolo Alto
conducted a study examining the use of hot water recirculation [40]. ey estimated that
nearly 1–3 gallons of potable water could be drained as a user waits for water to reach a
comfortable level. While they assert that the water wastage can virtually be eliminated by hot
water recirculation, it is noted several times that this is an “ideal” situation where user behav-
ior encourages immediate use of the hot water. Water saving estimates for this study ranged
from 900–3000 gallons per point-of-use per year. However, it should be noted that study
86 Journal of Green Building
had many limitations including small sample size, inconsistent study parameters, inconclusive
results, and a narrow range of home age not comparable to current age distribution of homes
in the U.S.
It was also asserted that the use of recirculation pumps would probably save energy. But
e Paolo Alto study used pump systems that were consumer activated just before use, and a
heat sensor was employed to turn o the pump when a desired temperature is reached[40].
e assertion that the system would save energy failed to consider energy demands to run the
pump and possible increased heat loss from the recirculation system [40], with a pump that
runs continuously. It has been suggested that intermittent use of the pump consistent with
reduced energy use could cause damage to the pump and system [38].
Another study in a multi-family building analyzed four different pump operations:
1) pump continuously on, 2) pump o at night (between 11:50 pm–5:20 am), 3) pump o
during “peak” use (5:45 am–8:15 am and 5:45 pm–9:15 pm), and 4) pump activated when
return line water temperature falls below a set point (i.e., 43 °C in study) [39]. While the
study was limited in scope to one unit and short study period, it was found that compared to
the baseline pump operation of case 1, scenario 2 and 3 reduced energy consumption by 5%
and scenario 4 reduced energy consumption by 11%. No comparison was made to a situation
without a return line. Moreover, it was determined that hot water recirculation congured
FIGUR E 6. Hot Water Systems and Energy Balance. There are four different water heater
congurations for the standard and recirculation systems: 1) STAND (no recirculation),
2) RECIRC-C (hot water recirculation line, pump continuously operating), 3) RECIRC-T (no pump,
thermosiphon return line to tank), and 4) RECIRC-O (optimized pump operation, hot water
recirculation or thermosiphon). EIN represents the energy required to heat the tank to the desired
temperature. EPUMP represents the added energy of running a pump. ETANK and EPIPE correspond
to the heat loss from the tank and pipes, respectively. “HIGH” and “LOW” represent EXPECTED
energy consumption/loss where “HIGH” refers to a higher temperature differential between
internal (tank and pipe) water temperature and ambient (i.e., due to stratication or heat loss
through the pipes, the temperature at the bottom of the tanks and pipes are cooler and thus
have a lower ∆T between internal temperature and ambient temperature. “LOW” also represents
the lower energy input expected to heat the partial tank from stratication as opposed to the
entire tank (i.e., “HIGH”) due to pipe recirculation. “HIGH” and “LOW” are simply expected
energy inputs/losses and will be fully developed through experimentation.
Volume 6, Number 4 87
with a pump operating continuously consumes nearly 40% of the total fuel used to heat
domestic hot water under that condition. e researchers also noted increased user complaints
of decreased hot water and lower temperature water during condition 3 [39]. It is imperative
that more research be conducted on various pump use and hot water recirculation relative to
consumer behavior and improved energy audits. e local code requiring recirculating systems
in MCWD mentioned earlier required that the pump not run more than “10 minutes in any
hour” [5]. While these types of recommendations may optimize system performance, there is
little uniformity in recommendations and it requires the users to maintain a regimented use
schedule.
Another potential problem that arises with recirculation systems is rapid cooling due to
mixing and backow from the return line [22]. As discussed previously, in storage type sys-
tems the cold water enters at the top of the tank and is delivered to the bottom of the tank via
a closed pipe. Since recirculation systems have a return line that also enters at the bottom of
the tank, fresh cold water entering the tank during ushing can “short circuit” the tank and
immediately backow through the return line to the faucet without any storage. is is likely
due to the pressure dierential in the tank versus atmospheric pressure at the tap. Addition-
ally, it is hypothesized that the pump and return line create a mixing eect within the tank
where colder water mixes with the heated water lowering the overall temperature of the water
within the tank as opposed to the more plug ow conditions of a standard storage tank. ere
have been no studies characterizing the temperature proles within a recirculation system tank
during ushing. e backow issue can be eliminated through installation of a check valve at
the end of the return line [5, 41]. Again, proper installation and optimization of this system
could have dramatic eects on the overall eciency of the design.
Other considerations with water recirculation loops include pin hole leaks and copper
corrosion due to high velocity ow through copper pipes. A hotel near Lake Tahoe experi-
enced near total water pipe failure due to a recirculation pump installed to eliminate long
waits for hot water in multiple rooms. Flow-accelerated corrosion (a.k.a. erosion corrosion) is
a common occurrence where owing hot water erodes the oxide lm formed by the reaction
of the copper pipe and dissolved oxygen which causes a thinning of the pipe wall and overall
scaling eect. is scaling eect can cause increased turbulence and increased failures [42].
2.2.3. Tankless On-Demand Systems: Centralized and Distributed (Point-of-Use)
Residential storage water heaters are the current U.S. standard. Storage type systems are prone
to heat loss during stagnation (i.e., stand-by losses). On-demand tankless water heater systems
have no anode, virtually no hot water storage and eliminate standby losses which can be as
much as 50% of the total energy demand in storage systems [26]. e DOE estimates that use
of electric, centralized on-demand systems can result in energy savings between 8–34% ver-
sus electric tank storage units depending on average daily water use and using a point-of-use
demand system can reduce energy use by 27–50% (Table 2) [24]. ese savings are dependent
on ow rate, total water use, and the installation of low-ow devices. Additionally, the data
for point-of-use heaters do not include energy savings from water use at the production phase
and could potentially underestimate total energy savings. e downside of on-demand heaters
include high cost, high peak energy use, limited ow potential, variable temperatures with tap
distance, and increased possibility of scalding at taps near the heater (Table 2) [24].
On-demand, tankless, or instantaneous water heaters eliminate storage tank heating by
using heat exchange coils that raise water to a set temperature only when needed. e cold
88 Journal of Green Building
water from the distribution line passes through the unit where a gas burner or electric ele-
ment heats the water to a pre-set temperature. [26] Two types of tankless water heaters will be
dened in this review: located central in the building or distributed or point-of-use (Figure 3).
While both types of on-demand systems function similarly, they have marked dierences with
regards to advantages, disadvantages, energy consumption, and public health considerations
(Table 2).
In centralized systems, a large central demand system would be located somewhere in
the residence and a hot water distribution system would deliver hot water to various fau-
cets within the house (Figure 3). Water would be heated through the heat exchange coils
rather than stored in a tank thus eliminating the standby losses of a storage type system; how-
ever, with this type of system, the same heat losses through the pipe network would need to
be considered. e distributed or point-of-use models consist of a series of smaller tankless
units installed directly at the faucet (Figure 3). ese systems will either provide single-source
use (i.e., one shower/tub) or small multiple point use (i.e., all faucets in a given bathroom
including the sink and shower/tub). is type of system eliminates both standby losses from
the storage tank and pipe loss through the network since the heated water does not need to
“travel” to get to the tap.
ere are several limitations with the use of on-demand systems (Table 2). Even using the
largest model, gas-red unit which should theoretically provide the most “power” for water
heating, on-demand systems typically cannot provide enough hot water to supply multiple
faucets and simultaneous uses at any given time [24]. Additionally, the maximum ow rate of
on-demand systems are limited by several variables including the water temperature setting,
cold water inuent, and the heat input to the unit itself. is will lower the maximum rate at
which hot water can be delivered when compared to a tank system.
Water temperature can also be inconsistent when using an on-demand system leading to
consumer complaints. If a centralized demand water heater is being used, the temperature
setting needs to be high enough to negate any heat loss in the pipe to the farthest faucet
without causing scalding at the nearest tap. Lower temperature settings and ow rate can be
more acceptable with low-ow devices, when point-of-use systems are in place, in a washing
machine where comfort and scalding are not a consideration, or if the water later gets elec-
trically heated by the appliance, as in the case of a dishwasher [43]. While gas-red demand
water heaters provide higher ow rates than electrical demand heaters, ow rates average
between 2–5 gallons per minute and still may not provide hot water to multiple locations
throughout a household.
With electric systems, increased power may be an issue. Even the smallest on-demand
heaters require more power (i.e., energy input) than tank systems. While standby losses are
virtually eliminated, the peak energy draw during use can be a problem in neighborhoods
with a taxed power grid [25, 27], and may require the homeowner to upgrade wiring or the
utility to upgrade the grid. Finally, scale buildup in the system causing damage to the unit
has been noted in areas with hard water in as little as four months, leading to costly repairs
and/or replacement [26]. To combat the limitations of on-demand water heaters, consum-
ers could consider using multiple units in parallel or point-of-source heaters that supply hot
water directly to the tap used. Other solutions include installing ultralow-ow showerheads
(which may lead to consumer dissatisfaction) and water softeners. Furthermore, the energy
costs associated with running multiple electric tankless water heaters simultaneously has not
been reviewed.
Volume 6, Number 4 89
2.2.4. Alternate Energy Water Heaters
Incentive tax credits exist for solar, electric heat pump, and on-demand natural gas residen-
tial water heater infrastructure (Table 2). However, the practical long-term performance of
these devices under scaling conditions, or in terms of pathogen control, has never been rig-
orously assessed. Concerns have been expressed about pathogen re-growth in solar applica-
tions although limited data available to date is inconclusive [44, 45]. In general, it might be
expected that electric heat pump and solar systems would behave like electric tank systems
relative to possible growth of premise plumbing pathogens, but with much higher storage
volumes. However, more practical performance data must be obtained. Due to variation in
temperature, climate, and weather, solar systems will typically have a back-up non-renewable
energy source (i.e., electric or natural gas) and are thus susceptible to the same detriments and
benets of these systems. Since solar water heaters require maximum sun exposure to be most
eective, there may be regional, climate limitations to this type of system.
2.3. Scaling
In certain “hard” or other waters, calcium and silica can precipitate and coat the surface of the
heating elements and pipe surfaces. ese deposits can cause water heater noise, increase cor-
rosion, clog pipes, reduce heater life and dramatically reduce energy eciency via formation of
scale layers that reduce heat transfer from the energy source to the water. e reduced energy
eciency is attributed to internal insulating properties and reduced heat transfer from the scale
layer to the tank. On-demand systems are especially prone to scaling problems (Table 2) because
of the small diameter tubes required for maximum heat transfer and constant ow of water over
the heating element. In some cases these devices can be rendered virtually inoperative in a matter
of months due to clogging, and acidic solutions must be used to clean the scale and maintain
eciency and ow [20, 26].
e Water Quality Research Council (WQRC) in conjunction with New Mexico State Uni-
versity found that the eects of hard water scaling on a gas red water heater was an increase in
energy demand of 30% in just 14 days; moreover, after the scale was cleaned out, only 5% of the
increase was reversed (Table 2) [20]. Water scaling and liming (i.e., calcite precipitation) are most
common in hard water, although silicates, sulfates, and waters with high total suspended solids
can also form sediment or “scale” layer at the bottom of the tank for gas-red systems and around
the electrical heating components in electric water heaters. e WQTC study also showed that in
a head to head comparison, scaling had a worse overall eect on energy eciency of the gas-red
heaters than the electric water heaters by nearly 8% [20]. In a recent practical study that examined
this issue, eciency of on-demand systems dropped dramatically in just a few months and some
were even rendered inoperative, practical trends that might make on-demand less ecient than
comparable tank systems [26]. us, benets of on-demand systems will not be possible in all
waters, and its use in heavily scaling waters might not save energy without frequent maintenance.
2.4. Overall Implications of Various Water Heating Systems
Given the multitude of variables and characteristics of the dierent types of water heating sys-
tems (Table 2), it is expected that various chemical, microbial and physical properties would
dier from system to system dependent on the actual conguration of specic water heater
types (i.e., Figure 6). ere has been a noticeable lack of research that provides insights to
these important issues. Future research is needed to identify how these variables are aected
by altering the operation and congurations of dierent water heating systems.
90 Journal of Green Building
3. PUBLIC HEALTH CONSIDERATIONS
There are two serious public health concerns when it comes to water heating: pathogen
growth and scalding. e former has already been described as a major area of concern for
new research and the latter may become a high priority with new “green” advice in water heat-
ing systems.
3.1. Pathogen Growth
Traditionally, control of pathogens in water leaving the treatment plant via disinfection, coag-
ulation, and ltration has been the paramount concern of water utilities and the U.S. Environ-
mental Protection Agency (U.S. EPA)—the successful mitigation of this hazard represents one
of the 10 greatest engineering achievements of the 20th century [17]. e CDC estimates that
between 8,000–18,000 people in the United States are hospitalized each year with Legion-
naires’ disease [1]. ere is also a similar growing concern with non-tuberculosis mycobacte-
rial (NTM) lung disease tied to drinking water [46-48]. Estimates of NTM disease incidence
range from 15–30 per every 100,000 persons with some 30,000 NTM infected patients in the
United States [18]. Because susceptibility to both NTM and Legionnaire’s disease increases
with age and diagnosis is improving, incidence of documented waterborne disease from prem-
ise plumbing pathogens will likely continue to increase [49, 50]. Representative opportunistic
pathogens of concern in premise plumbing include Legionella pneumophila, Acanthamoeba,
Mycobacterium avium complex and Pseudomonas aeruginosa (Table 3). Control of waterborne
disease from these and other premise plumbing pathogens will require a noteworthy paradigm
shift versus conventional water treatment practice and approaches.
Specically, “opportunistic” pathogens do not typically cause disease in healthy persons,
but can be fatal to humans with a compromised immune system such as the elderly, HIV
infected persons, or hospitalized patients. Premise plumbing pathogens grow in shower heads,
faucets, along pipe walls, or in water heaters, whereas conventional pathogens are naturally
present in the source water from fecal contamination and do not multiply in the water itself.
Finally, the primary mode of transmission and exposure is via inhalation or through wounds
as opposed to ingestion (Figure 7).
TABLE 3. Premise Plumbing Pathogens of Concern.
Pathogen Disease(s)
Host Organism
Required?
Mode of
Exposure Source
Legionella
pneumophila
Legionnaires’ Disease or
Pontiac Fever in Children
Yes Inhalation or
Aspiration
CDC, 2008 [51]
Pseudomonas
aeruginosa
Urinary Tract Infections,
Respiratory Infections,
Dermatitis, Soft Tissue
Infections, Bacteremia,
Bone and Joint Infections,
GI Infections
No Wound infection;
other modes of
transmission are
unknown
Todar, K, 2008 [52]
Mycobacterium
avium
Pulmonary Disease
Cervical Lymphadenitis
(children)
No Inhalation or
Aspiration
CDC, 2005 [53]
Acanthamoeba Acanthamoeba keratitis No Wound Infection CDC, 2008 [51]
Volume 6, Number 4 91
Systems that maintain a consistent inow of water from the main distribution line will
tend to have continuous levels of disinfectant; however, as water remains stagnant in the sys-
tem or recirculating for any length of time, such as the systems found in water heaters, disin-
fectants will decay and water quality will decrease [54, 55]. Chlorine decay is dependent on
several variables, including pipe material, inorganic and organic material in the water, and
hydraulic eects [56]. Since disinfectant decay over time will aect residual levels in the water,
it would also be expected that disinfectant decay can be directly associated to increased bio-
lm production and thus decreased water quality. “Biolm” in this paper and the research
conducted by Momba, et al. [57] describes “a layer of microorganisms in an aquatic environ-
ment held together in a polymetric matrix attached to a substratum such as pipes.” Biolms
are an integral part of microbial resistance to disinfectants [57]. If disinfectant residuals drop
below the normalized or designed level for any length of time, biolm can show substantial
re-growth with the new biolm more resistant to disinfectants [58]. It is important, therefore,
to understand how various water heating systems aect disinfectant decay and other chemical
parameters in premise plumbing as this will have a direct eect on biolm formation, resil-
ience, and re-growth potential.
Certain types of water heating systems may be linked to increased incidence of Legionella
in premise plumbing. e team of Moore, et al. [23] related the presence of hot water recir-
culation systems to increased occurrence of Legionella in Pinellas County, Florida. In fact, the
study found that buildings that contained a recirculation system were ve times more likely
to have viable Legionella in the plumbing. is type of study has been limited in nature and
pathogens such as Mycobacterium avium and Acanthamoeba also may be impacted by water
heater type. us far, there is a real, tangible gap in the research with respect to specic patho-
gen growth and water heating infrastructure.
3.2. Scalding
System operating temperature has profound implications for control of scalding and patho-
gens (Figure 8), and dierent countries have dierent strategies. e consumer product safety
commission estimates that scalding from hot tap water results in 3,800 injuries and 34 deaths
annually in homes, with children at special risk [59, 60]. To reduce energy costs, potential for
FIGU RE 7. Pathways of Pathogen Exposure.
Pathogen exposure in premise plumbing
systems. Acanthamoebae and other protists
occur in cyst (A) and trophozoite (B) forms.
Vesicles within trophozoites can harbor up
to 20–1500 pathogenic bacteria such as
L. pneumophila (C), which can eventually
burst and lead to pathogen occurrence in
tap water and shower water (D). Contact
lens wearers are vulnerable to keratitis
infection from Acanthamoebae (E–F).
Inhaling mists containing L. pneumophila
and nontuberculosis mycobacteria (NTM)
can cause lung infections (G). Exposure to
Acanthamoebae, P. aeruginosa and NTM
through skin lesions can cause infection (H).
92 Journal of Green Building
scaling and scalding, the EPA recommends
that water storage tanks be set at 48 °C [61].
Unfortunately, this increases the likelihood of
pathogen growth in water heaters relative to
higher temperatures (Figure 8). Other coun-
tries and the World Health Organization
(WHO) recommend setting temperatures for
tanks systems above 60 °C to control patho-
gens, and then reduce dangers of scalding by
requiring installation of mixing valves at all
fixtures to maintain dispensed water below
48 °C [61, 62]. A preliminary cost-benefit
analysis of the higher temperature and mix-
ing valve requirement in Canada indicated a
benet of $0.7–4.2 million in reduced scald-
ing versus a cost of $48–119 million per year
[61]; however, the estimated benet did not
include costs of reduced Legionella infections
and death.
5. CONCLUSIONS AND FUTURE WORK
ere are serious long-term public health and energy implications that arise from consumer
installation and operation of residential hot water heating systems. An “optimal” decision might
consider individual preferences, consumer susceptibility to problems (i.e., scalding and chil-
dren and immune-status for elderly), household hot water demand, climate, scaling potential,
presence of nutrients in the water supply, availability of natural gas connections, and the type/
concentration of residual disinfectant in the supply water. Unfortunately, due to a lack of prior
research, much of the evidence is anecdotal, and head-to-head comparisons have been absent.
Most data generated on water heaters has been provided by manufacturers, plumbers,
consumers, and government agencies. ere has been surprisingly little practical research on
head-to-head performance on water heating infrastructure despite its relevance in the water-
energy nexus. Yet, municipalities and agencies are mandating certain water heaters or pro-
viding incentive for consumer selection based largely on manufacturer claims of water con-
servation or other “green” initiatives. Given that water heating infrastructure has important
implications for green engineering, energy efficiency, water conservation, environmental
microbiology, and public health, it is imperative that more research be done to quantify actual
dierences in these systems. e discrepancy between WHO and U.S. temperature setting
recommendations has implications on energy eciency, scalding potential, scale build-up and
microbiological growth. Nevertheless, no applied direct measurements are available to under-
stand the practical extent of the dierence.
e ability of premise plumbing pathogens to amplify is controlled by water temperature,
residual disinfectant concentrations, water nutrient levels and water age: factors directly inu-
enced by water heating infrastructure type, design and operation. Hence, there will be inextri-
cable direct linkages between goals of reducing energy demand and maintaining public health,
both antagonistic and synergistic, which are only beginning to be appreciated and studied.
FIGUR E 8. Pathogen Growth and Scalding
Concerns with Temperature. Higher water
heater storage temperature decreases the time
required for Legionella death and the time to
acquire severe burns from scalding. Data from
National Research Council (16).
Volume 6, Number 4 93
REFERENCES
1. Centers for Disease Control and Prevention (CDC), Possible link of poor POU devices, plumbing to dis-
ease, AWWA Annual Conference and Exposition, Atlanta, GA, 2008.
2. United States Environmental Protection Agency (EPA), Energ y and water/wastewater infrastruc-
ture, Updated, August 6, 2009, Retrieved, May 2, 2010, from http://www.epa.gov/region1/eco/energy/
ew-infrastructure.html.
3. Energy Star (United States Department of Energy and United States Environmental Protection Agency),
Federal tax credits for energy eciency, Updated, Retrieved, May 26, 2009, from http://www.energystar.
gov/index.cfm?c=products.pr_tax_credits#c4.
4. United States Environmental Protection Agency (EPA), WaterSense, Updated, August 3, 2009, Retrieved,
May 4, 2010, from http://www.epa.gov/watersense/index.htm.
5. Marina Coast Water District (MCWD), Water conservation rules—new requirements: Engineering pro-
cedures, guidelines, and design requirements, Updated, Retrieved, June 29, 2009, from http://www.mcwd.
org/docs/conservation/HotWaterRecircSystems_100105.pdf.
6. G.C. Wedding, D. Crawford-Brown, An Analysis of Variation in the Energy-Related Environmental
Impacts of Leed Certied Buildings, J Green Build, 2 (4) (2007) 151-170.
7. S. Lu, J. Wu, Research on dierent domestic energy consumption patterns based on heat pump and their
exergy analysis, Energy Sustainability 2008, Jacksonville, FL, 2008.
8. Energy Star (United States Deparment of Energy and United States Environmental Protection Agency),
Save Energy at Home, Updated, Retrieved, July 8, 2010, from http://www.energystar.gov/index.
cfm?c=products.pr_save_energy_at_home.
9. Energy Information Administration (EIA), Annual Energy Review, Updated, Retrieved, May 4, 2010,
from http://www.eia.doe.gov/emeu/aer/consump.html.
10. Energy Information Administration (EIA), Regional energy prole: U.S. household electricity report,
Updated, Retrieved, May 4, 2010, from http://www.eia.doe.gov/emeu/reps/enduse/er01_us.html
11. Energy Information Administration (EIA), U.S. household electricty report, Updated, Retrieved, May 4,
2010, from http://www.eia.doe.gov/emeu/reps/enduse/er01_us.html.
12. United States Department of Energy (DOE), Buildings Energy Data Book, Updated, March 2011,
Retrieved, June 8,, 2011, from http://buildingsdatabook.eren.doe.gov/ChapterView.aspx?chap=2.
13. United States Department of Energy (DOE), Water Heater Market Prole, 2009, pp. 3.
14. Energy Star (United States Department of Energy and United States Environmental Protection Agency),
Save Energy at Home, Updated, 2010, Retrieved, July 8, 2010, from http://www.energystar.gov/index.
cfm?c=products.pr_save_energy_at_home.
15. Energy Information Administration (EIA), Average retail price of electricity to ultimate consumers: total
by end use sector, Updated, Retrieved, May 4, 2010, from http://www.eia.doe.gov/cneaf/electricity/epm/
table5_3.html.
16. Energy Information Administration (EIA), Emissions of greenhouse gases report, Updated, Retrieved,
May 4, 2010, from http://www.eia.doe.gov/oiaf/1605/ggrpt/methane.html.
17. National Research Council (NRC), Alternatives to premise plumbing, in: Drinking Water Distribution
Systems: ASsessing and Reducing Risk, 2006, pp. 316-340.
18. J.C. Rushing, M. Edwards, Eect of aluminium solids and chlorine on cold water pitting of copper,
Corros Sci, 46 (12) (2004) 3069-3088.
19. American Council for an Energy-Ecient Economy (ACEEE), Water Heating, Updated, January 2011,
Retrieved, May 26, 2011, from http://www.aceee.org/consumer/water-heating.
20. W.P. Isaacs, G.R. Stockton, Water softeners as energy conserving investments, Water Quality Reserach
Council (WQRC).
21. M. Lacroix, Electric water heater designs for load shifting and control of bacterial contamination, Energ
Convers Manage, 40 (12) (1999) 1313-1340.
22. R.H. Lieberman, M.A. Edwards, S. Masters, Sustainability of Residential Hot Water Infrastructure: Pub-
lic Health, Environmental Impacts, and Consumer Drivers, AWWA Annual Conference and Exposition
(ACE) Chicago, IL, 2010.
23. M.R. Moore, M. Pryor, B. Fields, C. Lucas, M. Phelan, R.E. Besser, Introduction of monochloramine into
a municipal water system: Impact on colonization of buildings by Legionella spp., Appl Environ Microb,
72 (1) (2006) 378-383.
94 Journal of Green Building
24. United States Department of Energy (DOE), Energy savers – Demand (tankless or instantaneous) water
heaters, Updated, March 24, 2009, Retrieved, August 9, 2010, from http://www.energysavers.gov/your_
home/water_heating/index.cfm/mytopic=12820?print
25. SRP Designer, Email about demand water heaters in new AZ subdivision, R. Lieberman (Ed.), 2010.
26. M. omas, A.C.S. Hayden, D. MacKenzie, Reducing GHG emissions through ecient water heating
technologies, Integrated Energy Systems (IES) Laboratory, Ottawa, 2006.
27. Green Energy Ecient Homes, Electrical tankless water heater: Do these heaters really make a dierence?,
Updated, Retrieved, December 17, 2010, from http://www.green-energy-ecient-homes.com/electric-
tankless-water-heater.html.
28. United States Department of Energy (DOE), Energy savers - heat pump water heaters, Updated, March 24,
2009, Retrieved, from http://www.energysavers.gov/your_home/water_heating/index.cfm/mytopic=12840.
29. United States Department of Energy (DOE), Energy Savers: Solar Water Heating, Updated, 2/09/11,
Retrieved, May 26, 2011, from http://www.energysavers.gov/your_home/water_heating/index.cfm/
mytopic=12850.
30. T. Klenck, How It Works: Water Heater, Popular Mechanics, Hearst Communication, Inc., 1997, http://
www.popularmechanics.com/home/improvement/interior/1275141.
31. E. Hirst, R. Hoskins, Residential water heaters: energy cost and analysis, Energy and Buildings, 1 (1977)
393-400.
32. United States Department of Energy (DOE), Water Heating, Updated, Retrieved, June 5, 2009, from
http://www.energy.gov/waterheating.htm#tip2007
33. C. Sole, M. Medrano, A. Castell, M. Nogues, H. Mehling, L.F. Cabeza, Energetic and exergetic analysis
of a domestic water tank with phase change material, Int J Energ Res, 32 (3) (2008) 204-214.
34. E. Dewailly, J.R. Joly, Contamination of Domestic Water-Heaters with Legionella-Pneumophila—Impact
of Water Temperature on Growth and Dissemination of the Bacterium, Environ Toxic Water, 6 (2) (1991)
249-257.
35. California Energy Commission (CEC), Water Heaters, Updated, Retrieved, July 16, 2009, from www.
consumerenergycenter.org/home/appliances/waterheaters.html.
36. M.A. Delucci, A lifecycle emissions analysis: urban air pollutants and greenhouse-gases from petroleum,
natural gas, lpg, and other fuels for highway vehicles, forklifts, and household heating in the U.S., World
Resource Review, 13 (1) (2001) 25-51.
37. J. Glater, J. Louis York, K. Campbell, Scale Formation and Prevention, in: K.S. Spiegler, A.D.K. Laird
(Eds.) Principles of Desalination, 2nd Ed., Part B, Academic Press, New York, 1980, pp. 627-678.
38. M.S. Lobenstein, Controlling recirculation loop heat losses, Home Energy Magazine Online, 1993, www.
homeenergy.org/archive/hem.dis.anl.gov/eehem/93/930106.html.
39. F. Goldner, Money Down the Drain: Controlling Hot Water Recirculation Costs, Home Energy Maga-
zine Online, 1999, http://www.homeenergy.org/archive/hem.dis.anl.gov/eehem/99/991109.html.
40. M.R. Ally, J.J. Tomlinson, Water and Energy Savings using Demand Hot Water Recirculating Systems in
Residential Homes: A Case Study of Five Homes in Palo Alto, California, Oak Ridge National Laboratory
(ORNL),, 2002, http://www.osti.gov/bridge.
41. T. Carter, Hot Water Recirculating System—Installation Tips, Updated, 2010, Retrieved, August 11, 2010,
from http://www.askthebuilder.com/B413_Hot_Water_Recirculating_System_-_Installation_Tips.shtml.
42. J. Villalobos, Recirculation hot water can corrode pipes, Consulting-Specifying Engineer, Reed Business
Information, Inc., 2007.
43. R.K. Johnson, C.A. Clark, Field evaluation of two demand electric water heaters, American Society of
Heating, Refrigerating and Air-Conditioning Engineers, Chicago, IL, 2006.
44. W. Mathys, J. Stanke, M. Harmuth, E. Junge-Mathys, Occurrence of Legionella in hot water systems of
single-family residences in suburbs of two German cities with special reference to solar and district heat-
ing, Int J Hyg Envir Heal, 211 (1-2) (2008) 179-185.
45. C. Garnier, J. Currie, T. Muneer, Integrated collector storage solar water heater: Temperature stratica-
tion, Appl Energ, 86 (9) (2009) 1465-1469.
46. Centers for Disease Control and Prevention (CDC), Patient Facts: Learn More about Legionnaires’ dis-
ease. Updated, June 27, 2008, Retrieved, from http://www.cdc.gov/L. pneumophila/patient_facts.htm.
Volume 6, Number 4 95
47. T.K. Marras, P. Chedore, A.M. Ying, F. Jamieson, Isolation prevalence of pulmonary non-tuberculous
mycobacteria in Ontario, 1997-2003, orax, 62 (8) (2007) 661-666.
48. J.O. Falkinham, M.D. Iseman, P. de Haas, D. van Soolingen, Mycobacterium avium in a shower linked to
pulmonary disease, J Water Health, 6 (2) (2008) 209-213.
49. D.S. Prince, D.D. Peterson, R.M. Steiner, J.E. Gottlieb, R. Scott, H.L. Israel, W.G. Figueroa, J.E. Fish,
Infection with Mycobacterium-Avium Complex in Patients without Predisposing Conditions, New Engl J
Med, 321 (13) (1989) 863-868.
50. A. Garcia-Fulgueiras, C. Navarro, D. Fenoll, J. Garcia, P. Gonzalez-Diego, T. Jimenez-Bunuales, M.
Rodriguez, R. Lopez, F. Pacheco, J. Ruiz, M. Segovia, B. Baladron, C. Pelaz, Legionnaires’ disease out-
break in Murcia, Spain, Emerg Infect Dis, 9 (8) (2003) 915-921.
51. Centers for Disease Control and Prevention (CDC), Legionellosis, Updated, June 27, 2008, Retrieved,
May 4, 2010, from http://www.cdc.gov/ncidod/dbmd/diseaseinfo/legionellosis_g.htm.
52. K. Todar, Todar’s Online Textbook of Bacteriology: Psuedomonas aeruginosa, Updated, Retrieved, Octo-
ber 20, 2010, from http://www.textbookofbacteriology.net/pseudomonas.html.
53. Centers for Disease Control and Prevention (CDC), Mycobacterium Avium complex. National Center for
Immunization and Respiratory Diseases, Updated, October 12, 2005, Retrieved.
54. L.K. Bagh, H.J. Albrechtsen, E. Arvin, K. Ovesen, Distribution of bacteria in a domestic hot water system
in a Danish apartment building, Water Res, 38 (1) (2004) 225-235.
55. L.K. Bagh, H.J. Albrechtsen, E. Arvin, K. Ovesen, Biolm formation in a hot water system, Water Sci
Technol, 46 (9) (2002) 95-101.
56. G. Mutoti, J.D. Dietz, J. Arevalo, J.S. Taylor, Combined chlorine dissipation: Pipe material, water quality,
and hydraulic eects, J Am Water Works Ass, 99 (10) (2007) 96-106.
57. M.N.B. Momba, R. Kr, S.N. Venter, T.E. Cloete, An overview of biolm formation in distribution sys-
tems and its impact on the deterioration of water quality, Water Sa, 26 (1) (2000) 59-66.
58. F. Codony, J. Morato, J. Mas, Role of discontinuous chlorination on microbial production by drinking
water biolms, Water Res, 39 (9) (2005) 1896-1906.
59. National SAFE KIDS Campaign (NSKC), Burn Injury Fact Sheet, N.S.K. Campaign (Ed.), Washington
D.C. , 2004.
60. Consumer Product Safety Commission (CPSC), Tap Water Scalds, Updated, 2005, Retrieved, August 9,
2010, from http://www.cpsc.gov/cpscpub/pubs/5098.html.
61. Centers for Disease Control and Prevention (CDC), Development of an action plan to address surveil-
lance, epidemiologic, laboratory and environmental issues related to disease caused by Nontuberculous
Mycobacteria., Centers for Disease Control and Prevention External Consultation Summary, 2007-2008.
62. A.T. Spinks, R.H. Dunstan, P. Coombes, G. Kuczera, ermal destruction analyses of water related
pathogens at domestic hot water system temperatures, e Instituation of Engineers: 28th International
Hydrology and Water Resources Symposium, 2003.
