Hot Water Heating
Technology
WATER HEATING
Written and prepared for
the U.S. Department of
Energy by:
cool
water
side
Sensor
NAHB Research Center
800-898-2842
www.nahbrc.org
Southface Energy Institute
404-872-3549
www.southface.org
U.S. Department of Energy’s
Oak Ridge
National Laboratory
Buildings Technology Center
865-574-5178
www.ornl.gov/ORNL/BTC
U.S. Department of Energy’s
National Renewable Energy
Laboratory
(303) 275-3000
www.nrel.gov/building_thermal
NOTICE: Neither the United
States government nor any
agency thereof, nor any of their
employees, makes any warranty,
express or implied, or assumes
any legal liability or responsibility
for the accuracy, completeness,
or usefulness of any information,
apparatus, product, or process
disclosed. The views and opinions of authors expressed herein
do not necessarily state or reflect
those of the United States government or any agency thereof.
Pump
Controls
REDUCING CONSUMPTION
Simple use of water-saving appliances such as
high-efficiency washing machines and
dishwashers (look for the ENERGY STAR® label),
combined with flow-restricting devices such as
faucet aerators and low-flow showerheads,
lowers household hot water consumption.
A recirculation system powers a pump to cycle
water through the plumbing loop between
water heater and faucet. This lessens the
amount of tepid water wasted down the drain
while waiting for hot water. On-demand DHW
systems recirculate water when a user trips a
switch at the faucet. Once water of the desired
temperature reaches the faucet, the pump
shuts off.
WATER HEATING SYSTEM SAFETY
Backdrafting—the pressure-induced spillage of
exhaust gases into interior living space—is a
health and safety concern where combustion
water heaters are employed. To reduce such
risk, employ one of the systems described
below, install a hardwired carbon monoxide
(CO) detector nearby, and annually inspect
(clean, adjust, or repair if necessary) the
burners, combustion chamber, and flue.
•
Outside of the home’s conditioned space
(e.g., in garage).
•
In a sealed, indoor mechanical room having
adequate exterior ventilation.
•
Inside the conditioned space when a power
venting system using a fan is incorporated to
expel combustion gases through the flue,
and/or a sealed combustion system
separately ducts in outside air for combustion
and ducts out combustion gas exhaust.
INSTALLATION AND MAINTENANCE
Always meet the minimum applicable
mechanical, plumbing, electrical, and/or other
code requirements when installing a water
heating system. Initial installation
improvements include installing a second anode
rod (combination anode/hot outlet nipple) and
replacement drain valve (full-port ¾-inch
threaded brass ball valve with hose adapter).
Water heater placement can impact operating
cost, especially if the unit is poorly insulated.
Unit placement to facilitate short, insulated
piping runs to bathrooms and kitchens is most
effective. To improve energy efficiency,
storage-type water heaters are best located in
conditioned space, except in extremely hot
climates where tank heat loss increases the
cooling load.
Periodic water heater maintenance can
significantly extend water heater life and
minimize loss of efficiency over time. Routine
maintenance involves flushing a quart of water
from the storage tank every three months,
checking the temperature and pressure relief
valve every six months, and inspecting the
anode rod every three to four years.
Energy-efficient strategies for
supplying hot water in the home
Buildings for
the 21st Century
Buildings that are more
energy efficient, comfortable,
and affordable…that’s the
goal of DOE’s Office of Building
Technology, State and
Community Programs (BTS).
To accelerate the development
and wide application of energy
efficiency measures, BTS:
• Conducts R&D on
technologies and concepts
for energy efficiency,
working closely with the
building industry and with
manufacturers of materials,
equipment, and appliances
• Promotes energy/money
saving opportunities to both
builders and buyers of homes
and commercial buildings
• Works with state and local
regulatory groups to improve
building codes, appliance
standards, and guidelines for
efficient energy use
• Provides support and grants
to states and communities
for deployment of energyefficient technologies and
practices
ENERGY-EFFICIENT WATER
HEATING
Domestic water heating accounts for between
15 and 25 percent of the energy consumed in
homes. Water-heating energy costs can be
managed by selecting the appropriate fuel and
water heater type, using efficient system
design, and reducing hot water consumption.
TYPES OF WATER HEATERS
Storage-type water heaters, the primary focus
within this fact sheet, are the most common
domestic hot water (DHW) heating system
selected today. However, other types of water
heaters may be very cost effective.
Storage water heaters—heat and store water
in a tank ranging in size from 20 to 80 gallons.
They offer a ready reservoir of hot water,
although “standby” energy losses are higher
than with some other types. Conventional fuel
sources include natural gas, electricity,
propane, and fuel oil.
Heat pump water heaters are electric storage
water heaters that are two to three times as
efficient as conventional electric resistance
units. Because they remove heat from the
surrounding air, they are most effective in
warm climates.
Combination space and water heating
systems—are storage water heating systems
providing space heating plus DHW. Separate
water heaters and forced-air or hydronic
systems may be combined, or a single-source
system may be purchased. Because heating
needs of small, well-insulated homes often are
low, combination systems can be an
excellent choice.
Demand (tankless or instantaneous) water
heaters—heat water directly without use of a
storage tank. Demand systems produce a
limited amount of hot water—a 70°F water
temperature rise is possible at a flow rate of
five gallons per minute through gas water
heaters and two gallons per minute through
electric water heaters. These are best suited
for low-demand or remote applications.
Tankless coil water heaters—use a heat
exchanger integrated with a space-heating
boiler to heat water instantaneously. These are
generally recommended for use only in an
extremely cold climate.
An indirect water heater is a tankless coil
water heater with a separate storage tank to
reduce boiler cycling. When matched with a
high-efficiency boiler, this becomes a most
efficient hot water system.
HEAT PUMP WATER HEATER
Heat pump technology can provide costeffective water heating in mild climates.
Cold water in
Or visit the BTS Web site at
www.eren.doe.gov/buildings
Hot and cold
water service
to sink
Heat
trap
Hot water out
Access
cover
Thermostat
Drain valve
Storage tank
Heat pump
Printed with a renewable-source ink on paper containing at
least 50% wastepaper, including 20% postconsumer waste.
August 2001
DOE/GO-102001-0785
Sheet
WATER HEATING
When a combustion-type hot water storage
tank system is used, placing the tank in the
following places may improve resistance to
backdrafting:
Sacrificial annode rod
Energy Efficiency and
Renewable Energy
Clearinghouse (EREC)
1-800-DOE-3732
www.eren.doe.gov
HOT WATER
RECIRCULATION SYSTEMS
These devices
provide “instant”
hot water at
point sources
and provide
water
conservation
warm
water
benefits.
side
Temp/pressure relief valve
For more information, contact:
Fact
OFFICE OF BUILDING TECHNOLOGY, STATE AND COMMUNITY PROGRAMS
ENERGY EFFICIENCY AND RENEWABLE ENERGY • U.S. DEPARTMENT OF ENERGY
WATER HEATING
To determine the local cost, multiply the annual energy use
(in kWh or therms) found on the EnergyGuide label by local
energy rates. Based on local utility rates, one type of water
heater may operate much more cheaply than another.
WATER HEATER INSULATING JACKETS
Insulating the water heater is one of the best dollar-fordollar energy-saving measures.
FIRST HOUR RATING/PEAK HOUR DEMANDS
•
Fuel availability, including natural gas, electricity, oil, and
propane
Hot water use
Avg. gal.
hot water
per usage
Heat trap
Times
used in
hour
Gal. used
in hour
•
Cost—equipment, installation, and expected annual fuel cost
showering
15
x
=
•
Capability—system capacity, including first hour rating (FHR)
bathing
20
x
=
•
Longevity—expected equipment life
shaving
2
x
=
•
Safety, including possible combustion gas backdrafting
concerns
washing hands & face
2
x
=
shampooing hair
4
x
=
The FHR is the amount of hot water the heater can supply per
hour (starting with the tank full of hot water). The FHR depends
on the tank capacity, source of heat (burner or element), and
size of the burner or element. To select the correct size water
heater, use the FHR—not tank capacity. Using the table
provided, first estimate the daily peak one-hour hot water
demand for the house. Then try to choose a model with a FHR
within about 2 gallons of this peak demand.
The energy factor (EF) indicates overall unit efficiency based on
the amount of hot water produced per unit of fuel consumed
over a typical day. While higher EF values generally equate with
higher efficiency, they do not always mean lowered operating
cost, especially when fuel sources are compared.
Information on annual energy use and average operating cost
can be found on the EnergyGuide label on new water heaters.
hand dishwashing
2
x
=
automatic dishwashing
14
x
=
preparing food
5
x
=
clothes washing
32
x
=
PEAK HOUR DEMAND (total gal used in hour)
Drainwater heat recovery devices improve efficiency by warming
incoming cold water with heat absorbed from draining warm
water. Recovery systems reclaim energy and can enhance hot
water system performance by increasing effective FHR and
capacity. Benefits ultimately depend on many variables
including installation, fuel type, system geometry, exchanger
characteristics, and usage patterns.
Flue dampers seal off the flues of naturally drafted gas water
heaters when there is no exhaust to reduce heat loss up the flue.
Timers can shut off electric water heaters for a set time period,
such as when time-of-use electric rates are highest or when
occupants normally do not use hot water. Depending on shutoff
time, setpoint, and tank insulation level, timers may or may not be
cost-effective. In addition, they may be inconvenient when much
hot water is needed while the water heater is turned off.
Cut-outs for
heating coil
elements
=
EXAMPLE
A typical household's peak hour of hot water usage might entail:
Electric
2 showers
15
x
2
=
30
2 shampoos
4
x
2
=
8
1 shaving
2
x
1
=
2
2 hand/face washing
2
x
2
=
4
1 food prep
5
x
1
=
5
=
49
PEAK HOUR DEMAND (total gal used in hour)
inexpensive, do-it-yourself alternative to plumber-installed
heat traps.
Source: Gas Appliance Manufacturers Association
*The above chart assumes no water conservation measures.
STORAGE WATER HEATER COMPARISON
Cut-out for
combustion air
Gas
IMPROVING WATER HEATER EFFICIENCY
With any DHW system, methods for reducing hot water cost
start with lowering the thermostat. Set the thermostat low
enough to minimize safety concerns (scalding) yet high enough
for washing (dishes, clothes) and bathing. A setting of 120°F
generally meets most household needs.
Installing water heater insulating “jackets” of R-11 or higher can
be cost effective in reducing standby losses on storage-type units,
especially older or poorly insulated ones. Greater benefit is
achieved when the hot water tank is installed outside of conditioned space (e.g., garage). For safety, however, never block any
controls, valves, or airways for combustion or exhaust.
Pipe insulation is inexpensive and easy to install. Add it to hot
and cold water pipes in unconditioned spaces such as garages,
attics, and crawlspaces and for a distance of at least six feet
from the tank.
Load management devices enable utilities to turn off electric
water heaters remotely during periods of heavy electrical
demand. Allowing this option to be installed, where available,
typically results in a monthly credit from the utility.
DRAINWATER HEAT RECOVERY
This system captures waste heat through heat exchange
from the wastewater drains.
Faucet
Preheated cold water to
plumbing fixtures and to water
heater
Hot water
STORAGE TANK WATER HEATER SELECTION
The lowest-priced water heater may be the most expensive to
operate and maintain over its lifetime. And while an oversized
unit may be alluring, it carries a higher purchase price and
increased energy costs due to increased stand-by losses.
Consider the following factors when buying a water heater:
Heat exchanger
Cold water in
Heat traps prevent heated water in a storage tank from mixing
with cooled water in pipes, a process called thermosiphoning.
Some new water heaters have built-in heat traps, although they
can be added to new or existing water heaters. Flexible
connectors with a loop in the vertical line offer an effective,
Based on a family of four, electricity at $0.08 per kWh, natural gas at $0.60 per therm, and propane at $1.00 per gallon (prices often vary seasonally).
Drain water
Hot water tank
WATER HEATING
To determine the local cost, multiply the annual energy use
(in kWh or therms) found on the EnergyGuide label by local
energy rates. Based on local utility rates, one type of water
heater may operate much more cheaply than another.
WATER HEATER INSULATING JACKETS
Insulating the water heater is one of the best dollar-fordollar energy-saving measures.
FIRST HOUR RATING/PEAK HOUR DEMANDS
•
Fuel availability, including natural gas, electricity, oil, and
propane
Hot water use
Avg. gal.
hot water
per usage
Heat trap
Times
used in
hour
Gal. used
in hour
•
Cost—equipment, installation, and expected annual fuel cost
showering
15
x
=
•
Capability—system capacity, including first hour rating (FHR)
bathing
20
x
=
•
Longevity—expected equipment life
shaving
2
x
=
•
Safety, including possible combustion gas backdrafting
concerns
washing hands & face
2
x
=
shampooing hair
4
x
=
The FHR is the amount of hot water the heater can supply per
hour (starting with the tank full of hot water). The FHR depends
on the tank capacity, source of heat (burner or element), and
size of the burner or element. To select the correct size water
heater, use the FHR—not tank capacity. Using the table
provided, first estimate the daily peak one-hour hot water
demand for the house. Then try to choose a model with a FHR
within about 2 gallons of this peak demand.
The energy factor (EF) indicates overall unit efficiency based on
the amount of hot water produced per unit of fuel consumed
over a typical day. While higher EF values generally equate with
higher efficiency, they do not always mean lowered operating
cost, especially when fuel sources are compared.
Information on annual energy use and average operating cost
can be found on the EnergyGuide label on new water heaters.
hand dishwashing
2
x
=
automatic dishwashing
14
x
=
preparing food
5
x
=
clothes washing
32
x
=
PEAK HOUR DEMAND (total gal used in hour)
Drainwater heat recovery devices improve efficiency by warming
incoming cold water with heat absorbed from draining warm
water. Recovery systems reclaim energy and can enhance hot
water system performance by increasing effective FHR and
capacity. Benefits ultimately depend on many variables
including installation, fuel type, system geometry, exchanger
characteristics, and usage patterns.
Flue dampers seal off the flues of naturally drafted gas water
heaters when there is no exhaust to reduce heat loss up the flue.
Timers can shut off electric water heaters for a set time period,
such as when time-of-use electric rates are highest or when
occupants normally do not use hot water. Depending on shutoff
time, setpoint, and tank insulation level, timers may or may not be
cost-effective. In addition, they may be inconvenient when much
hot water is needed while the water heater is turned off.
Cut-outs for
heating coil
elements
=
EXAMPLE
A typical household's peak hour of hot water usage might entail:
Electric
2 showers
15
x
2
=
30
2 shampoos
4
x
2
=
8
1 shaving
2
x
1
=
2
2 hand/face washing
2
x
2
=
4
1 food prep
5
x
1
=
5
=
49
PEAK HOUR DEMAND (total gal used in hour)
inexpensive, do-it-yourself alternative to plumber-installed
heat traps.
Source: Gas Appliance Manufacturers Association
*The above chart assumes no water conservation measures.
STORAGE WATER HEATER COMPARISON
Cut-out for
combustion air
Gas
IMPROVING WATER HEATER EFFICIENCY
With any DHW system, methods for reducing hot water cost
start with lowering the thermostat. Set the thermostat low
enough to minimize safety concerns (scalding) yet high enough
for washing (dishes, clothes) and bathing. A setting of 120°F
generally meets most household needs.
Installing water heater insulating “jackets” of R-11 or higher can
be cost effective in reducing standby losses on storage-type units,
especially older or poorly insulated ones. Greater benefit is
achieved when the hot water tank is installed outside of conditioned space (e.g., garage). For safety, however, never block any
controls, valves, or airways for combustion or exhaust.
Pipe insulation is inexpensive and easy to install. Add it to hot
and cold water pipes in unconditioned spaces such as garages,
attics, and crawlspaces and for a distance of at least six feet
from the tank.
Load management devices enable utilities to turn off electric
water heaters remotely during periods of heavy electrical
demand. Allowing this option to be installed, where available,
typically results in a monthly credit from the utility.
DRAINWATER HEAT RECOVERY
This system captures waste heat through heat exchange
from the wastewater drains.
Faucet
Preheated cold water to
plumbing fixtures and to water
heater
Hot water
STORAGE TANK WATER HEATER SELECTION
The lowest-priced water heater may be the most expensive to
operate and maintain over its lifetime. And while an oversized
unit may be alluring, it carries a higher purchase price and
increased energy costs due to increased stand-by losses.
Consider the following factors when buying a water heater:
Heat exchanger
Cold water in
Heat traps prevent heated water in a storage tank from mixing
with cooled water in pipes, a process called thermosiphoning.
Some new water heaters have built-in heat traps, although they
can be added to new or existing water heaters. Flexible
connectors with a loop in the vertical line offer an effective,
Based on a family of four, electricity at $0.08 per kWh, natural gas at $0.60 per therm, and propane at $1.00 per gallon (prices often vary seasonally).
Drain water
Hot water tank
Technology
WATER HEATING
Written and prepared for
the U.S. Department of
Energy by:
cool
water
side
Sensor
NAHB Research Center
800-898-2842
www.nahbrc.org
Southface Energy Institute
404-872-3549
www.southface.org
U.S. Department of Energy’s
Oak Ridge
National Laboratory
Buildings Technology Center
865-574-5178
www.ornl.gov/ORNL/BTC
U.S. Department of Energy’s
National Renewable Energy
Laboratory
(303) 275-3000
www.nrel.gov/building_thermal
NOTICE: Neither the United
States government nor any
agency thereof, nor any of their
employees, makes any warranty,
express or implied, or assumes
any legal liability or responsibility
for the accuracy, completeness,
or usefulness of any information,
apparatus, product, or process
disclosed. The views and opinions of authors expressed herein
do not necessarily state or reflect
those of the United States government or any agency thereof.
Pump
Controls
REDUCING CONSUMPTION
Simple use of water-saving appliances such as
high-efficiency washing machines and
dishwashers (look for the ENERGY STAR® label),
combined with flow-restricting devices such as
faucet aerators and low-flow showerheads,
lowers household hot water consumption.
A recirculation system powers a pump to cycle
water through the plumbing loop between
water heater and faucet. This lessens the
amount of tepid water wasted down the drain
while waiting for hot water. On-demand DHW
systems recirculate water when a user trips a
switch at the faucet. Once water of the desired
temperature reaches the faucet, the pump
shuts off.
WATER HEATING SYSTEM SAFETY
Backdrafting—the pressure-induced spillage of
exhaust gases into interior living space—is a
health and safety concern where combustion
water heaters are employed. To reduce such
risk, employ one of the systems described
below, install a hardwired carbon monoxide
(CO) detector nearby, and annually inspect
(clean, adjust, or repair if necessary) the
burners, combustion chamber, and flue.
•
Outside of the home’s conditioned space
(e.g., in garage).
•
In a sealed, indoor mechanical room having
adequate exterior ventilation.
•
Inside the conditioned space when a power
venting system using a fan is incorporated to
expel combustion gases through the flue,
and/or a sealed combustion system
separately ducts in outside air for combustion
and ducts out combustion gas exhaust.
INSTALLATION AND MAINTENANCE
Always meet the minimum applicable
mechanical, plumbing, electrical, and/or other
code requirements when installing a water
heating system. Initial installation
improvements include installing a second anode
rod (combination anode/hot outlet nipple) and
replacement drain valve (full-port ¾-inch
threaded brass ball valve with hose adapter).
Water heater placement can impact operating
cost, especially if the unit is poorly insulated.
Unit placement to facilitate short, insulated
piping runs to bathrooms and kitchens is most
effective. To improve energy efficiency,
storage-type water heaters are best located in
conditioned space, except in extremely hot
climates where tank heat loss increases the
cooling load.
Periodic water heater maintenance can
significantly extend water heater life and
minimize loss of efficiency over time. Routine
maintenance involves flushing a quart of water
from the storage tank every three months,
checking the temperature and pressure relief
valve every six months, and inspecting the
anode rod every three to four years.
Energy-efficient strategies for
supplying hot water in the home
Buildings for
the 21st Century
Buildings that are more
energy efficient, comfortable,
and affordable…that’s the
goal of DOE’s Office of Building
Technology, State and
Community Programs (BTS).
To accelerate the development
and wide application of energy
efficiency measures, BTS:
• Conducts R&D on
technologies and concepts
for energy efficiency,
working closely with the
building industry and with
manufacturers of materials,
equipment, and appliances
• Promotes energy/money
saving opportunities to both
builders and buyers of homes
and commercial buildings
• Works with state and local
regulatory groups to improve
building codes, appliance
standards, and guidelines for
efficient energy use
• Provides support and grants
to states and communities
for deployment of energyefficient technologies and
practices
ENERGY-EFFICIENT WATER
HEATING
Domestic water heating accounts for between
15 and 25 percent of the energy consumed in
homes. Water-heating energy costs can be
managed by selecting the appropriate fuel and
water heater type, using efficient system
design, and reducing hot water consumption.
TYPES OF WATER HEATERS
Storage-type water heaters, the primary focus
within this fact sheet, are the most common
domestic hot water (DHW) heating system
selected today. However, other types of water
heaters may be very cost effective.
Storage water heaters—heat and store water
in a tank ranging in size from 20 to 80 gallons.
They offer a ready reservoir of hot water,
although “standby” energy losses are higher
than with some other types. Conventional fuel
sources include natural gas, electricity,
propane, and fuel oil.
Heat pump water heaters are electric storage
water heaters that are two to three times as
efficient as conventional electric resistance
units. Because they remove heat from the
surrounding air, they are most effective in
warm climates.
Combination space and water heating
systems—are storage water heating systems
providing space heating plus DHW. Separate
water heaters and forced-air or hydronic
systems may be combined, or a single-source
system may be purchased. Because heating
needs of small, well-insulated homes often are
low, combination systems can be an
excellent choice.
Demand (tankless or instantaneous) water
heaters—heat water directly without use of a
storage tank. Demand systems produce a
limited amount of hot water—a 70°F water
temperature rise is possible at a flow rate of
five gallons per minute through gas water
heaters and two gallons per minute through
electric water heaters. These are best suited
for low-demand or remote applications.
Tankless coil water heaters—use a heat
exchanger integrated with a space-heating
boiler to heat water instantaneously. These are
generally recommended for use only in an
extremely cold climate.
An indirect water heater is a tankless coil
water heater with a separate storage tank to
reduce boiler cycling. When matched with a
high-efficiency boiler, this becomes a most
efficient hot water system.
HEAT PUMP WATER HEATER
Heat pump technology can provide costeffective water heating in mild climates.
Cold water in
Or visit the BTS Web site at
www.eren.doe.gov/buildings
Hot and cold
water service
to sink
Heat
trap
Hot water out
Access
cover
Thermostat
Drain valve
Storage tank
Heat pump
Printed with a renewable-source ink on paper containing at
least 50% wastepaper, including 20% postconsumer waste.
August 2001
DOE/GO-102001-0785
Sheet
WATER HEATING
When a combustion-type hot water storage
tank system is used, placing the tank in the
following places may improve resistance to
backdrafting:
Sacrificial annode rod
Energy Efficiency and
Renewable Energy
Clearinghouse (EREC)
1-800-DOE-3732
www.eren.doe.gov
HOT WATER
RECIRCULATION SYSTEMS
These devices
provide “instant”
hot water at
point sources
and provide
water
conservation
warm
water
benefits.
side
Temp/pressure relief valve
For more information, contact:
Fact
OFFICE OF BUILDING TECHNOLOGY, STATE AND COMMUNITY PROGRAMS
ENERGY EFFICIENCY AND RENEWABLE ENERGY • U.S. DEPARTMENT OF ENERGY
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