Domestic hot water (DHW) is hot water that is used for
bathing, clothes washing, washing dishes, and many
other things, but not for heating building spaces. Domestic
hot water is sometimes called building service
hot water in nonresidential uses. Sometimes, when a
well-insulated building uses very little water for space
heating but uses a lot of hot water for other purposes,
a single large hot water heater supplies both.
HOT WATER TEMPERATURES
Excessively hot water temperatures can result in scalding.
People generally take showers at 41°C to 49°C
(105°F–120°F), often by blending hot water at 60°C
(140°F) with cold water with a mixing valve in the
shower. Most people experience temperatures above
43°C (110°F) as uncomfortably hot.
Some commercial uses require higher temperatures.
The minimum for a sanitizing rinse for a commercial
dishwasher or laundry is 82°C (180°F). Generalpurpose
cleaning and food preparation requires 60°C
(140°F) water. Temperatures above 60°C can cause serious
burns, and promote scaling if the water is hard.
However, high temperatures limit the growth of the
harmful bacterium Legionella pneumophila, which causes
Legionnaire’s disease. Water heaters for high temperature
uses have larger heating units, but the tanks can
be smaller because less cold water has to be mixed in.
Some appliances, such as dishwashers, heat water at the
point of use. Codes may regulate or limit high water
temperatures.
Lower temperatures are less likely to cause burns,
but may be inadequate for sanitation. Lower temperature
water loses less heat in storage and in pipes, saving
energy. Smaller heating units are adequate, but larger
storage tanks are needed. Solar or waste heat recovery
sources work better with lower temperature water
heaters. For energy conservation, use the lowest possible
temperatures.
WATER HEATERS
Water heating accounts for over 20 percent of the average
family’s annual heating bill. Hot water is commonly
heated using natural gas or electricity. It is also possible
to use heat that would be wasted from other systems,
or heat from steam, cogeneration, or wood-burning
systems.
Hot Water
Solar Water Heaters
Solar energy is often used for the hot water needs of
families in sunny climates. In temperate climates with
little winter sun, solar water heaters can serve as preheating
systems, with backup from a standard system.
The solar water heater raises the temperature of the
water before it enters the standard water-heating tank,
so that the electric element or gas burner consumes less
fuel. Solar water heaters can cut the average family’s
water-heating bill by 40 to 60 percent annually, even in
a cold climate. Heavy water users will benefit the most.
Although initial costs of solar water heaters may be
higher than for conventional systems, they offer longterm
savings. A complete system costing under $3000
can provide two-thirds of a family’s hot water needs even
in New England. This is competitive with the still less
expensive gas water heater. Some states offer income tax
credits, and some electric utilities give rebates for solar
water heaters. Solar water heaters are required on new
construction in some parts of the United States.
Solar water heating isn’t always the best choice.
When considering a decision to go solar, the existing
water heater should first be made as efficient as possible.
A careful analysis of the building site will determine
if there is adequate sun for solar collectors, which will
need to face within 40 degrees of true south. Trees,
buildings, or other obstructions should not shade the
collectors between 9 a.m. and 3 p.m.
Solar water heaters use either direct or indirect systems.
In a direct system, the water circulates through a
solar collector (Fig. 9-1). Direct systems are simple, efficient,
and have no piping or heat exchanger complications.
In an indirect system, a fluid circulates in a
closed loop through the collector and storage tank. With
an indirect system, the fluid is not mixed with the hot
water, but heat is passed between fluids by a heat exchanger.
This allows for the use of nonfreezing solutions
in the collector loop.
Solar water heater systems are categorized as either
active or passive. In passive systems, gravity circulates
water down from a storage tank above the collector. The
heavy tanks may require special structural support.
These systems tend to have relatively low initial installation
and operating cost and to be very reliable mechanically.
Active systems use pumps to force fluid to
the collector. This leaves them susceptible to mechanical
breakdown and increases maintenance and energy
costs. Active systems are more common in the United
States.
Solar energy can heat outdoor swimming pools during
the months with most sun. Solar pool heating extends
the swimming season by several weeks and pays
for itself within two years. The pool’s existing filtration
system pumps water through solar collectors, where
water is heated and pumped back to the pool. More
complex systems are available for heating indoor pools,
hot tubs, and spas in colder climates.
Heat Pump Water Heaters
A heat pump water heater takes excess heat from the air
in a hot place, like a restaurant kitchen or hot outdoor
air, and uses it to heat water. In the process, the heat
pump cools and dehumidifies the space it serves. Because
the heat pump water heater moves the heat from
one location to another rather than heating the water
directly, it uses only one-half to one-third of the amount
of energy a standard water heater needs. Heat pump
water heaters can run on the heat given off by refrigeration
units such as ice-making machines, grocery refrigeration
display units, and walk-in freezers.
Because a heat pump water heater uses refrigerant
fluid and a compressor to transfer heat to an insulated
Solar water heater.
storage tank, they are more expensive than other types
of water heaters to purchase and maintain. Some units
come with built-in water tanks, while others are added
onto existing hot water tanks. The heat pump takes up
a small amount of space in addition to the storage tank,
and there is some noise from the compressor and fan.
Storage Tank Water Heaters
Residential and small commercial buildings usually use
centrally located storage tank water heaters. Some buildings
combine a central tank with additional tanks near
the end use to help reduce heat lost in pipes. Circulating
storage water heaters heat the water first by a coil,
and then circulate it through the storage tank.
Storage-type water heaters are rated by tank capacity
in gallons, and by recovery time, which is the time
required for the tank to reach a desired temperature
when filled with cold water. This shows up as the time
it takes to get a hot shower after someone takes a long
shower and empties the tank. Storage water heaters usually
have 20- to 80-gallon capacities, and use electricity,
natural gas, propane, or oil for fuel. The water enters at
the bottom of the tank, where it is heated, and leaves
at the top. The heat loss through the sides of the tank
continues even when no hot water is being used, so storage
water heaters keep using energy to maintain water
temperature. The tanks usually are insulated to retain
heat, but some older models may need more insulation.
Local utilities will sometimes insulate hot water tanks
for free. High-efficiency water heaters are better insulated
and use less energy.
Tankless Water Heaters
Small wall-mounted tankless water heaters (Fig. 9-2) are
located next to plumbing fixtures that occasionally need
hot water, like isolated bathrooms and laundry rooms.
They can be easily installed in cabinets, vanities, or closets
near the point of use. Although they use a great
amount of heat for a short time to heat a very limited
amount of water, these tankless heaters can reduce energy
consumption by limiting the heat lost from water
storage tanks and long piping runs. Because they may
demand a lot of heat at peak times, electric heaters are
usually not economical over time where electric utilities
charge customers based on demand.
These small tankless water heaters (also called instantaneous
or demand heaters) raise the water temperature
very quickly within a heating coil, from which
it is immediately sent to the point of use. A gas burner
or electrical element heats the water as needed. They
have no storage tank, and consequently do not lose heat.
With modulating temperature controls, demand water
heaters will keep water temperatures the same at different
rates of flow.
Without a storage tank, the number of gallons of
hot water available per minute is limited. The largest
gas-fired demand water heaters can heat only 3 gallons
of water per minute (gpm), so they are not very useful
for commercial applications, but may be acceptable for
a residence with a low-flow shower and limited demand.
Gas heaters must be vented.
The largest electric models heat only 2 gpm, and are
used as supplementary heaters in home additions or remote
locations, or as boosters under sinks. Electric
heaters require 240V wiring.
Instant hot water taps use electric resistance heaters
to supply hot water up to 88°C (190°F) at kitchen and
bar sinks. They are expensive and waste energy. Instant
hot water dispensers require a 120V fused, grounded
outlet within 102 cm (40 in.) from the hot water dispenser
tank, plus a water supply.
Some tankless coil water heaters take their heat from
an older oil- or gas-fired boiler used for the home heating
system. The hot water circulates through a heat exchanger
in the boiler. The boiler must be run for hot
water even in the summer when space heating isn’t
needed, so the boiler cycles on and off frequently just
to heat water. These inefficient systems consume 3 Btus
of heat energy from fuel for each Btu of hot water they
produce.
Point-of-use water heater.
Indirect Water Heaters
Indirect water heaters also use a boiler or furnace as the
heat source, but are designed to be one of the least expensive
ways to provide hot water when used with a
new high-efficiency boiler. Hot water from the boiler is
circulated through a heat exchanger in a separate insulated
tank. Less commonly, water in a heat exchanger
coil circulates through a furnace, then through a water
storage tank. These indirect water heaters are purchased
as part of a boiler or furnace system, or as a separate
component. They may be operated with gas, oil, or
propane.
Integrated Water Heating
and Space Heating
Some advanced heating systems combine water heating
with warm air space heating in the same appliance. A
powerful water heater provides hot water for domestic
use and to supplement a fan-coil unit (FCU) that heats
air for space heating. The warmed air is then distributed
through ducts. Integrated gas heaters are inexpensive to
purchase and install. They take up less space and are more
efficient at heating water than conventional systems.
Water Heater Safety
and Energy Efficiency
Either sealed combustion or a power-vented system will
assure safety and energy efficiency in a water heater. In
a sealed combustion system, outside air is fed directly
to the water heater and the combustion gases are vented
directly to the outside. Power-vented equipment can
use house air for combustion, with flue gases vented
by a fan. This is not a safe solution in a tightly sealed
building.
In 1987, the National Appliance Energy Conservation
Act set minimum requirements for water heating
equipment in the United States. Equipment is labeled
with energy conservation information. The U.S. Department
of Energy (DOE) developed standardized energy
factors (EF) as a measure of annual overall efficiency.
Standard gas-fired storage tank water heaters may
receive an EF of 0.60 to 0.64. Gas-fired tankless water
heaters rate up to 0.69 with continuous pilots, and up
to 0.93 with electronic ignition. The 2001 DOE standards
for water heaters will increase efficiency criteria,
and should result in significant utility savings over the
life of gas-fired water heaters and electric water heaters.
Water heaters lose less heat if they are located
in a relatively warm area, so avoid putting the water
heater in an unheated basement. By locating the
water heater centrally, you can cut down on heat lost in
long piping runs to kitchens and bathrooms.
Existing water heaters can be upgraded for improved
efficiency. By installing heat traps on both hot and cold
water lines at a cost of about $30 each, you will save
about $15 to $30 per year in lost heat. The cold water
pipe should be insulated between the tank and the heat
trap. If heat traps are not installed, both hot and cold
pipes should be insulated for several feet near the water
heater.
Low-flow showerheads and faucet aerators save
both heat and water. United States government standards
require that showerheads and faucets use less than
2.5 gpm. Low-flow showerheads come in shower massage
styles. Faucets with aerators are available that use
to 1 gpm. By lowering water temperatures to around
49°C (120°F), you save energy and reduce the risk of
burns.
A relatively inexpensive counterflow heat exchanger
can save up to 50 percent of the energy a home uses to
heat water. It consists of a coil of copper tubing that’s
tightly wrapped around a 76- to 102-mm (3–4-in.) diameter
copper pipe, and installed vertically in the
plumbing system. As waste water flows down through
the vertical pipe section, more than half the water’s heat
energy is transferred through the copper pipe and tubing
to the incoming cold water. There is no pump, no
storage tank, and no electricity used. The counterflow
heat exchanger only works when the drain and supply
lines are being used simultaneously, as when someone
is taking a shower.
Spas and hot tubs must be kept tightly covered and
insulated around the bottom and sides. Waterbeds are
found in up to 20 percent of homes in the United States,
and are sometimes the largest electrical use in the home.
Most waterbeds are heated with electric coils underneath
the bed. Your clients can conserve energy by keeping
a comforter on top, insulating the sides, and putting
the heater on a timer
.
HOT WATER DISTRIBUTION
Hot water is carried through the building by pipes
arranged in distribution trees. When hot water flows
through a single hot water distribution tree, it will cool
off as it gets farther from the hot water heater. To get
hot water at the end of the run, you have to waste the
cooled-off water already in the pipes. With a looped hot
water distribution tree, the water circulates constantly.
There is still some heat loss in the pipes, but less water
has to be run at the fixture before it gets hot. Hot water
is always available at each tap in one to two seconds.
Hot water is circulated by use of the thermosiphon
principle. This is the phenomenon where water expands
and becomes lighter as it is heated. The warmed water
rises to where it is used, then cools and drops back down
to the water heater, leaving no cold water standing in
pipes. Thermosiphon circulation works better the higher
the system goes.
Forced circulation is used in long buildings that are
too low for thermosiphon circulation, and where friction
from long pipe runs slows down the flow. The water
heater and a pump are turned on as needed to keep
water at the desired temperature. It takes five to ten seconds
for water to reach full temperature at the fixture.
Forced circulation is common in large one-story residential,
school, and factory buildings.
Computer controls can save energy in hotels, motels,
apartment houses, and larger commercial buildings.
The computer provides the hottest water temperatures
at the busiest hours. When usage is lower, the
supply temperature is lowered and more hot water is
mixed with less cold water at showers, lavatories, and
sinks. Distributing cooler water to the fixture results in
less heat lost along the pipes. The computer stores and
adjusts a memory of the building’s typical daily use
patterns. Hot water pipes expand. Expansion bends are installed
in long piping runs to accommodate the expansion
of the pipes due to heat.
Where the pipes branch out to a fixture, capped
lengths of vertical pipe about 0.6 meters (2 ft) long provide
expansion chambers to dampen the shock of hot
water expansion. Rechargeable air chambers on branch
lines adjacent to groups of fixtures are designed to deal
with the shock of water expansion. They require service
access to be refilled with air.
bathing, clothes washing, washing dishes, and many
other things, but not for heating building spaces. Domestic
hot water is sometimes called building service
hot water in nonresidential uses. Sometimes, when a
well-insulated building uses very little water for space
heating but uses a lot of hot water for other purposes,
a single large hot water heater supplies both.
HOT WATER TEMPERATURES
Excessively hot water temperatures can result in scalding.
People generally take showers at 41°C to 49°C
(105°F–120°F), often by blending hot water at 60°C
(140°F) with cold water with a mixing valve in the
shower. Most people experience temperatures above
43°C (110°F) as uncomfortably hot.
Some commercial uses require higher temperatures.
The minimum for a sanitizing rinse for a commercial
dishwasher or laundry is 82°C (180°F). Generalpurpose
cleaning and food preparation requires 60°C
(140°F) water. Temperatures above 60°C can cause serious
burns, and promote scaling if the water is hard.
However, high temperatures limit the growth of the
harmful bacterium Legionella pneumophila, which causes
Legionnaire’s disease. Water heaters for high temperature
uses have larger heating units, but the tanks can
be smaller because less cold water has to be mixed in.
Some appliances, such as dishwashers, heat water at the
point of use. Codes may regulate or limit high water
temperatures.
Lower temperatures are less likely to cause burns,
but may be inadequate for sanitation. Lower temperature
water loses less heat in storage and in pipes, saving
energy. Smaller heating units are adequate, but larger
storage tanks are needed. Solar or waste heat recovery
sources work better with lower temperature water
heaters. For energy conservation, use the lowest possible
temperatures.
WATER HEATERS
Water heating accounts for over 20 percent of the average
family’s annual heating bill. Hot water is commonly
heated using natural gas or electricity. It is also possible
to use heat that would be wasted from other systems,
or heat from steam, cogeneration, or wood-burning
systems.
Hot Water
Solar Water Heaters
Solar energy is often used for the hot water needs of
families in sunny climates. In temperate climates with
little winter sun, solar water heaters can serve as preheating
systems, with backup from a standard system.
The solar water heater raises the temperature of the
water before it enters the standard water-heating tank,
so that the electric element or gas burner consumes less
fuel. Solar water heaters can cut the average family’s
water-heating bill by 40 to 60 percent annually, even in
a cold climate. Heavy water users will benefit the most.
Although initial costs of solar water heaters may be
higher than for conventional systems, they offer longterm
savings. A complete system costing under $3000
can provide two-thirds of a family’s hot water needs even
in New England. This is competitive with the still less
expensive gas water heater. Some states offer income tax
credits, and some electric utilities give rebates for solar
water heaters. Solar water heaters are required on new
construction in some parts of the United States.
Solar water heating isn’t always the best choice.
When considering a decision to go solar, the existing
water heater should first be made as efficient as possible.
A careful analysis of the building site will determine
if there is adequate sun for solar collectors, which will
need to face within 40 degrees of true south. Trees,
buildings, or other obstructions should not shade the
collectors between 9 a.m. and 3 p.m.
Solar water heaters use either direct or indirect systems.
In a direct system, the water circulates through a
solar collector (Fig. 9-1). Direct systems are simple, efficient,
and have no piping or heat exchanger complications.
In an indirect system, a fluid circulates in a
closed loop through the collector and storage tank. With
an indirect system, the fluid is not mixed with the hot
water, but heat is passed between fluids by a heat exchanger.
This allows for the use of nonfreezing solutions
in the collector loop.
Solar water heater systems are categorized as either
active or passive. In passive systems, gravity circulates
water down from a storage tank above the collector. The
heavy tanks may require special structural support.
These systems tend to have relatively low initial installation
and operating cost and to be very reliable mechanically.
Active systems use pumps to force fluid to
the collector. This leaves them susceptible to mechanical
breakdown and increases maintenance and energy
costs. Active systems are more common in the United
States.
Solar energy can heat outdoor swimming pools during
the months with most sun. Solar pool heating extends
the swimming season by several weeks and pays
for itself within two years. The pool’s existing filtration
system pumps water through solar collectors, where
water is heated and pumped back to the pool. More
complex systems are available for heating indoor pools,
hot tubs, and spas in colder climates.
Heat Pump Water Heaters
A heat pump water heater takes excess heat from the air
in a hot place, like a restaurant kitchen or hot outdoor
air, and uses it to heat water. In the process, the heat
pump cools and dehumidifies the space it serves. Because
the heat pump water heater moves the heat from
one location to another rather than heating the water
directly, it uses only one-half to one-third of the amount
of energy a standard water heater needs. Heat pump
water heaters can run on the heat given off by refrigeration
units such as ice-making machines, grocery refrigeration
display units, and walk-in freezers.
Because a heat pump water heater uses refrigerant
fluid and a compressor to transfer heat to an insulated
Solar water heater.
storage tank, they are more expensive than other types
of water heaters to purchase and maintain. Some units
come with built-in water tanks, while others are added
onto existing hot water tanks. The heat pump takes up
a small amount of space in addition to the storage tank,
and there is some noise from the compressor and fan.
Storage Tank Water Heaters
Residential and small commercial buildings usually use
centrally located storage tank water heaters. Some buildings
combine a central tank with additional tanks near
the end use to help reduce heat lost in pipes. Circulating
storage water heaters heat the water first by a coil,
and then circulate it through the storage tank.
Storage-type water heaters are rated by tank capacity
in gallons, and by recovery time, which is the time
required for the tank to reach a desired temperature
when filled with cold water. This shows up as the time
it takes to get a hot shower after someone takes a long
shower and empties the tank. Storage water heaters usually
have 20- to 80-gallon capacities, and use electricity,
natural gas, propane, or oil for fuel. The water enters at
the bottom of the tank, where it is heated, and leaves
at the top. The heat loss through the sides of the tank
continues even when no hot water is being used, so storage
water heaters keep using energy to maintain water
temperature. The tanks usually are insulated to retain
heat, but some older models may need more insulation.
Local utilities will sometimes insulate hot water tanks
for free. High-efficiency water heaters are better insulated
and use less energy.
Tankless Water Heaters
Small wall-mounted tankless water heaters (Fig. 9-2) are
located next to plumbing fixtures that occasionally need
hot water, like isolated bathrooms and laundry rooms.
They can be easily installed in cabinets, vanities, or closets
near the point of use. Although they use a great
amount of heat for a short time to heat a very limited
amount of water, these tankless heaters can reduce energy
consumption by limiting the heat lost from water
storage tanks and long piping runs. Because they may
demand a lot of heat at peak times, electric heaters are
usually not economical over time where electric utilities
charge customers based on demand.
These small tankless water heaters (also called instantaneous
or demand heaters) raise the water temperature
very quickly within a heating coil, from which
it is immediately sent to the point of use. A gas burner
or electrical element heats the water as needed. They
have no storage tank, and consequently do not lose heat.
With modulating temperature controls, demand water
heaters will keep water temperatures the same at different
rates of flow.
Without a storage tank, the number of gallons of
hot water available per minute is limited. The largest
gas-fired demand water heaters can heat only 3 gallons
of water per minute (gpm), so they are not very useful
for commercial applications, but may be acceptable for
a residence with a low-flow shower and limited demand.
Gas heaters must be vented.
The largest electric models heat only 2 gpm, and are
used as supplementary heaters in home additions or remote
locations, or as boosters under sinks. Electric
heaters require 240V wiring.
Instant hot water taps use electric resistance heaters
to supply hot water up to 88°C (190°F) at kitchen and
bar sinks. They are expensive and waste energy. Instant
hot water dispensers require a 120V fused, grounded
outlet within 102 cm (40 in.) from the hot water dispenser
tank, plus a water supply.
Some tankless coil water heaters take their heat from
an older oil- or gas-fired boiler used for the home heating
system. The hot water circulates through a heat exchanger
in the boiler. The boiler must be run for hot
water even in the summer when space heating isn’t
needed, so the boiler cycles on and off frequently just
to heat water. These inefficient systems consume 3 Btus
of heat energy from fuel for each Btu of hot water they
produce.
Point-of-use water heater.
Indirect Water Heaters
Indirect water heaters also use a boiler or furnace as the
heat source, but are designed to be one of the least expensive
ways to provide hot water when used with a
new high-efficiency boiler. Hot water from the boiler is
circulated through a heat exchanger in a separate insulated
tank. Less commonly, water in a heat exchanger
coil circulates through a furnace, then through a water
storage tank. These indirect water heaters are purchased
as part of a boiler or furnace system, or as a separate
component. They may be operated with gas, oil, or
propane.
Integrated Water Heating
and Space Heating
Some advanced heating systems combine water heating
with warm air space heating in the same appliance. A
powerful water heater provides hot water for domestic
use and to supplement a fan-coil unit (FCU) that heats
air for space heating. The warmed air is then distributed
through ducts. Integrated gas heaters are inexpensive to
purchase and install. They take up less space and are more
efficient at heating water than conventional systems.
Water Heater Safety
and Energy Efficiency
Either sealed combustion or a power-vented system will
assure safety and energy efficiency in a water heater. In
a sealed combustion system, outside air is fed directly
to the water heater and the combustion gases are vented
directly to the outside. Power-vented equipment can
use house air for combustion, with flue gases vented
by a fan. This is not a safe solution in a tightly sealed
building.
In 1987, the National Appliance Energy Conservation
Act set minimum requirements for water heating
equipment in the United States. Equipment is labeled
with energy conservation information. The U.S. Department
of Energy (DOE) developed standardized energy
factors (EF) as a measure of annual overall efficiency.
Standard gas-fired storage tank water heaters may
receive an EF of 0.60 to 0.64. Gas-fired tankless water
heaters rate up to 0.69 with continuous pilots, and up
to 0.93 with electronic ignition. The 2001 DOE standards
for water heaters will increase efficiency criteria,
and should result in significant utility savings over the
life of gas-fired water heaters and electric water heaters.
Water heaters lose less heat if they are located
in a relatively warm area, so avoid putting the water
heater in an unheated basement. By locating the
water heater centrally, you can cut down on heat lost in
long piping runs to kitchens and bathrooms.
Existing water heaters can be upgraded for improved
efficiency. By installing heat traps on both hot and cold
water lines at a cost of about $30 each, you will save
about $15 to $30 per year in lost heat. The cold water
pipe should be insulated between the tank and the heat
trap. If heat traps are not installed, both hot and cold
pipes should be insulated for several feet near the water
heater.
Low-flow showerheads and faucet aerators save
both heat and water. United States government standards
require that showerheads and faucets use less than
2.5 gpm. Low-flow showerheads come in shower massage
styles. Faucets with aerators are available that use
to 1 gpm. By lowering water temperatures to around
49°C (120°F), you save energy and reduce the risk of
burns.
A relatively inexpensive counterflow heat exchanger
can save up to 50 percent of the energy a home uses to
heat water. It consists of a coil of copper tubing that’s
tightly wrapped around a 76- to 102-mm (3–4-in.) diameter
copper pipe, and installed vertically in the
plumbing system. As waste water flows down through
the vertical pipe section, more than half the water’s heat
energy is transferred through the copper pipe and tubing
to the incoming cold water. There is no pump, no
storage tank, and no electricity used. The counterflow
heat exchanger only works when the drain and supply
lines are being used simultaneously, as when someone
is taking a shower.
Spas and hot tubs must be kept tightly covered and
insulated around the bottom and sides. Waterbeds are
found in up to 20 percent of homes in the United States,
and are sometimes the largest electrical use in the home.
Most waterbeds are heated with electric coils underneath
the bed. Your clients can conserve energy by keeping
a comforter on top, insulating the sides, and putting
the heater on a timer
.
HOT WATER DISTRIBUTION
Hot water is carried through the building by pipes
arranged in distribution trees. When hot water flows
through a single hot water distribution tree, it will cool
off as it gets farther from the hot water heater. To get
hot water at the end of the run, you have to waste the
cooled-off water already in the pipes. With a looped hot
water distribution tree, the water circulates constantly.
There is still some heat loss in the pipes, but less water
has to be run at the fixture before it gets hot. Hot water
is always available at each tap in one to two seconds.
Hot water is circulated by use of the thermosiphon
principle. This is the phenomenon where water expands
and becomes lighter as it is heated. The warmed water
rises to where it is used, then cools and drops back down
to the water heater, leaving no cold water standing in
pipes. Thermosiphon circulation works better the higher
the system goes.
Forced circulation is used in long buildings that are
too low for thermosiphon circulation, and where friction
from long pipe runs slows down the flow. The water
heater and a pump are turned on as needed to keep
water at the desired temperature. It takes five to ten seconds
for water to reach full temperature at the fixture.
Forced circulation is common in large one-story residential,
school, and factory buildings.
Computer controls can save energy in hotels, motels,
apartment houses, and larger commercial buildings.
The computer provides the hottest water temperatures
at the busiest hours. When usage is lower, the
supply temperature is lowered and more hot water is
mixed with less cold water at showers, lavatories, and
sinks. Distributing cooler water to the fixture results in
less heat lost along the pipes. The computer stores and
adjusts a memory of the building’s typical daily use
patterns. Hot water pipes expand. Expansion bends are installed
in long piping runs to accommodate the expansion
of the pipes due to heat.
Where the pipes branch out to a fixture, capped
lengths of vertical pipe about 0.6 meters (2 ft) long provide
expansion chambers to dampen the shock of hot
water expansion. Rechargeable air chambers on branch
lines adjacent to groups of fixtures are designed to deal
with the shock of water expansion. They require service
access to be refilled with air.
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