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To provide space heating, water is heated by the gas
burner and then pumped through a coil of pipe. Afan blows
air over this heated coil and the warm air produced is distrib-
uted to the house through ducts. The hot water inside the coil
is distributed through a separate piping system to the kitchen,
bathrooms and laundry.
Properly sized, these "combo-heaters" can provide
adequate space and water heating for homes in any part of
the country.
Hydronic or Boiler Systems
Homes that use hot water or steam from a gas boiler
for space-heating can also use a coil in the boiler to produce
hot water for the home. These coils are long pieces of copper
tubing that absorb heat from the boiler water to heat the water
inside the coil. The difference between this system and the
combo-heater is that the hot water or steam from the boiler is circulated through the home to provide heat, rather than
warm air. With this system, the boiler has to be operated all year, which could be somewhat inefficient in summer
months when a boiler large enough to provide space heating is being used only for water heating.
Asimilar system uses the same technology to heat the water, but also includes a separate storage tank to
hold the heated water. These are sometimes called indirect water heaters.
No matter which type of natural gas water heater you chose, be sure the models you are considering are
design-certified by a nationally recognized laboratory that tests to national standards.
Size and Capacity
The best way to determine the size of the equipment you need is the water heater’s "first-hour rating" or
"recovery rate." This is the maximum amount of hot water that can be heated and supplied in one hour, starting with a
cold tank of water. First-hour ratings for natural gas storage water heaters range from 41 to over 131.
Use the chart below to select the first-hour rating that is most appropriate for your home. For example, if your
peak-hour demand for hot water is 70 gallons, you should buy a gas water heater that has a recovery rate of 68 to 72
g a l l o n s .
G e n e r a l l y, natural gas water heaters have higher first-hour recovery rates than electric water heaters with the
same storage capacity, so when you buy a gas water heater by its first-hour recovery rating, its tank size is usually
smaller than a similar electric water heater’s tank. Natural gas storage water heaters range in size from 20 to 80 gal-
lons of capacity.
E f ficiency Ratings
The energy efficiency of a water heater is stated in terms of its "energy factor" or EF. The EF is the ratio of the
energy delivered as hot water compared with the total energy used by the water heater over a typical 24-hour period.
The rating takes into consideration the efficiency of the energy source that heats the water, the unit’s standby energy
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No matter which type of natural
gas water heater you chose,
be sure the models you are
considering are design-certified
by a nationally recognized
laboratory that tests to national
s t a n d a r d s .