Demand (Tankless or
Instantaneous) Water Heaters Demand (tankless or instantaneous) water heaters
provide hot water only as it is needed. They don't
produce the standby energy losses associated with
storage water heaters, which can save you money.
Here you'll find basic information about how they
work, whether a demand water heater might be right
for your home, and what criteria to use when
selecting the right model.
How They Work Demand water heaters heat water directly without
the use of a storage tank. Therefore, they avoid the
standby heat losses associated with storage water
heaters. When a hot water tap is turned on, cold
water travels through a pipe into the unit. Either a
gas burner or an electric element heats the water.
As a result, demand water heaters deliver a constant
supply of hot water. You don't need to wait for a
storage tank to fill up with enough hot water.
However, a demand water heater's output limits the
flow rate.
Typically, demand water heaters provide hot water at
a rate of 2–5 gallons (7.6–15.2 liters) per minute.
Gas-fired demand water heaters produce higher flow
rates than electric ones. Sometimes, however, even
the largest, gas-fired model cannot supply enough
hot water for simultaneous, multiple uses in large
households. For example, taking a shower and running
the dishwasher at the same time can stretch a demand
water heater to its limit. To overcome this problem,
you can install two or more demand water heaters,
connected in parallel for simultaneous demands of
hot water. You can also install separate demand
water heaters for appliances—such as a clothes
washer or dishwater—that use a lot of hot water in
your home.
Other applications for demand water heaters
include the following:
•Remote bathrooms or hot tubs
•Booster for appliances, such as dishwashers or
clothes washers
•Booster for a solar water heating system.
Although gas-fired demand water heaters tend to have
higher flow rates than electric ones, they can waste
energy if they have a constantly burning pilot
light. This can sometimes offset the elimination of
standby energy losses when compared to a storage
water heater. In a gas-fired storage water heater,
the pilot light heats the water in the tank so the
energy isn't wasted. The cost of operating a pilot
light in a demand water heater varies from model to
model. Ask the manufacturer how much gas the pilot
light uses for the model you're considering. If you
purchase a model that uses a standing pilot light,
you can always turn it off when it's not in use to
save energy. Also consider models that have an
intermittent ignition device (IID) instead of a
standing pilot light. This device resembles the
spark ignition device on some gas kitchen ranges and
ovens.
For homes that use 41 gallons or less of hot water
daily, demand water heaters can be 24%–34% more
energy efficient than conventional storage tank
water heaters. They can be 8%–14% more energy
efficient for homes that use a lot of hot
water—around 86 gallons per day. You can achieve
even greater energy savings of 27%–50% if you
install a demand water heater at each hot water
outlet.
Selecting a Demand Water Heater Demand water heaters cost more than conventional
storage water heaters. However, you may find that a
demand water heater may have lower operating and
energy costs, which could offset its higher purchase
price.
Before buying a demand water heater, you also need
to consider the following:
•Size
•Fuel type and availability.
•Energy efficiency (energy factor)
•Estimate costs.
For information about specific demand water heater
models, see the Product Information resources listed
on the right side of this page (or below if you've
printed out this page).
Installation and Maintenance
Proper installation and maintenance of your demand
water heater can optimize its energy efficiency.
Proper installation depends on many factors. These
factors include fuel type, climate, local building
code requirements, and safety issues, especially
concerning the combustion of gas-fired water
heaters. Therefore, it's best to have a qualified
plumbing and heating contractor install your demand
water heater. Do the following when selecting a
contractor:
•Request cost estimates in writing
•Ask for references
•Check the company with your local Better Business
Bureau
•See if the company will obtain a local permit if
necessary and understands local building codes, etc.
If you're determined to install your water heater
yourself, first consult the manufacturer.
Manufacturers usually have the necessary
installation and instruction manuals. Also, contact
your city or town for information about obtaining a
permit, if necessary, and about local water heater
installation codes.
Most tankless water heaters have a life expectancy
of more than 20 years. They also have easily
replaceable parts that extend their life by many
more years. In contrast, storage water heaters last
10–15 years. Periodic water heater maintenance can
significantly extend your water heater's life and
minimize loss of efficiency. Read your owner's
manual for specific maintenance recommendations.
Improving Energy Efficiency
After your demand water heater is properly installed
and maintained, try some additional energy-saving
strategies to help lower your water heating bills.
Some energy-saving devices and systems are more
cost-effective to install with the water heater.
Other Water Heater Options
•Conventional storage water heaters
•Heat pump water heaters
•Solar water heaters
•Tankless coil and indirect water heaters.
