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SOURCE: Consumer
Energy Information: EREC Reference Briefs
Select a tankless water heater based on the maximum amount
of hot water to meet your peak demand. Use the following assumptions
on water flow for various appliances to find the size of unit
that is right for your purposes:
- Faucets: 0.75 gallons (2.84 liters) to 2.5 gallons (9.46
liters) per minute.
- Low-flow shower heads: 1.2 gallons (4.54 liters) to 2
gallons (7.57 liters) per minute.
- Older standard shower heads: 2.5 gallons (9.46 liters)
to 3.5 gallons (13.25 liters) per minute.
- Clothes washers and dishwashers: 1 gallon (3.79 liters)
to 2 gallons (7.57 liters) per minute.
Unless you know otherwise, assume that the incoming potable
water temperature is 50° F (10° C). You will want your water heated to 120° F (49° C) for most uses, or
140° F (60°
C) for dishwashers without internal heaters. To determine
how much of a temperature rise you need, subtract the incoming
water temperature from the desired output temperature. In
this example, the needed rise is 70°
F (39° C).
List the number of hot water devices you expect to have open
at any one time, and add up their flow rates. This is the
desired flow rate for the tankless water heater. Select a
manufacturer that makes such a unit. Most demand water heaters
are rated for a variety of inlet water temperatures. Choose
the model of tankless water heater that is closest to your
needs.
As an example, assume the following conditions: One hot water
faucet open with a flow rate of 0.75 gallons (2.84 liters)
per minute. One person bathing using a shower head with a
flow rate of 2.5 gallons (9.46 liters) per minute. Add the
two flow rates together. If the inlet water temperature is
50° F (10° C), the needed flow rate through the demand water heater
would need to be no greater than 3.25 gallons (12.3 liters)
per minute. Faster flow rates or cooler inlet temperatures
will reduce the water temperature at the most distant faucet.
Using low-flow shower heads and water-conserving faucets are
a good idea with demand water heaters.
Some types of tankless water heaters are thermostatically
controlled. They can vary their output temperature according
to the water flow rate and the inlet water temperature. This
is useful when using a solar water heater for preheating the
inlet water. If, using the above example, you connect this
same unit to the outlet of a solar system, it only has to
raise the water temperature a few degrees more, if at all,
depending on the amount of solar gain that day.
Cost
Demand water heaters cost more than conventional storage
tank-type units. Small point-of-use heaters that deliver 1
gallon (3.8 liters) to 2 gallons (7.6 liters) per minute sell
for about $200. Larger gas-fired tankless units that deliver
3 gallons (11.4 liters) to 5 gallons (19 liters) per minute
cost $550-$1,000.
The appeal of a tankless water heater is not only the elimination
of the tank standby losses and the resulting lower operating
costs, but also the fact that the heater delivers hot water
continuously. Gas models with a standing (constantly burning)
pilot light, however, offset some of the savings achieved
by the elimination of tank standby losses with the energy
consumed by the pilot light. Moreover, much of the heat produced
by the pilot light of a tank-type water heater heats the water
in the tank; most of this heat is not used productively in
a demand water heater. The exact cost of operating the pilot
light will depend on the design of the heater and price of
gas, but could range from $12 to $20 per year. Ask the manufacturer
of the unit how much gas the pilot light uses for the models
you consider. It is a common practice in Europe to turn off
the pilot light when the unit is not in use.
An alternative to the standing pilot light is an intermittent
ignition device (IID). This resembles the spark ignition device
on some gas kitchen ranges and ovens. Not all demand water
heaters have this electrical device. You should check with
the manufacturer for models that have this feature.
Bibliography
This list does not cover all available sources
of information on tankless water heaters, nor is the mention
of any publication, product, service, or organization to be
considered a recommendation or endorsement.
"Efficiency of Tankless Domestic Water Heaters,"
Energy Design Update,
(7:4) pp. 6-9, April 1988.
Extended Range Tankless Water Heater,
J. Harris, Harmony Thermal Co., 1993. Available from National
Technical Information Service (NTIS), 5285 Port Royal Road,
Springfield, VA 22161; Phone: (800) 553-6847 or (703) 605-6000;
Fax: (703) 605-6900; Email: orders@ntis.gov; World Wide Web: http://www.ntis.gov/. 33 pp., $19.50, Order
Number DE 93013327.
"Going Tankless," P. du Pont, Home Energy, (6:5) pp. 34-37,
September/ October 1989.
"Indirect-Fired Water Heaters," J. Vastyan
and R. McNally, Journal of
Light Construction, (20:8) pp. 85-90, May 2002.
"Instant Hot Water-Maybe," A. Wilson, Journal of
Light Construction, (7:2) pp. 55-56, November 1988.
"Never-Ending Hot Water and Energy Savings,
Too," R. Layne, Popular Science, (228:4) pp.
106-08, 150-51, April 1986.
"On-Demand Water Heaters," J. Wagner, Journal of
Light Construction, (15:4) pp. 51-54, January 1997.
Performance of Instantaneous Gas-Fired
Water Heaters, National Bureau of Standards, 1987. Available
from NTIS, (see above). 66 pp., $27.00, Report
Number PB-87200390.
"Seisco Tankless Electric Water Heater Sets
New Standard," N. Nisson, Energy
Design Update, (17:5) pp. 14-16, May 1997.
"A Tankless Job," Z. Gaulkin, This Old House,
pp. 60-62, June 2001.
"Tankless Water Heaters," Consumer Reports,
(51:1) pp. 53-55, January 1986.
The information in this brief was reviewed
for accuracy and updated in January 2003.
EREC is operated by NCI Information Systems,
Inc. for the National Renewable Energy Laboratory/U.S. Department
of Energy. The content of this brief is based on information
known to EREC at the time of preparation. No recommendation
or endorsement of any non-US Government product or service
is implied if mentioned by EREC.
NOTICE
This report was prepared as an account of work sponsored by
an agency of the United States government. 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, or represents that its use would not infringe privately
owned rights. Reference herein to any specific commercial
product, process, or service by trade name, trademark, manufacturer,
or otherwise does not necessarily constitute or imply its
endorsement, recommendation, or favoring by the United States
government or any agency thereof. The views and opinions of
authors expressed herein do not necessarily state or reflect
those of the United States government or any agency thereof.
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