How to select a tankless water heater

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Selecting a Demand Water Heater:


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 showerheads: 1.2 gallons (4.54 liters) to 2 gallons (7.57 liters) per minute.

Older standard showerheads: 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 the incoming potable water temperature is 50°F (10° C).  You 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.

Example: One hot water faucet open with a flow rate of 0.75 gallons (2.84 liters) per minute.  One person bathing using a showerhead 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 showerheads 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 the same unit to the outlet of a solar system, it only has to raise the water temperature a few degrees more, depending on the amount of solar gain that day.

Cost: 

Demand water heaters cost more than conventional storage tank-type units did.  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 the elimination of the tank standby losses and the resulting lower operating costs, as well as the heater delivers hot water
continuously.  Gas models with a standing (constantly burning) pilot light offset some of the savings achieved by the elimination of tank standby losses because of the energy consumed by the pilot light.  The heat produced by the pilot light of a tank-type system 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.  It 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 are considering.  In Europe, it is a common practice 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 could check with
the manufacturer for models that have this feature.


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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.


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