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You must know how much hot water you are going to use ar one time and you must know the lowest possible temperature of your incoming potable water temperature in order to properly select a tankless on demand water heater. Tankless water heaters are designed to supply a certain amount of water and they can supply more hot water depending on the incoming water temperature. As such, homowners in southern states are able to size a smaller tankless water heater than a homeowner in an northern state because the incoming temperature to the tankless water heater would be higher.
In order to select a tankless water heater based on the maximum amount of hot water to meet your peak demand, you shoudl use these assumptions. We reccomend that you check with the manufacturer of the tankless water heater to make sure they do not have any additional assumptions for your to consider. 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.
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