Selecting Tankless Water Heaters - How to Select the Correct Size Tankless Water Heater For Your Home.
Selecting tankless hot water heaters isn't very difficult. Select one based on the maximum amount of hot water to meet your peak demand.
Selecting Tankless Water Heaters
Selecting tankless hot water heaters isn't very difficult. Select one based on the maximum amount of hot water to meet your peak demand. Use the following assumptions on 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 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 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 an internal heater. To determine how much of a temperature rise you need, subtract the incoming 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 heater. Selecting a tankless water heater is nearly impossible without knowing the flow rate you need.
As an example, assume the following conditions: One hot 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 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 temperature at the most distant faucet.
Some units can vary their output temperature according to the flow rate and the inlet temperature. This is useful when using a solar 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 temperature a few degrees more, if at all, depending on the amount of solar gain that day.
They also make it take longer for you to get your hot water, since they don't start heating the water until you turn on the faucet. This problem can be solved by using a specialized pump, which in combination with the tankless unit can get your hot water to you at less than half the time it would take running the faucet full blast.
The tankless water heaters also cause an increase in water wastage since you have to let the water run longer to get your hot water. This problem is also solved when using the specialized pumping system. Water conservation is an important advantage to the pumping system.