Tankless Water Heaters: Basic Guidelines For Proper Sizing

Tankless water heaters can provide a nearly endless supply of hot water. Traditional water heaters have large tanks that hold water and keep it at a set, hot temperature. When the tank runs empty, the system is out of hot water until the tank is refilled and reheated. When the tank is not being used, it wastes energy maintaining the temperature of the water. In a tankless system, water is heated only as it is needed.

The best sizing factor for a tankless water heater is its flow per minute rate. A 3.2 gpm, tankless heater can heat 3.2 gallons of water per minute, which is roughly enough to provide hot water to a shower and a sink. To find the correct capacity, add up all of the fixtures that you expect to use at the same time. For example, if you know that two people will be showering while another person runs a sink, you would need a 5.75 gpm unit, based on two 2.5 gallon-per-minute showers and one 0.75 gallon-per-minute faucet.

In addition to flow rate, you should also consider how much heat the tankless heater can put into the water. To find out how much capacity you need, subtract the incoming water temperature from your desired temperature. For instance, if water comes into your house at 60 degrees and you want to heat it to 120 degrees, you will need a tankless water heater that can add 60 degrees. In winter when water temperatures drop, though, your water heater might need to add 75 or 80 degrees to the water to bring it to 120. Be sure that the tankless heater you buy can provide the correct amount of heating at your desired flow rate.

Sizing a tankless water heater is an exacting science. Contact the pros at Sobieski Services for help in choosing the right unit.

Our goal is to help educate our customers in Delaware, Pennsylvania, Maryland and New Jersey about energy and home comfort issues (specific to HVAC & plumbing systems).

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