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How to Size a Tankless Water Heater


A tankless water heater will provide an endless stream of hot water. Photo: Dreamstime

When sizing a tankless water heater, you need to think in terms of flow, not capacity. A tankless water heater doesn’t run out of hot water like a storage water heater can, but it may not be able to heat water fast enough to serve multiple fixtures at once.

A tankless water heater is given BTU (British Thermal Unit) input and efficiency ratings. These determine its flow rate, expressed in gallons per minute (GPM).

One BTU is equal to the amount of energy required to raise the temperature of 1 pound of water 1 degree F. The higher a water heater’s BTU rating, the higher the flow rate. Under normal circumstances, it takes about 31,000 BTUs to deliver 1.2 GPM; 190,000 BTUs deliver 5.7 GPM.

Flow rates vary from about 1.2 to 6 GPM. Point-of-use models, such as those that provide hot water to a single bathroom sink, are rated 1.2 GPM. A 2.6-GPM tankless water heater will handle one shower at a time, a 4-GPM model one shower and one sink, and a 6-GPM two showers.

Homeowners who are concerned about energy savings also need to pay attention to a unit’s efficiency ratings. These ratings, which range from about 78 percent to 87 percent, indicate how much of the fuel is converted to heat by the appliance—the higher the better. You can shop for tankless water heaters online at Amazon.com.

NEXT SEE: Tankless Water Heater Buying Guide

About Don Vandervort
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Don Vandervort developed his expertise more than 30 years ago as Building Editor for both Sunset Books and Home Magazine. He has written more than 30 home improvement books and countless magazine articles. He appeared regularly on HGTV’s “The Fix,” and served as MSN’s home expert. Don founded HomeTips in 1996.

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