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 typically 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.

Why we like it: • 199,000 BTU output but very compact • Electronic ignition • Can be converted to direct-vent

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.