Ask an Expert: Sizing a Tankless Water Heater

How do I correctly size a tankless water heater?

In terms of sizing a water heater, the tankless type is sold using maximum gallons per minute (GPM) flow at a given temperature rise. First, list the number of hot water devices you expect to have open at any one time and add up their flow rates. An experienced plumber can provide an accurate estimate, but here are some average figures:

• Shower: 2.5 GPM
• Clothes washer: 3.3 GPM
• Dishwasher: 1.3 GPM

The total expected GPM draw is the desired flow rate for the water heater. Next, match that with the temperature rise provided by the water heater at that flow rate.

Unless you know otherwise, assume that the incoming water temperature is 55°F. You'll want water heated to 120°F for most uses. To determine how much of a temperature rise you need, subtract the supply temperature from the desired exiting water temperature. In the following example, the needed rise is 65°F.

Exit temperature - Supply Temperature = Temperature Rise (120°F - 55°F = 65°F)

Most tankless water heaters are rated for a variety of flow rates at different water temperature rises. By estimating the total flow rate in your home with your expected temperature rise, you can get a good idea of what size unit you'll need. Many tankless unit manufacturers provide online sizing charts that can help you get a more detailed estimate.

Pensacola Energy offers rebates up to $1,200 when you switch to natural gas tankless water heating. To learn more, visit pensacolaenergy.com or call 850-436-5050.