Tankless Water Heater Sizing Guide

Determine what water fixtures you are likely to run at the same time. If you have a large family, you might need to run two showers at the same time. If you often do laundry at night, you might need to run the washing machine and shower at the same time.

Add up the flow rates of the fixtures you want to be sure you can run simultaneously. Here is a chart of common flow rates:

You should be realistic at this step. You don’t want to undersize your unit and run out of hot water, but you also don’t want to spend extra money on a high flow rate that you’ll never need.

Look at the map below to find your average groundwater temperature. If you are on the edge of two areas, go with the lower temperature to be safe. The average groundwater temperature tells you how hard your tankless water heater is going to need to work to heat your water. If you live in a warmer climate, such as Austin or San Antonio, you might not need as high powered of a tankless water heater. If you live further north in a colder climate, you will need a higher powered tankless water heater.

Most manufacturers provide a chart for each model that shows the maximum flow rate it can provide at each groundwater temperature. You’ll first find your groundwater temperature on the chart and then go across to the maximum flow rate. You must be sure the flow rate is at or higher than the rate you determined in step 2.

That is it! If you have any questions, please feel free to contact Reliant Plumbing.