Before you can start shopping for a tankless water heater, you need to determine what size unit you need. This is a very important step! Unlike a tank style water heater that is limited by how much water it can store in its tank, tankless water heaters are limited by their flow rate. That is, the amount of water that can flow through the unit. Tankless water heaters heat up water instantly as it flows through, but the amount of water that flows through varies by unit.
You should never try to save money by undersizing a tankless water heater. This will only lead to wasted money and a very disappointing experience when you try to take a shower!
If you don’t want to get into the nitty gritty of the math, you can call a licensed plumber to determine what size tankless water heater you need. If you want to determine your tankless water heater size on your own, keep reading below.
How to Size a Tankless Water Heater by Flow Rate
Step 1
Determine the Fixtures You Will Run at the Same Time
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.
Step 2
Add Up The Flow Rates
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.
Step 3
Determine Your Temperature Rise
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 further north in a colder climate, you will need a higher powered tankless water heater.
Step 4
Shop For Your 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.
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