How to Size a Tankless Water Heater

By The Tankless Expert | Last Updated: 11 October 2023

Now that you decided to ditch the 'ol tank and install a tankless water heater, you're probably questioning, "what size water heater do I need?" Finding the size tankless water heater that is suitable for your needs is as easy as 1-2-3!

Here's some helpful lingo:

Flow Rate - the amount of hot water that comes out of the tankless water heater.
Gallons per Minute (GPM) - the amount of hot water the tankless water heater produces per minute. Or the measurement of flow rate.

Pounds per Square Inch (PSI) - the water pressure or the amount of pressure needed to force the water through the pipes. The higher the PSI, the higher the GPM.

Delta T - the change in temperature.

Temperature Rise - the temperature difference between incoming ground water temperature and the source for hot water.
Tools you'll need -

Bucket
Thermometer

Now, let's begin!

Ground Water Temperature:

Desired Temperature at Showerhead = Temperature Rise

What temperature rise do you require? Measure the ground temperature, or the temperature of your incoming water as it enters the house. To do this, take a bucket and fill it with water from the cold water tap. We recommend doing this during the coldest months. Measure the water with a thermometer. In warmer climates, the ground temperature can range from 65F -- 90F. In colder climates, it can get as low as 35F. Now determine how hot you'd prefer the temperature to be as it comes out of the showerhead or faucet. Anything above 120F is scalding.
Calculate peak demand and GPM. How many simultaneous uses do you run throughout the house? This is the flow rate, or the maximum demand. Now add the total GPM per simultaneous use:

Filling a standard bathtub uses ~ 3.0 GPM
Running a dishwasher uses ~ 2.0 GPM
Filling a clothes washer uses ~2.0 GPM

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