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What size tankless water heater do I need? – iWP Foundation

If the water is really cold, you’ll need a more powerful water hater

So you’ve decided to make the switch to a tankless water heater, and we are the iWP Foundation salute you! We’ve put together this simple guide to help you figure out what size tankless water heater you need.

There are 3 primary factors that influence the size, in terms of the flow rate you need for your needs. Flow rates are measured in gallons per minute (GPM).

Incoming Water Temperature

The most important factor in determining the size of your tankless water heater is the incoming water temperature. The colder the water, the bigger the water heater you will need. This map gives you an idea of the incoming water temperatures in your area.

Depending on how much of a temperature rise you need in the winter, you will need to adjust accordingly. Northern states will generally require the largest GPM flow rate available powered by natural gas, whereas Southern states might be able to get away with little electric tankless water heaters.

Peak Demand

Use the following table to determine the peak demand by adding all your demand types together:

Demand TypeSize
Bathroom Sink0.5 GPM
Standard Shower2.5 GPM
Low-Flow Shower1.5 GPM
Dishwasher1.0 GPM
Washing Machine1.5 GPM
Kitchen Sink0.5 GPM
Laundry Sink1.0 GPM

It’s important to bear in mind that you can easily make house hold policies to not run multiple water sources at the same time in order to save on the size of water heater you need. In addition, if your incoming water temperature is above 67F, you can get away with slightly less since you don’t need as much of a temperature rise.

Dwelling Size

The size of your house and the amount of people who live there make a big impact on the hot water demand. We’ve put together an easy table to help guide the size, for approximation purposes.

Apartment / Condo13.0 GPM
Small House (<1000 sq. ft.)14.0 GPM
Average House (<1600 sq. ft)27.5 GPM
Large House (1600+ sq. ft)3+10+ GPM
A Large house will require multiple water heaters.

Power Source

For most homes in the United States, there will be natural gas available directly to the home. Natural gas is both cheaper and more efficient. If you are looking to provide hot water for an entire home, you should go with a natural gas powered water heater.

If natural gas isn’t an option, you could go with electric, but you need to bear in mind that you will have to purchase multiple units. Additionally, if your incoming water temperature is cold and you don’t have access to natural gas, we would recommend that you stick with a traditional hot water tank.

Water Hardness

All tankless water heaters will require maintenance at some frequency, depending on how hard the water is. While not directly related to size, it’s an important factor to consider when deciding to switch to a tankless water heater.

You can see a map of water hardness across the United States here.

If your incoming water is especially hard, you want want to consider an infrared electric tankless water heater, which doesn’t build up calcium and limescale.

How to Size a Tankless Water Heater

After considering the incoming water temperature, your peak demand needs, the water hardness and the power source you have available to you, you can make an informed decision about what size tankless water heater you need.

If you need help selecting a tankless water heater, please see our comprehensive buyer guide.


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