What Size Tankless Water Heater Do I Need? [Full Guide]

The first thing you need to decide before choosing a tankless water heater is calculating the dimensions of your heater. While your family size calculates traditional tank water heaters, a tankless hot-water heater is calculated by the number and kinds of features or appliances it’ll serve.

The resulting figure is named “GPM” (Gallons Per Minute), or sometimes “flow rate,” which helps you to determine heater size.

Step by Step Guide:

What size tankless water heater do I need Infographic

The best tankless water heaters are rated by the temperature rise possible at a given flow. Therefore, to decide about the size of the water heater, you need to know the flow and temperature rise.

You’ll need this for its application (whole house or just a single-use, like just a bathroom) in your home. It’s important to ensure that you should never attempt to save money by under-sizing your tankless hot-water heater.

Step 1: Identify the Number of Devices

Identify the number of devices you would like to run and their total flow. Then, add up their flow rates. This is the specified flow you will want for the demand hot-water heater.

For example, let’s say you expect to instantaneously run a water heater faucet with a flow of 0.75 gallons per minute and a shower head with a flow of two 0.6 gallons per minute.

The flow through the water heater demand would need to be a minimum of 3.26 gallons per minute. To scale flow rates, install low-flow water fixtures.

Step 2: Determine Required Temperature Rise

To work out temperature rise, subtract the incoming water temperature from the specified output temperature. Unless you identify otherwise, assume that the inward water temperature is 50°F.

For many uses, you’ll want your water heated to around 105–115°. During this example, you’d need a hot-water heater that produces a temperature rise of 55°.

Step 3: Determine Required Size

Sizing Example: the shower will be between 104–106° and uses 2.6 gallons of water. Assuming your water temperature is 40° coming into your home, you would have to supply enough heat to run 2 showers simultaneously. What temperature rise would you like to supply to accomplish this?

Answer: You’ll have to raise the inward water temperature from 45 degrees to 105 degrees. You’ll have to be ready to heat a minimum of 5.2 gallons of water.

This means you’ll need a tankless hot-water heater which will produce a minimum of a 60-degree rise in temperature at 5.2 gallons per minute.

Gas Tankless Hot-Water Heater or Electric Tankless Water Heater?

Electric tankless water heaters can heat only 2 gallons of water to 70 degrees Fahrenheit. In comparison, gas tankless water heaters can heat up to five to six gallons of water up to 70 degrees Fahrenheit in one minute at the same time. We chose 70°F because that’s the specified temperature rise for normal groundwater of 40°F to succeed in a steamy 110°F.

Although the power varies for various manufacturers and models, tankless gas water heaters are more prevailing than their electric competitors. Always check the temperature flow rise and specifications for GPM before purchase.

What Size Tankless Water Heater to Installed by Self?

None, this is often not a DIY activity as you would need the expertise of a trained installer. The installation process is kind of complicated. It involves electrical circuits, gas connections, propane units, etc. In some cases, the shop always recommends a technician.

Which Sizes Last Longer?

Size isn’t the biggest determinant of the tankless water heater. Gas tankless water heaters tend to possess longer lifespans than electrical water heaters.

However, the product and the model brand play as regards the lifespan. Still, gas-fired tankless heaters typically tend to last for over twenty years, while electric water heaters have a 7-10-year lifespan.

What Size Tankless Water Heater Is Simpler to Maintain?

Smaller model sizes would be easier to take care of as compared to the larger models. Regardless, at timely intervals, you’ll need an expert to run some check-ups on your tankless water heater. This includes checking the air vents, checking for leakages, cleaning the burner, etc.

If you reside in a water area, you have to think about using vinegar to flush your tankless water heater every 3 weeks to keep deposits from clogging the warmth exchanger, especially for small sizes.


After you’ve calculated your water flow, required temperature rise for normal usage, make sure you adjust usage so your heater can serve you at its optimal capacity. The size matters a lot.

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