When it comes to tankless water heaters, size matters. If you install a unit that’s too small, you won’t have enough hot water to meet demand. On the other hand, an oversized unit will cost more to purchase and operate than necessary, negating one of the biggest benefits of tankless water heaters—energy efficiency!

In short, the tankless water heater size must be just right to meet your expectations. Consider the two factors that go into sizing a tankless unit so you know what to install in your home.


The flow rate is how much hot water the tankless unit can produce, expressed in gallons per minute (gpm). You want the unit you install to provide a high enough flow rate to meet your needs. Keep in mind that tankless water heaters will typically restrict your family to one or two hot water activities at a time, depending on the size you install.

Examples of efficient, low-flow hot water plumbing fixtures include:

  • Bathroom sink faucet – 1.0 gpm
  • Showerhead – 2.5 gpm
  • Tub faucet – 4.0 gpm
  • Kitchen faucet – 1.5 gpm
  • Dishwasher – 1.5 gpm
  • Washing machine – 2.0 gpm

Now, determine how many of these hot water fixtures you expect to use during peak times. For example, if you have two bathrooms and want two people to be able to shower at the same time every morning, you’ll need a flow rate of around 5 gpm, depending on the precise flow of your showerheads.


Knowing the flow rate you want is a good place to start, but it’s not enough. You must also consider the temperature of the water when it goes in and how hot it should be when it comes out. This is known as the temperature rise. Seasonal shifts change the incoming water temperature, but for the purposes of sizing a tankless water heater, you just need to know the average groundwater temperature in your region.


As you shop for tankless water heaters, you’ll see that their sizes are listed as a combination of flow rate and temperature rise. A particular unit may provide a flow rate of 8.0 gpm where the temperature rise is only 30 degrees. That same unit may drop to a flow rate of 5.3 gpm where the temperature rise is 65 degrees.

Clearly, both of these factors are critical considerations when sizing a tankless water heater.


We perform careful calculations to help you achieve a balance between purchase price, performance, and energy efficiency. If you move forward with the tankless water heater installation, rest assured that we are highly qualified for the job. We are fully trained, licensed, certified, and bonded, and our plumbers are background-checked and drug-tested for your peace of mind.

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