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What Size Tankless Water Heater Do I Need? 3 Things to Consider

Are you considering switching from a tank water heater to a tankless water heater? If so, you’re probably wondering what size you’ll need for your home. One thing that many homeowners don’t realize is that the “sizing” for tankless water heaters doesn’t work the same way as it does for tank heaters. There are a few different factors that you’ll need to consider to determine what tankless water heater size will work best for your household.


1. Flow Rate

If you’re used to tank water heaters, then you probably know that their size is equal to how many gallons of water the tank holds. Tankless water heaters are a little different. These water heaters do not store water. Instead, they supply hot water “on demand.” Rather than gallon capacity, you’ll need to look at a tankless water heater’s flow rate.

A tankless water heater’s flow rate is measured in gallons per minute (GPM). The higher the GPM, the more hot water the fixture can supply at a time to various areas throughout your home.

2. Temperature Rise

The temperature rise is the difference between the cold water entering your water heater and the hot water leaving it. Tankless water heaters must be able to heat water rapidly without storing it or warming it up in advance (like a tank water heater). If you install a model that does not have the temperature rise you need, then it may not heat water enough for your liking—especially during winter.

3. Your Hot Water Usage

Once you understand flow rate and temperature rise, the most important thing to consider is your household’s hot water usage. Consider:

  • Do you typically use hot water from one tap at a time?
  • Do you often need to use hot water at multiple locations in your home simultaneously?
  • Do you like using hot water in your dishwasher and washing machine?

The more places you’re using hot water simultaneously, the more your tankless water heater may struggle to provide hot water to all of those areas at once. For busier households, it can be worth the investment to choose a tankless model with a high flow rate to keep up with your hot water needs.


If you don’t have on-the-job experience, there is definitely room for error when determining the flow rate and temperature rise a tankless water heater should have. Involving a plumber can save you some major headaches that would come with buying and installing an incorrectly-sized model.

Not to mention, a tankless water heater is a 20+ year investment in energy savings: involving an expert is the best way to ensure you get your money’s worth in the long run. Additionally, if you’re considering other home improvements, like a water softening system to extend the life of your tankless water heater, your plumber can work with you on the best way to integrate them.

Looking to install a tankless water heater in Knoxville? Consult our plumbing experts at The Plumbing Authority: (865) 238-2280.

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