Don't Get Cold Feet: Mastering Tankless Water Heater Sizing

Don't Get Cold Feet: Mastering Tankless Water Heater Sizing

Tankless water heaters are gaining popularity for their energy efficiency and space-saving design. But unlike traditional storage models, size truly matters with tankless. A heater that's too small won't keep up with demand, leaving you with lukewarm showers. Conversely, an oversized unit is a waste of money.

So, how do you find the Goldilocks zone of tankless water heater sizing? Here are some tips and tricks to help you out:

Understanding Flow Rate and Temperature Rise

  • Flow Rate: This refers to the number of gallons of hot water your heater can deliver per minute (GPM). Imagine two people showering simultaneously - that's your peak demand. To calculate this, find the GPM of each hot water appliance (showerheads, faucets, dishwashers) and add them together.
  • Temperature Rise: This is the difference between your incoming cold water temperature and your desired hot water temperature. For example, if your incoming water is 50°F and you want it at 120°F, the rise is 70°F.

Key Sizing Considerations:

  • Number of Fixtures: A single person or couple with a basic bathroom might be fine with a 3.5 GPM heater. Families with multiple bathrooms and high hot water usage will need a higher GPM (think 5-7 GPM or even more).
  • Climate: Colder climates require a larger temperature rise, impacting the GPM output. Factor this in, especially if you live in an area with significant seasonal temperature variations.
  • Future Needs: Considering an addition to the family or a hot tub down the line? Opt for a slightly larger heater to accommodate future growth.

Beyond the Basics:

  • Tankless Water Heater Sizing Calculators: Many manufacturers offer online calculators that help estimate your needs based on zip code and fixture selections.
  • Multiple Units: For very high hot water demands, consider installing two tankless heaters in a cascade system.
  • Consult a Professional: A licensed plumber can assess your specific situation and recommend the ideal heater size.


  • Think about peak usage: Size your heater for the scenario where multiple hot water fixtures are running simultaneously.
  • Don't forget temperature limitations: Even a powerful heater might struggle with a large temperature rise at high flow rates. This could result in cooler water at distant faucets.

By following these tips and consulting with a professional if needed, you can ensure your tankless water heater keeps your showers hot and your utility bills under control. Now go forth and conquer hot water worries!

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