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What Size Tankless Water Heater Do I Need?

If you look at tankless water spec sheets, you’ll see terms like GPM and BTU. What do they mean and why are they important? 

So you’re looking to buy a tankless water heater. You’re convinced that you’d enjoy having endless hot water and benefit from the long term cost savings of switching to a tankless water heater. But with so many sizes, choices, and brands, how do you know what size of tankless water heater you need?

If you look at tankless water spec sheets, you’ll see terms like GPM and BTU. What do they mean and why are they important? 

GPM is “Gallons Per Minute” and it’s exactly what you might expect: how many gallons of water can flow through the system in one minute. You’ll also see BTU (British Thermal Unit) which is a measurement of how much energy is required to raise the temperature of your water by 1 degree Fahrenheit. 

A common misconception is that you just need a high flow rate. But to get the endless water you want, you need more than a high GPM. You need to have the thermal capacity (high BTUs) to raise the water temperature at the same time. 

Tankless water heater output is based on Delta temperature, which is the difference between the incoming groundwater temperature and the desired output temperature. The higher you need the temperature to rise, the harder it is for your tankless unit to have a higher flow of water. 

If you live in Minnesota, the groundwater is roughly 40 degrees, so you’ll want the temperature to rise by almost 80 degrees to get the water up to an acceptable average temp of 120 - which is what you have coming out of your traditional tank now. You’ll need a tankless unit with up to 200,000 BTU’s to raise the water temperature 80 degrees at a 5 GPM flow rate. 

How do I calculate my peak hot water demand?

To make sure you have hot water when you need it most, take note of what your average use is per water fixture for the highest demand point in the day. For example, let’s say every morning you have 2 showers running and 1 load of laundry going. Calculate the flow rate of both showers and the laundry, and you have your highest demand point of the day. Here’s an average flow rate table:

  • Shower: 1.5 GMP for low flow rate or 3.0 GPM for high flow
  • Kitchen faucet: 2-3 GPM
  • Bathroom Sink: 0.5 - 1 GPM
  • Washing machine: 2-3 GPM
  • Bathtub: 4-6 GPM

Tip: to get the flow rate for your shower fixture, fill a measuring bowl the size of a gallon with your shower on full blast. Time how quickly it takes to reach 1 gallon, divide that by 60 seconds, and you have your GPM rate. 

In this example, if you use low flow shower heads, your peak use is 5 GPM. But, take into consideration the temperature rise needed of your groundwater. If you want your hot water to be 120 degrees Fahrenheit and you’re starting at 40 degrees from the groundwater, you’ll need a tankless unit that can match it. 

In a Minnesota winter, a tankless unit with 200,000 BTUs should be able to easily supply 3 fixtures simultaneously.

Consult an expert before you buy.

Purchasing a tankless water heater is a long term investment. If you need help to figure out what configuration is right for you, our expert technicians are standing by ready to answer your questions.

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