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If you’re in the market for home upgrades or renovations, you’re probably wondering, “What size tankless water heater do I need?” To answer this question, there are a variety of things to consider, like the climate you live in and how many people are in the home. Read on to learn more about tankless water heater sizes, how to calculate tankless water heater capacity, and what tankless water heater size will fit your needs.

When Sizing a Tankless Water Heater, Consider This

There is no one-size-fits-all water heating solution. The best tankless water heater for your home depends on many factors, including:

  • Size of your home.
  • Climate in your area.
  • Groundwater temperature.
  • How many appliances will be running at once.
  • How many people are in the home.

Getting the right size tankless water heater for your home is important. Choosing one that’s too small could leave you without enough cold water. One that’s too large will cost more than you need to spend.

How to Size a Tankless Water Heater for Your Home

To find the right size tankless water heater for your home, you’ll need to determine what the peak hot water flow demand for your home is, and your required water temperature rise for the incoming water.

The factors you need to calculate are Flow Rate (GPM) and Temperature Rise. When you’re shopping around, each tankless water heater will list the maximum GPM Flow Rate at different required temperature rises.

Calculating Tankless Water Heater Capacity

It’s not difficult to calculate the Flow Rate (GPM) and Temperature Rise for your home. It involves simple addition and subtraction.

First, add up the GPM for all the major appliances (or fixtures) that you expect may be used at the same time. For example, during peak use in your house, you may be running the dishwasher, sink faucet, and shower all at the same time. Add up the different Flow Rates of those appliances/fixtures. A chart of typical Flow Rates (GPM) is below.

  • Sink faucet—1 GPM
  • Bathtub—3 GPM
  • Shower—2.5 GPM
  • Dishwasher—3 GPM
  • Clothes washer—3 GPM

Using the chart above, running the dishwasher, sink faucet, and shower at the same time adds up to 6.5 GPM (3 + 1 + 2.5 GPM).

Then, you’re ready to calculate Temperature Rise — or, how cold the incoming water is for your home compared with how hot you need it. Reference the map of average groundwater temperatures below. If you live in Iowa, your groundwater temperature is 40–50 degrees Fahrenheit. For most uses, you’ll want your water heated to around 100–115 degrees Fahrenheit. Subtracting your desired temperature from your groundwater temperature, your required Temperature Rise is 60–65 degrees Fahrenheit.

Check the product details for the tankless water heater you’re considering. It will list the maximum GPM Flow Rate at different required Temperature Rises. You want a water heater with a Flow Rate at 60–65 degrees Fahrenheit of at least 6.5 GPM. If the listed GPMs are less than 6.5, it may be too small for your home, and you won’t have enough hot water at peak use times.

As another example, say you plan to run the clothes washer and a sink faucet at the same time. Using the chart above, your GPM is 4 (1 + 3). If you’re located in Florida, your minimum required Temperature Rise is 35 degrees Fahrenheit. You need a tankless water heater sized for a Flow Rate of 4 GPM at 35 degrees Fahrenheit.

What Size Tankless Water Heater Do I Need to Replace a 50 Gallon Storage Tank?

You may have a 50-gallon storage tank water heater and wonder what size of tankless water heater is right for you. Tankless water heaters heat water on demand, rather than storing the water for when you need it. If you need a lot of hot water (for multiple showers, for example), you’ll want to make sure you have enough hot water to go around — in other words, that your tankless water heater is powerful enough. If you live in the northern region of the U.S., you should begin by browsing for a 10 GPM propane tankless water heater. If you live in the southern U.S., begin by browsing for propane tankless water heaters with a GPM of 7.

What Size Tankless Water Heater do I Need for a Family of Five?

The size of the tankless water heater you would need for a family of five depends largely on your location. In a northern state, your tankless water heater will need to heat water more powerfully than if you live in a southern state, where the groundwater is already warmer. If you have a family of five in one household, you could potentially have a high hot water demand, so you’ll want to be sure your tankless water heater can keep up. In general, look for a tankless water heater of 10 GPM, if you live in a northern region of the U.S. If you live in the south, look for a tankless unit of about 7 GPM.

Learn more about propane-powered water heaters here.