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What Size Water Heater Do I Need?

If you’re looking to buy a new water heater, choosing the right size makes all the difference when it comes to your comfort and utility bills.

Go too small and you’ll run out of hot water before the end of your showers. Go too big and you’ll pay more for hot water you’ll never use.

So how do you ensure you’re getting the right size water heater?

Well, it depends on the type of water heater you are purchasing: tank or tankless.

Tank and tankless water heaters are measured differently. Read on for step-by-step guides on how to choose the right size for both tank and tankless water heaters.

Sizing a tank water heater

To determine the correct size tank water heater you only need to look at 2 numbers:

  1. Your peak hour demand (how much hot water your household uses during a busy hour)
  2. The tank’s FHR (how many gallons of hot water a heater can produce at any given hour)

The tanks FHR or “1st-hour rating” will be listed on the yellow EnergyGuide label. Be sure that your tank’s FHR falls within 1-3 gallons of your peak hour demand.


Photo Courtesy: Care2

Your peak hour demand can be calculated using the chart and steps below:

peak hour demand

Step 1: Pick an hour of the day that your house uses the most hot water. Determine how many activities in the chart above will be performed during that one hour.

Hint: If 2 showers are running at the same time be sure to multiply the average gallons for that activity by 2 (20x2=40).

Step 2: Add up the avg. gallons of hot water that will be used during that hour. This is your household’s peak hour demand.

peak hour demand

Example of a typical peak hour of demand for a family of three.

Step 3: Choose a tank with an FHR that is close to your peak hour demand figure. For example, the family referenced in the chart above should look for a tank water heater with an FHR of around 70.

Sizing a tankless water heater

Tankless water heaters are sized differently than tank water heaters because they work differently. Instead of heating a large amount of water that is readily available at all times, they heat incoming water only as you need it.

Tankless water heaters are measured in two ways:

  1. Flow rate (how many gallons of hot water per minute you expect to use at any one time)
  2. Temperature rise (how many degrees the incoming water needs to be heated before it’s used)

Calculate your flow rate and temperature rise using the chart and directions below:

water flow rate

Step 1: Select the hot water appliances that you expect to ever run at the same time.

Step 2: Add those appliances’ average flow (gallons per minute) together. This is your home’s flow rate.

Step 3: Determine the maximum water temperature you’ll want to use and subtract the incoming water temperature. In Minnesota, the incoming water temperature is around 40 degrees Fahrenheit.

Example: Let’s say you expect to use your dishwasher, kitchen sink and shower at the same time. Your flow rate would then be 4-5 GPM.

Now let’s say that the highest temperature water you’ll need is 120 degrees for your washing machine. Subtract the incoming water temperature from this and you get your temperature rise (120 - 40 = 80).

So, you would need a tankless water heater that provides at least an 80 degree rise in temperature at up to 5 gallons of hot water per minute.

NOTE: A temperature rise of 80 degrees at a flow rate of 5 GPM is at the high end of most residential tankless water heaters. To get a more affordable tankless water heater, you may need to not run as many hot water-using appliances at once.

Need help sizing and installing your water heater?

Live in the Minneapolis-St. Paul area and have more questions about your water heater? For professional advice and an easy installation process, contact MSP Plumbing today.