How Big Of A Water Heater Do I Need

Updated March 15th, 2020

Here’s how to get the sizing right before you make the purchase.

You need to install a new water heater. It’s a significant expense. You want to get the details right. So how do you know what size water heater to invest in?

Installing a water heater that’s the wrong size for you household can result in a string of unpleasant problems. You could end up never having hot water: it always feels lukewarm, at best. Your overworked water heater can wear through much faster than you expected, resulting in endless bills for parts replacements. Or, you could spend unnecessary amounts of money heating up water that you never use.

So how do you know what size water heater is the perfect fit for your household?

We’re here with a simple, step-by-step guide on how to work out what size water heater you need to service your home. We’ll work through some easy calculations for figuring out your household’s hot water use, and cover what you should look for in both a storage tank and a tankless water heater.

Figuring Out What Size Water Heater Your Household Needs

There’s a wide variety of water heaters available on the market, and each type functions differently. In this article, we’ll cover the two most popular types of water heaters: water heaters with a storage tank, and tankless water heaters.

Calculating What Size Storage Tank Water Heater You Need

    1. Figure out what hour of the day is your busiest hour for using hot water. Each household has that time of day when every person and appliance appears to be using the hot water. It could be in the morning before everyone leaves for school or work, or later on when the kids come home from the sports field. Showers generally use more water than appliances, so keep this in mind when you’re figuring out when your hot water usage peaks.
    2. Now, write a list of all the activities that use water within that hour. You’re going to use this list to figure out your hot water consumption during “hot water hour”. Activities usually include:
      • Shaving
      • Showering
      • Handwashing dishes
      • Food prep
      • Running a dishwasher
      • Running a clothes washer

      In your list, include how many times each activity is performed in this hour. For an example, if you have two bathrooms and you, your spouse and three kids all take a shower first thing in the morning, that could be 5 showers in your peak hot water hour.

    3. Time to figure out your peak hot water consumption. Here’s how many gallons of hot water each activity usually takes:
      • Shaving = 2 gallons
      • Handwashing dishes and food prep = 4 gallons, or 2 gallons per minute
      • Dishwasher = 6 gallons
      • Clothes washer = 7 gallons
      • Showering = 10 gallons

      Multiply each activity’s water consumption by the amount of times that activity is performed in your “hot water hour”. For an example, if your family is taking 5 showers in your peak hour, that’s 50 gallons of water being used. Add it all together. The resulting number is the maximum amount of hot water you need your water heater to be able to produce in an hour.

  1. Find a water heater that has a “First Hour Rating” that matches your peak hot water consumption. A “First Hour Rating”, or FHR, is how many gallons of hot water a water heater can produce in a hour, if it starts off with a full tank of hot water. All storage tank water heaters have an FHR: if you’re shopping in-store, you can find it on the packaging or in the manual, or by chatting to store assistant. If you’re shopping online, you can find it under the item’s “specifications,” “features” or “performance” sections. If you purchase a water heater that can provide just enough water to cover your needs when your household is using the most hot water, you’ll find the right balance between having enough hot water, and not using excess power.

Calculating What Size Tankless Water Heater You Need

  1. Firstly, figure out how many, and which, hot water devices you expect your household to use at any given time. For an example, if your home has two bathrooms with showers and you tend to run the dishwasher and clothes washer simultaneously, while prepping tonight’s supper, your household may end up running hot water through two showers, a dishwasher, a clothes washer, and a prep bowl tap, all at the same time.
  2. Now, you need to calculate each device’s flow rate of gallons per minute, or gpm. Don’t worry: it’s easier than you think. Most appliances that use water include the flow rate in their list of specs, or state how much water they use in a cycle. Even showerheads often come with their flow rate advertised on the packaging. To calculate the flow rate of a tap, you need a bucket and a timer. Run water from the tap for a set amount of time. Then use this convenient online flow rate calculator to determine your tap’s flow rate.
  3. Add the flow rates together. The resulting number is the flow rate you want your tankless water heater to be able to produce. For an example, if you’re simultaneously running two showers at 2 gpm each, a dishwasher at 3 gpm and a clothes washer at 4 gpm, plus a faucet at 2 gpm, that’s 11 gpm altogther.This brings up an important point: around 11 gpm is the highest flow rate you can expect from a gas tankless water heater. If you’re installing a tankless heater, and plan on running a bunch of hot water devices at once, you’ll want to make sure that you use water efficient appliances and showerheads. Either that or establish some household guidelines about who uses hot water when.
  4. Now to find a tankless heater that matches your needs. Tankless water heater gpm changes according to the incoming water’s temperature. It takes longer to heat water that’s coming in at 60 degrees Fahrenheit than water that’s 80 degrees, so the water heater can put out less water per minute in cold conditions. To find your starting water temperature, hold a thermometer under a running cold water faucet in your home. It’s ideal if you can do this in the dead of winter. Then decide what temperature you want your hot water to be. Subtract the incoming cold water temperature from your ideal hot water temperature. This is your water temperature “rise”. Tankless water heaters have charts that show what gpm you can expect from the heater at a particular “rise”. You need to pick a water heater that can give you your desired gpm at your ideal “rise”.

Useful Water Heater Information

If you’re shopping for tankless water heaters, here’s how to know whether an electric or gas water heater will be the best fit for your household.
Here’s a review of our current favorite brand of tankless water heaters.
And here are the very best water heaters Amazon has to offer at the moment.

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