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How to Size a Hot Water Tank

Sizing a hot water tank properly will ensure you won't be stuck in a cold shower and that you keep your utility bills down. Hot water tanks work by heating the water in the tank and keeping it warm until you need to use it. When you use hot water, it is taken out of the top of the tank and cool water enters the bottom of the tank to be heated. Hot water heaters are sized by their first hour rating which takes into account the capacity of the hot water tank and how quickly it can replace used water.

  1. Determine which hour during the day your household uses the most hot water. Remember to take into account all hot water usage, such as food preparation and laundry, when making your estimate.

  2. Determine how much water you use in your bathroom during that hour. According to the U.S. Department of Energy, a shower takes about 12 gallons, baths take about 9 gallons and shampooing your hair takes an additional 4 gallons. Each time you wash your hands or face you use about 4 gallons and each shave takes an average of 2 gallons.

  3. Estimate the water usage of your kitchen during the busiest hour of the day. The U.S. Department of Energy estimates that food prep takes about 5 gallons. Hand-washing dishes takes an average of 4 gallons and automatic dishwashers take about 14 gallons of hot water.

  4. Estimate the total hot water usage of your laundry room during the busiest hour. You can check how much hot water your washer model uses, but the Department of Energy estimates it is between 26 and 32 gallons per load.

  5. Add the totals from Steps 2 through 4 to get your first hour rating. For example, if you use 11 gallons in the bathroom, 5 gallons in the kitchen and 28 gallons for laundry, you would need a tank with a first hour rating of 44 gallons.

About the Author

Mark Kennan is a writer based in the Kansas City area, specializing in personal finance and business topics. He has been writing since 2009 and has been published by "Quicken," "TurboTax," and "The Motley Fool."