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Sizing Calculator

Water Heater Sizing

Heating water is the second biggest energy expense in your home.  Knowing your family's first hour water usage will help you get the right size heater. Getting a heater that is too small will create havoc in your household, but getting a heater that is too big will cause you to waste money on the initial purchase and costs to keep all of that water hot 24 hours a day 365 days a year.  A water heaterís first-hour rating (FHR) is the most important feature to consider when purchasing a new heater, since it tells you how much hot water the heater can deliver in an hour of use.

Take a minute to calculate the amount of hot water your household needs on the busiest one hour period of time of any average day.  You can quickly calculate it using the following chart: Figure on roughly 2 gallons for shaving, 4 gallons for washing face and hands, 5 gallons for preparing food, 10 gallons for a dishwasher, and 20 gallons for each 10-minute shower and each load of laundry. Factor in growing children and other issues that can increase your water needs.

Then apply a fudge factor to consider growing children, children that will be leaving home, resale value, and water efficient products you may install in the future. 

Then check the FHR (first hour recovery) rating when considering a replacement heater.



Know your flow Posted: September 2008 ó Consumer Reports Magazine issue: October 2008

If your shower head is relatively new, the gallons-per-minute consumption rating is inscribed somewhere on the outside of the fixture. If it's not, do this quick test.

Calculate your family's first hour water heater rating by  multiplying the average water use (in gallons per minute) for each fixture used during the single busiest hour of water usage in your home.

Any shower head installed after 1992,  probably has the flow rate etched into the showerhead. If you want to determine just how many gallons of water a fixture uses in one minute, you can place an empty gallon milk jug under the faucet spout, turn on the faucet handles open as far as possible, and time the number of seconds it takes to completely fill the tub Divide 60 by the number of seconds it took to completely.  If the jug took 40 seconds to fill, it would be rated at 1.5 gpm.

Make excel spread sheet of chart from Consumer Reports here.

Water Heater Disclaimer: Customer has been advised that the water heater should remain set at 120 degrees as determined by the manufacturer and Gas Appliance Manufacturers Association (GAMA) standards.

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