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Convert Gallons to GPM

Question: I am considering replacing my hot water tank with an on-demand tankless water heater. I currently have a 50 gal water tank how do I convert that to GPM (gallons per minute), so that I know what size water heater I need to purchase?

Answer: There is no direct formula for converting the current size of a hot water tank to the required size of an on-demand hot water heater.

The principle problem is that hot water tanks are sold by the size of their storage capacity in gallons. While on-demand hot water heaters are sold based on flow – how many gallons of water passing through the water heater in one minute can it heat.

In a hot water tank once you have consumed the heated water in the tank, for all intent purposes you have to wait for the cold water in the tank to heat up before you will have more hot water.

A tankless hot water system will continually heat a quantity of water up to a certain flow rate. If you turn on all the faucets in your home and exceed the maximum flow rate of the tankless water heater, the water will not be heated to full capacity.

You will have to determine the hot water flow rates at each of your faucets:

  1. Turn the hot water on at each faucet, one at a time to a normal position.
  2. Collect the water in a container for 10 seconds.
  3. Measure the amount of water in the container and convert it to a gallon (it will most likely be a fraction of a gallon).
  4. Multiply the amount of water determined in Step 3 by 6 to obtain GPM (gallons of water per minute).

You can take it one step further. As the maximum number of hot water faucets open at any time is a function of the number of people in the home. You can take the faucets yielding the highest flow rate for the number of people in the home plus 1 and add those together. In other words if there are four people in the home take the five highest flow rates and add them together. If there are only two people in the home take the three highest flow rates and add them together.

Note: The "plus 1" covers washing machines and dishwashers.



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