Tankless Sizing – Total GPM

    Buying a tankless system can be a tricky task as there is a number of different units on the market today. In addition you need to take into account numerous different factors to determine the correct sizing and venting.

    You will first want to figure out what the total GPM that you need supplied at your house, do this to calculate the total.

    If your showerhead 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.

    Hold a 1-gallon container under a faucet or fixture and time how many seconds it takes to fill the container with the faucet opened all the way. Then divide 60 by the number of seconds it took to fill the container. For example, a fixture that filled the container in 30 seconds would be rated at 2 gpm.

    While you’re at it, it’s a good idea to clean any screens in the fixtures of any scale deposit.

    The maximum gpm rating your water heater needs depends on how many fixtures and appliances will be drawing hot water at the same time. Add up the gpm of your fixtures and then check off the fixtures and appliances that will be in use simultaneously. This should provide you with the Total Gallons per Minute that a tankless will need to provide.

    Now the last thing to consider is your location, if you are in the southern US the incoming water supply will be much warmer than in the dead of winter in southern Ontario. This plays a significant factor as the tankless unit needs to heat the water almost twice the amount, inturn reducing the output. A tankless rated at 6.8 gpm may only be able to produce 4.8 when it has to heat the water from 38 Fahrenheit instead of 64.

    Of course if this is too complicated you can always have a professional come out for a free quote and have them size one out for you. Click here if you are interested.


    Author: John

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