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  • domestic HWH/HWT expansion tanks

    okay, i gotta ask.

    i understand the therory behind the expansion tank for heating systems, but what's up with putting them on DHW?

    we don't use them up here without any problems.

    is there something i'm missing?

    Vince

  • #2
    Re: domestic HWH/HWT expansion tanks

    If there is a check valve or some form of backflow device installed on a domestic system (commonly found on re circ lines) the system becomes in essence, a closed system. Meaning, water may enter at the check valve, but may not leave through it. This causes a problem when dealing with the expansion properties of water when it is heated. In an open system , when water is heated, it may travel back through the system, and this allows the water pressure to remain the same. but when a check valve is in place the expanding water has no where to go, which causes a pressure spike, an often times the T and P valve on a HWT will go off. Installing an expansion tank gives the water somewhere to go when it expands.
    West Trail Mechanical Ltd
    Service. Commitment. Expertise.

    www.westtrailmechanical.ca

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    • #3
      Re: domestic HWH/HWT expansion tanks

      I should add that this is my understanding of it, I'm sure others will have stuff to add
      West Trail Mechanical Ltd
      Service. Commitment. Expertise.

      www.westtrailmechanical.ca

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      • #4
        Re: domestic HWH/HWT expansion tanks

        Water expands as its heated, and therefore it needs a place to go. This usually means it gets pushed back into the cold side. If you have higher water pressure, even if its regulated by prv's the check valves in some of them prevent the pathway for the expasion to go. This puts extra pressure in the system and your t&p valve is going to go bonkers. The expasion tank added to the system temporary takes the expasion between hot water useage. Darn I type too slow.........

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        • #5
          Re: domestic HWH/HWT expansion tanks

          i forget about the PRV all the time.

          we are at sea level here and basically the town is the same elevation throughout.

          the water main pressure is about 50 psi.

          is the check valve on the cold water another form of backflow prevention?

          Vince

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          • #6
            Re: domestic HWH/HWT expansion tanks

            is the check valve on the cold water another form of backflow prevention?
            Yes. And as such would require an expansion tank if the system is connected to a water heater.

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            • #7
              Re: domestic HWH/HWT expansion tanks

              thanks guys.

              makes sense.

              Vince

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              • #8
                Re: domestic HWH/HWT expansion tanks

                Originally posted by Vince the Plumber View Post
                i forget about the PRV all the time.

                we are at sea level here and basically the town is the same elevation throughout.

                the water main pressure is about 50 psi.

                is the check valve on the cold water another form of backflow prevention?

                Vince
                Here we are required to use pressure regulators with a bypass for thermal expansion. Because we do not have dual checks at the meters, when the thermal expansion causes the water pressure to rise it is pushed back into the City line. If the City pressure is greater than the T&P relief valve pressure (150 psi) we have to install an expansion tank to absorb the expansion.

                Mark
                "Somewhere a Village is Missing Twelve Idiots!" - Casey Anthony

                I never lost a cent on the jobs I didn't get!

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                • #9
                  Re: domestic HWH/HWT expansion tanks

                  "If the City pressure is greater than the T&P relief valve pressure (150 psi) we have to install an expansion tank to absorb the expansion."



                  That's when it gets tricky. City pressure may be below 70psi during the daytime when you test the line and then it can shoot up over 100 during low use in the middle of the night. That's when a gauge with a lazy hand or some local knowledge comes in handy.

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