Thermal expansion queston

Discussion in 'Plumbing Forum, Professional & DIY Advice, Tips & ' started by DIY, Mar 12, 2010.

  1. DIY

    DIY New Member

    Messages:
    153
    Location:
    Florida
    Hi everyone,
    What is ,and what does a thermal expansion valve do in a plumbing system? (water plumbing system that is hot/cold etc.)

    To all thank you
  2. Gary Swart

    Gary Swart In the Trades

    Messages:
    7,328
    Location:
    Yakima WA
    To be proper, it is not a valve. Here's what they do. When you have a PRV or other check valve between your water heater and the city water main, it creates what is termed a closed system. When the water heater heats, the water in the tank expands and needs someplace to go. If the system is an open system, this expansion is absorbed by the city main with no problems. However, with the closed system this can not occur and the rapid rise in pressure occurs in the tank. This causes the T/P valve on the tank to open to relieve that pressure and prevent the tank of exploding. When the pressure is reduced, the T/P should close, but doesn't always. The thermal expansion tank, just like it's name implies, absorbs this expansion. It is placed in the cold water supply between the water heater and PRV.
  3. FloridaOrange

    FloridaOrange Plumbing Designer

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    1,317
    Location:
    SW Florida
    Last edited: Mar 12, 2010
  4. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,485
    Location:
    Cave Creek, Arizona
    The link does not work, but any "expansion valve" would be a version of pressure relief valve, which would merely duplicate the function of the heater's T&P relief valve.
  5. FloridaOrange

    FloridaOrange Plumbing Designer

    Messages:
    1,317
    Location:
    SW Florida
    The link works for me, anyway it's a PDF to some watt's thermal expansion "products". Yes it does act as a T&P but it satisfies the code requirement for thermal expansion control. We typically use tanks but there have been a few times where there was no room for a tank (under cabinet water heater, etc).
  6. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,485
    Location:
    Cave Creek, Arizona
    IF a relief valve satisfies the requirement for expansion control, and it would not here, then unless it is set at less than 150 psi, all it is doing is duplicating the T&P valve which would be a waste of time and money.
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