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Tankless WH - Expansion tank?

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ChrisJ
ChrisJ Member Posts: 15,915
I'm curious, is an expansion tank required with a tankless water heater when a RPZ is used?

If not required, is it beneficial?
Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment

Comments

  • pecmsg
    pecmsg Member Posts: 4,955
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    Doesn't hurt.
    SuperTech
  • hot_rod
    hot_rod Member Posts: 22,472
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    I think all the plumbing codes require a thermal expansion “device” on water heaters

    A small tank like this works for tankless and combis with small water content 
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    GGross
  • ethicalpaul
    ethicalpaul Member Posts: 5,792
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    My municipality doesn't require a backflow preventer on the supply into the house. The city water system is my expansion device.

    NJ Steam Homeowner.
    Free NJ and remote steam advice: https://heatinghelp.com/find-a-contractor/detail/new-jersey-steam-help/
    See my sight glass boiler videos: https://bit.ly/3sZW1el

  • ChrisJ
    ChrisJ Member Posts: 15,915
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    My municipality doesn't require a backflow preventer on the supply into the house. The city water system is my expansion device.

    Ok.......

    :D
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
    SuperTech
  • ethicalpaul
    ethicalpaul Member Posts: 5,792
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    Laugh if you want :smiley:, but I believe this is commonplace!


    NJ Steam Homeowner.
    Free NJ and remote steam advice: https://heatinghelp.com/find-a-contractor/detail/new-jersey-steam-help/
    See my sight glass boiler videos: https://bit.ly/3sZW1el

    SuperTech
  • ChrisJ
    ChrisJ Member Posts: 15,915
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    Time to dig out the Good Grief meme again.

    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • Larry Weingarten
    Larry Weingarten Member Posts: 3,388
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    Hi, Thinking about the physics going on with a tankless around a hot water draw and afterwards, I think it would be a fun project to actually test pressure just before, during and for about 1/2 hour after the draw in a "closed" system. If anything, the pressure might drop as plumbing and heater cool down. In that case, more water would enter the system. I'm not seeing a way for high pressure to develop or become a problem. I almost never see expansion tanks used with tankless and never hear of over-pressure problems. Maybe I live in Shangri La? :)

    Yours, Larry
  • Jamie Hall
    Jamie Hall Member Posts: 23,640
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    Hi, Thinking about the physics going on with a tankless around a hot water draw and afterwards, I think it would be a fun project to actually test pressure just before, during and for about 1/2 hour after the draw in a "closed" system. If anything, the pressure might drop as plumbing and heater cool down. In that case, more water would enter the system. I'm not seeing a way for high pressure to develop or become a problem. I almost never see expansion tanks used with tankless and never hear of over-pressure problems. Maybe I live in Shangri La? :)

    Yours, Larry

    I tend to agree with you, @Larry Weingarten -- but the question isn't really the physics, it's the code and authority having jurisdiction. Isn't it always?
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • leonz
    leonz Member Posts: 1,180
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    I think the new 2023 very expensive Bradford White propane water heater I have has a check valve in it already to prevent ghost flow
  • ChrisJ
    ChrisJ Member Posts: 15,915
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    leonz said:

    I think the new 2023 very expensive Bradford White propane water heater I have has a check valve in it already to prevent ghost flow


    This is about expanding with temperature, not ghost flow.

    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • leonz
    leonz Member Posts: 1,180
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    I understand that, I mentioned the check valve in my Bradford White Water heater simply because the water flow stops there when the demand stops.
    ChrisJ
  • ChrisJ
    ChrisJ Member Posts: 15,915
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    leonz said:

    I understand that, I mentioned the check valve in my Bradford White Water heater simply because the water flow stops there when the demand stops.

    Very good point.....
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment