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Thermal Expansion Tank

nycpanycpa Posts: 108Member
Does anyone know if I need a thermal expansion tank for a house that has steam heating?  If so, do I also need to install a backflow preventer before the cold water line meets the thermal expansion tank?  Currently, I use a 50 gallon hot water tank.  Thanks
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Comments

  • Jamie HallJamie Hall Posts: 4,018Member ✭✭✭
    No

    steam heat certainly does not need a thermal expansion tank!  That's hydronic only.  Nor does it need a backflow preventer on the feed lines -- in general.  I can see some overzealous building inspector possibly requiring though, come to think of it.



    What are you using the 50 gallon hot water tank for?
    Jamie



    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.



    Hoffman Equipped System (all original except boiler), Weil-McClain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
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  • nycpanycpa Posts: 108Member
    Expansion Tank

    The 50 gallon hot water heater is what provides the hot water to my house, the typical gas powered tall cylinder.  I asked because I read if you have hydronic heat, then you need a thermal expansion tank but wasn't sure if this included heat expansion coming only from the hot water tank. 

    Is the consensus that steam boiler feed lines do not need their own back flow preventer?  I thought it was common practice to install a backflow preventer to boiler feed lines.  In my case, i would have two backflow preventers, one for steam boiler feed lines, one for the main water line right before the expansion tank. 

    Only a hot water heating system needs a thermal expansion tank?  The hot water produced from the hot water heater will not make the water expand and cause problems? 

    thanks
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  • Jamie HallJamie Hall Posts: 4,018Member ✭✭✭
    Good thinking...

    but not quite!  First, you don't need an expansion tank on domestic hot water because the system isn't closed -- that is, the hot water tank is connected to the water supply at all times, so if expansion does occur (and it does) the pressure doesn't increase.  In principle, a very small amount of water is forced back out of the hot water tank into the rest of the domestic water supply system.



    You do need one on a hydronic heating system, though, because the system is closed -- even if it is connected to your water supply, as most are, the connection is through a pressure reducing valve and a backflow preventer, so when expansion occurs there is nowhere for the water to go, and the pressure rises instead.



    I'm not sure what a consensus would be on backflow preventers on steam boilers.  Certainly on power boilers and many process boilers, you would need a backflow preventer (if they were left connected at all; power boilers usually aren't) as the pressure inside is high.  But for residential boilers, the pressure is very low -- never more than 2 psi -- so unless the domestic water line dropped essentially to a vacuum you couldn't get backflow anyway.  That said, some authorities do require a backflow preventer.  It varies.  It won't hurt anything, anyway, on the boiler.



    If you do indeed have a backflow preventer on your main domestic water line, then yes you would need an expansion tank for the hot water. 
    Jamie



    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.



    Hoffman Equipped System (all original except boiler), Weil-McClain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
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  • Paul48Paul48 Posts: 2,459Member ✭✭✭
    You

    absolutely do need an expansion tank for DHW, if there is a PRV present.
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  • Jamie HallJamie Hall Posts: 4,018Member ✭✭✭
    Quite true

    a pressure reducing valve or a backflow preventer or a check valve -- anything that prevents flow back from the system.
    Jamie



    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.



    Hoffman Equipped System (all original except boiler), Weil-McClain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
    ·
  • ChrisJChrisJ Posts: 3,556Member ✭✭✭
    Backflow preventer

    I believe most areas require a backflow preventer on the water supply to a steam boiler.  In fact, my inspector called me a few hours after he left just to double check that I had one, which I do. 
    Weil-McLain EG-40 connected to 392sqft of radiation via two 2" risers into a 3" drop header and 2" equalizer. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment Typical operating pressure 0.5 - 1.0 inch wc.

    Steam system pictures updated 1/25/15.
    https://picasaweb.google.com/thetube0a3/Boiler?authkey=Gv1sRgCImUxIqv9436MQ#

    Don't push the envelope, eliminate it.
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  • nycpanycpa Posts: 108Member
    Thanks

    Thanks for the advice
    ·
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