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gravity hot water systems

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Is there anyone out there old enough to helpus on this problem-we've got a gravity hw system that has just started overpressurizing and opening the relief valve. There doesn't appear to be leak-by in the closed boiler feed valve. There is a conventional type 30 gallon expansion tank in the basement, which we have isolated and drained down, only to fire the burner up and have the presure build right back up until the relief valve blows again. Remember there is no auto feeder. Can anyone out there help us
Thanks
adavis

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  • alan  davis
    alan davis Member Posts: 2
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    trouble with a gravity hw system

    Is there anyone out there old enough to helpus on this problem-we've got a gravity hw system that has just started overpressurizing and opening the relief valve. There doesn't appear to be leak-by in the closed boiler feed valve. There is a conventional type 30 gallon expansion tank in the basement, which we have isolated and drained down, only to fire the burner up and have the presure build right back up until the relief valve blows again. Remember there is no auto feeder. Can anyone out there help us
    Thanks
    adavis
  • Brad White_9
    Brad White_9 Member Posts: 2,440
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    I am 48 years old...

    is that old enough to qualify?

    :)

    Things to check (dumb questions but bear with me)

    1. When you isolated and drained down the expansion tank did you open the valve again?

    2. Is there an automatic vent on the tank (sometimes installed to regulate the air charge). Should be isolated and may be venting your air cushion.

    3. What pressure are you experiencing and how high (tall) is the house? I assume 30 psi is the relief valve but might it be leaking by below that?

    4. Is there still an attic-mounted expansion tank? Curious. Thinking it might have a feed valve of its own and yet have its vent valved off too. Saw that once on a 1928 house. It was forgotten.

    Anyway, need more information from you. Off to play shuffleboard and get some Geritol and Depends...

    :)
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928
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    42 years with the heart of a Dead Man.

    True gravity systems usually don't have a compression tank in the basement--they have an expansion tank in the attic. That attic tank also has an outlet--typically out the roof but sometimes routed back down into the basement. The ONLY pressure in such a system is from the weight of the water as it is open to the atmosphere.

    If there's a pump in the system it's a converted gravity system.

    As mentioned previously the valve leading to the tank MUST be open when the system is operating!!! Ideally that tank has a 50/50 mix of air/water when the system is cold. As the system heats water expands into the tank (the ONLY place it can expand to) and compresses the air.

    If the compression tank in the basement is full of water the ONLY way I know to remedy the situation is to drain the system below the level of the tank and re-fill. And no, it's NEVER fun to drain and re-fill a converted gravity system. 2-3 people and cell phones make it relatively easy...

    Plain compression tanks on gravity conversion systems tend to fill with water over time. You can STOP THIS by installing a Bell & Gossett Air Trol fitting on the tank. Follow the filling instructions to the letter. The instructions regarding disabling the circulator and heating the boiler to 220F or so don't seem truly applicable to gravity conversions.
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