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High Boiler Pressure

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dpasta
dpasta Member Posts: 5
Hello,

Just bought an older home (1906) with a very old boiler system. Having issues with high pressure I can't seem to alleviate. I'm getting a slow drip out of the relief valve when the boiler temp hits 170, and psi sits at about 28-30.

I've tried draining the expansion tank, but everytime I reopen the expansion valve, the water line seems to automatically refill what I've drained.

Any recommendations?

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  • Jamie Hall
    Jamie Hall Member Posts: 23,526
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    What type of "expansion tank" are we talking about here? Is it the newer bladder or diaphragm tank, hooked up somewhere on the piping -- it should be just before the main circulating pump, but probably isn't -- or is it the older style compression tank -- a big, cylindrical tank hung from the joists above the boiler, or even sometimes in the attic?

    The two have very different ways of making sure they are properly charged.

    Picture?
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • dpasta
    dpasta Member Posts: 5
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    It is the older style compression tank mounted to the joists above the boiler system.
  • dpasta
    dpasta Member Posts: 5
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  • Jamie Hall
    Jamie Hall Member Posts: 23,526
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    OK. Those are easy to manage -- if they are piped correctly. The procedure is to valve it off from the system and drain it completely. Close the drain (and any air release if there is one on the tank) and reopen the valve to the system. Add water to the system to bring the pressure up to where you want it. Done. The tank should refill about half way -- possibly a little more.

    If it fills all the way, there is an air leak in the tank or an air release valve which isn't closed.

    Now note that if some more recent plumber has installed an air removal device anywhere else on the system, that will defeat the tank which will waterlog very quickly. I don't see one in the photos, but... worth checking.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • dpasta
    dpasta Member Posts: 5
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    The water seems to keep refilling the same amount drained. It'll stop at about 10psi, but then at full temp it creeps up.

    Should I shut off the water line before it refills fully?

    I appreciate the help so far!
  • Jamie Hall
    Jamie Hall Member Posts: 23,526
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    No -- seems like you're fine. It will refill about the total amount you drain, if the tank had air in it to begin with. That doesn't mean the tank is full -- just that it has taken in enough water to compress the air to the system pressure.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    dpasta
  • dpasta
    dpasta Member Posts: 5
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    Thanks for the help Jamie. I think I narrowed down the problem. There is no air inlet in the tank. I only got 5 gallons to drain with the drain valve open, but blowing into a hose drained another 10 gallons and some really nasty gunk. Assuming maybe an air pocket or something. She seems to be humming along now at 20psi and 170 see degrees.

    Thanks again!