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Thread: Boiler pressure

  1. #1
    DIY Junior Member swcooper's Avatar
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    Default Boiler pressure

    I have a gas - hot water central heating system, with a regulator from the mains to maintain water level and an expansion tank inline close to the boiler (60k BTU Burnham "Holiday", good few years old). Recently the pressure (there's a gauge on the boiler) has been swinging alot when it is in use - it rises until the red zone on the gauge is reached, and the pressure relief valve kicks in. On the plus side, the relief valve looks beautifully calibrated right at the start of the red part of the gauge (I think it's 30PSI or so, it's also marked in altitude units). But on the downside, there's always a puddle on the floor of the basement when it's cold. This is a new problem this year - any idea what can be causing it? Can't imagine the expansion tank does anything active to go wrong.

    Thanks,
    Simon

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    Well, you'd likely be wrong! One of two things, both related to the expansion tank: it either has a blown bladder, or lost most or all of its air precharge. My guess is that if you knock on it, you'll find it sounds and feels like it is very heavy (i.e., full of water). It should be mostly full of air. They're simple to replace unless they didn't install a shutoff valve, then it gets a little messier.

    They come precharged with around 14-15# from the factory, but it is always good to check them before installation. Fill it carefully with a bike pump or small compressor. Carefully because they don't hold all that much air.

    A quick check is to remove the cap on the schrader vavle and see if water comes out. That is supposed to be the dry side of the air bladder. If no water comes out (it probably won't until you press the valve down inside), then you'd need to check the precharge. Can't do that while there is pressure on the wet side, so you need to shut things down, including the water makeup valve, then drain a little bit out. Check and adjust the pressure.

    My guess, though, is that the bladder is shot. They usually last longer than that, but it happens.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    In the Trades Bob NH's Avatar
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    Default

    You might also have a regulator failure. With the boiler off (not firing) and the supply valve off, let some water out of the system (there is usually a boiler drain valve) just until the pressure drops to zero (don't let air in at the valve). Close the drain valve.

    Knock on or shake the expansion tank. It should be empty. Measure the pressure at the Schrader valve if possible. It should be about 12 to 15 psi. If there is water in the tank and the pressure is low, that is another sign that the bladder has probably failed. Increase the air to about 12 psi and drain again until it stops without letting air into the valve.

    Turn on the supply valve. The pressure should increase to maybe in the range of 7 to 12 psi. If it goes to 20, the regulator is probably shot, or set too high.

    Now turn on the heat. See what happens to the pressure gauge. If the tank is good, it should be OK.

  4. #4
    DIY Junior Member swcooper's Avatar
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    Default

    Thanks for the suggestions. I've studied the expansion tank and associated hardware, but haven't drained the boiler to zero pressure as yet. Doesn't look like I have a convenient shutoff valve for the tank either.

    Noted:
    - Tank doesn't seem to be full, on shaking it a little there's plenty of sloshing and doesn't feel like enough inertia. It doesn't appear to have an air valve anywhere - the only orifice seems to be the top connection into the air scoop.
    - above the tank on the air scoop there's an airlock looking thing on one of the two upper ports that is evidently leaking a little bit and has been for some time - maybe that's just a natural consequence of the system pressure going up.

    I took a few pictures for clarity:
    Tank/airscoop
    The whole lot
    Regulator valve, I believe it's marked 12PSI and there's a shutoff just above it.
    Flow control valve - what's the role of this? Its inscription says to rotate one direction for a gravity system.

  5. #5
    In the Trades mattbee24's Avatar
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    If that relief valve has been popping, you may want to consider changing it soon. Usually after they pop once or twice, they will keep leaking.

  6. #6
    Moderator & Master Plumber hj's Avatar
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    Default tank

    The tank should not "slosh". If the air charge is intact, and the proper amount of water is in the tank, then it is a "compact" unit with nowhere for the water to slosh.

  7. #7
    Retired Defense Industry Engineer jadnashua's Avatar
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    Depending on the design of the tank, the water should be confined - with no movement as HJ said. Think of a water balloon or tank where there is pressurized air on one side, and contained water on the other...no sloshing allowed!

    Replace the tank, double-check and adjust the air as necessary before installation, think about replacing the T&P valve, since once they pop, it is sometimes hard to get them to reseal well again. The tanks are cheap and it sounds like it is the culprit. If it is over 5-years old, I wouldn't think twice about it. They can last a lot longer, but don't always. For most systems, somewhere around 14-15# of pressure is about right.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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