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Thread: High pressure in the system

  1. #1
    DIY Junior Member den_lab's Avatar
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    Default High pressure in the system

    Hello,

    I have a three zone gas water heating system. Two zones provide hot water for baseboards. Third zone is for hot water in house.
    After cold season started I began to see water on the floor. I found it is coming out of pressure relief valve.
    I can see that air pressure gets to 30-32 psi when couple of zones running. That is were water would flush out of the relief valve.
    When system is off, the pressure was around 20 psi. I thought it is high and that is why it gets to 30 when it is running.
    So, I adjusted the Fill Valve. And the pressure went to 15 psi. But still when running it gets to 30 psi and water leaks from relief valve.
    Then I checked the expansion tank. It had no pressure. So, I used air pump to get it to 15 psi.
    And I get the same result, 15 psi when nothing runs, get to 30 psi when running.

    Any advice on what else I need to check or could be wrong with the system?
    Thank you

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    The only way to check pressure in the expansion tank is when there's no water pressure in the system. If the tank is shot, when the water heats, it can raise the pressure enough to cause the pressure relief valve to open. The only other reasons for the pressure to rise that I can think of are: there's a hole in the heat exchanger of the indirect allowing water to pressurized the boiler circuit, or, the autofill valve is defective and letting water in regardless. If the high temp limit is not operating, you could be getting some steam generated, but that would (or should) shut the whole thing down. Because steam expands radically, the pressure will rise, but it has to be quite hot with that pressure for that to happen, and isn't likely.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    DIY Junior Member den_lab's Avatar
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    Quote Originally Posted by jadnashua View Post
    The only way to check pressure in the expansion tank is when there's no water pressure in the system. If the tank is shot, when the water heats, it can raise the pressure enough to cause the pressure relief valve to open. The only other reasons for the pressure to rise that I can think of are: there's a hole in the heat exchanger of the indirect allowing water to pressurized the boiler circuit, or, the autofill valve is defective and letting water in regardless. If the high temp limit is not operating, you could be getting some steam generated, but that would (or should) shut the whole thing down. Because steam expands radically, the pressure will rise, but it has to be quite hot with that pressure for that to happen, and isn't likely.
    Thanks for the reply
    I checked the pressure in the expansion tank, after I shutdown the system and made sure pressure was 0. I pressed the tip of the air valve on the tank and no air was coming out. So, I used air pump and air pump showed o psi from expansion tank. I added air and it was then reading around 15 psi. I have not checked it since.
    So, if expansion tank does not hold the air pressure, that will cause system pressure to go up while boiler is running?

    Would it make sense, if I say, air pressure rise, while heating zone, that supply water for baseboards, is on (the pump is working). But when heating zone, that supply hot water for shower and so on, air pressure stays the same?
    Last edited by den_lab; 12-10-2012 at 09:18 AM.

  4. #4
    Retired Defense Industry Engineer jadnashua's Avatar
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    The simple act of heating water causes it to expand. Since your pipes are pretty robust, they generally don't swell much under the available water pressure (the relief valve after all is set to 30psi, so that's not enough pressure to deform the pipes). The act of compressing the stored air in the expansion tank will raise its pressure slightly, but since the air is readily compressable, with the volume changes we're talking about here, that pressure change is small (you're likely only adding a pint or so of water from expansion).

    You need to drain some water, or at least have the drain valve open when adding air to the expansion tank - you need to inflate the bladder with nothing pressing in on the other side or the pressure will go up almost instantly, and there'll be no air cushion to absorb the expansion.

    If you didn't get any water out of the air valve, the tank will probably be okay once it is properly filled. Once properly filled and you repressurize the water in the boiler, the tank should still be mostly full of air and sound hollow when tapped. If it thuds when you tap it, it either isn't filled properly or the bladder is shot and needs to be replaced.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  5. #5
    In the Trades Tom Sawyer's Avatar
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    Change the expansion tank. It's got a ruptured bladder
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

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    DIY Junior Member den_lab's Avatar
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    Thank you both for replyes
    Expansion tank is on its way
    I will update once installed

  7. #7
    DIY Junior Member den_lab's Avatar
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    Quote Originally Posted by den_lab View Post
    Thank you both for replyes
    Expansion tank is on its way
    I will update once installed
    OK, new expansion tank is in place. No water leaks anymore and contained air pressure in the system.
    Thank you all for your help

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