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Thread: Boiler Pressure & T&P Relief Valve

  1. #16

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    I removed the pressure from the system. Well, mostly. I got sprayed with water when I changed out the automatic air vent on the manifold where the expansion tank resides. With the new air valve in position I checked the air pressure in the expansion tank.

    20 psi. How the heck did that happen?

    With the pressure dropped to 12 psi the boiler is now at 19-20 pounds. I really don't know how that tank was at 20 psi. I checked it before I installed it and it was at 11 psi. I hit it with some air and rechecked it. I thought I saw 13 psi. I must not have had the pressure gauge on correctly.

  2. #17
    Retired Defense Industry Engineer jadnashua's Avatar
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    Given the same volume, the pressure will go up about 1-psi for every 10-degrees temperature rise.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #18

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    Then that would explain the pressure difference.

    I think this is the first time in many years that my boiler wasn't leaking from some where. Thanks for the effort and support.

  4. #19
    DIY Junior Member tray's Avatar
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    A little food for thought, if owning a boiler you should purchase a water pressure gauge that reads up to 200psi with 3/4" threads. With this you can verify water pressure in the boiler and see if press/temp gauge is correct, if the pressure reducing valve is working, and verify house pressure is normal. This could've saved time and headaches .

  5. #20
    In the Trades Tom Sawyer's Avatar
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    200lbs? why?
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

  6. #21
    DIY Junior Member tray's Avatar
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    well for one in rochester, ny thats what is sold and second i have seen bad pressure reducing valves at the house with water pressure nearly 150psi. In that case the valve need to be replaced and water authority brought in, talk bout a real long cold day. So id rather be on the high side for worst case scenarios

  7. #22
    Retired Defense Industry Engineer jadnashua's Avatar
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    I could see possibly on the main water supply, but not on a boiler. Boilers for most residential situations run less than 20#, and the relief valve is often in the 30# range, so one with a scale up to 200# is very hard to read in the normal working range with any accuracy.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  8. #23
    In the Trades Tom Sawyer's Avatar
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    You put more than 30 lbs on the boiler and the relief valve is blowing off. There is absolutely no need for any other pressure gauge than the one installed on the boiler. If it's bad, change it. Jeeze talk about overkill.

    With any boiler system there are only a few things that will cause excess pressure.

    1 - bad or waterlogged expansion tank
    2 - High limit set over 220 degrees
    3 - bad or improperly adjusted auto fill (though it should be shut off after the boiler is pressurized)
    4 - A leak in the domestic hot water coil if the boiler is equipped with one.

    There are no other causes possible. Be sure you gauge is accurate before troubleshooting anything else and it can be checked by simply closing off all water feed sources to the boiler and draining the pressure off. The gauge should go to zero. Close the drain and open the feed. The pressure should rise to between 15 and 18 lbs and stay there.
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

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