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Thread: Boiler Pressure

  1. #1
    DIY Junior Member tombstone's Avatar
    Join Date
    Feb 2008
    New York

    Default Boiler Pressure

    Good Morning ...

    On Saturday I finally got around to installing a Runtal basebord radiator as a replacment for a cast iron radiator that was removed for a bathroom remodel a few years ago. I bled everything several times throughout the weekend and am getting good heat from the system but the pressure was at 30 psi this morning when I checked it.

    I have an older expansion tank and am planning on isolating it and draining it today after work. The boiler is in the basement and I have a two story cape cod.

    Is there anything else I should be looking at? A HVAC guy said the autofill seems to be working. Are there any adjustments that can be made in the autofill?

    What should the standing pressure be and what is an acceptable pressure when the boiler is running?

    Thanks for any and all advice.

  2. #2
    In the trades Dana's Avatar
    Join Date
    Jan 2009


    12-15psi would be more "normal", and adequate for 2 stories. For some low-mass boilers it needs to be a bit higher to limit micro-boil sizzle & hiss at the heat exchanger, but 20psi is usually more than enough. If it's running high it could be that the expansion tank has failed or needs the air charge to be adjusted. I should be big enough, since the Runtal most likely has a lower water volume than the cast iron it replaced.

    Most autofill valves have a screw adjustment for setting the pressure but if it's bleeding bu at all it could charge the system to whatever the water pressure is from the street. (I'm not a big fan of autofill valves- it's some thing of a "solution problem". If there's a leak in the system and the pressure goes too low the boiler gives you plenty of sizzle & bang warning well before there's a hazard.)

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