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Boiler pressure

bhop2920bhop2920 Member Posts: 8
Twin boilers, two circs, 1 zone, 1 circ fist stage, 1 circ 2nd stage, 40 psi relief valve on both boilers. 4 floors of heating w 2 indoor sensor on each floor which is averaged w 1 t stat on first floor boiler in basement, circs on return when boiler is at 130 degrees pressure is at about 12 psi and when at 160 is about 25 psi. For some reason when a-stat set for cold start relief blows off when it makes temp at 180 when I have it set for 160-180 relief doesn't. There's no real problem I'm just curious if anyone had any ideas why this is so.

Thanks in advance.


  • 40 psi Relief valve blowing

    Maybe your expansion tank is waterlogged with a bad diaphragm, or too small.--NBC
  • earl burnermannearl burnermann Member Posts: 126
    edited December 2013
    expansion tank may be failing.

    sorry, double posted.
    If the women don't find you handsome, they should at least find you handy!
  • earl burnermannearl burnermann Member Posts: 126
    expansion tank may be failing.

    Probably have spin-on expansion tank, or tanks to create an air cushion for the heated water. As the water in the system expands due to heat, the air behind the rubber diaphragm compresses and allows the water to enter the tank and keeps the boiler from flooding and dumping into the room. You need 1 psi of water pressure for every 2.31 feet the water has to travel up, plus 4 psi extra. That's what the fill valve should be set to  when cold. What ever pressure that comes out to should be the pressure your expansion tank is charged to when installed.
    If the women don't find you handsome, they should at least find you handy!
  • bhop2920bhop2920 Member Posts: 8
    Expansion tank

    Yea I know to drain expansion tank but I found it strange that when boiler a-stat is set for 130-180 relief valve blows off but when I have it at 160-180 it doesn't. Was just curious if this made any sense to anyone.
  • Jamie HallJamie Hall Member Posts: 6,708
    It does seem a little odd...

    but I can think of at least one possibility.  If the expansion tank is really waterlogged or failed, when the system cools to 130 the water contracts and the pressure drops -- if the pressure drops enough, it could even pull a vacuum.  Either way, air -- or water if there is a feed from the domestic line attached -- will get in there and replace the volume.  Now when it heats up, it all tries to expand and there's nowhere for it to go, so poof goes the relief valve.  But if it only has cooled to say 180 or so it doesn't contract as much, so it the pressure doesn't drop to pull in a little water or air...


    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-McClain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • icesailoricesailor Member Posts: 7,265
    Expansion Tanks.

    Sounds like it might be failing and needs to be replaced with one considerably larger if it is a cold start and 4 floors.

    Most expansion tanks are way undersized especially when used for a cold start system.
  • Paul48Paul48 Member Posts: 4,257
    Couple Thoughts

    It does sound like you are walking the tight-rope with expansion capacity, and 12 psi is not enough pressure to keep 4 floors full, to the highest point of radiation.
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