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Boiler pressure increase during warmup

Gravityhelp Member, Email Confirmation Posts: 2
My boiler static pressure is 14 lbs. How much should that pressure increase when the water is heated from say 60 degrees to 180 degrees? I suspect something is plugged somewhere... The altitude gauge show an increase to app. 28 lbs. Fortunately the relief valve is not blowing off water. It's 5 degrees outside!


  • Jamie Hall
    Jamie Hall Member Posts: 21,541
    A few pounds is normal. To 28 pounds is not. The expansion tank is supposed to take care of that... so the most likely culprit is that the expansion tank has failed. There is a valve on the end of the tank opposite the inlet; a standard Schrader valve, just like a car tire. If you take the cap off that, and depress it just a bit you should get air, not water. If you get water... problem. If not, find a tire pressure gauge and put that on the valve; it should read -- in your case -- about 14 psi when the boiler is cool. If that isn't true, then you need to isolate the tank and drain it, then drain it, then fill it with air (a bicycle pump will do) to your static pressure -- 14 psi -- and then bring it back on line and see what happens.

    The other obvious thing to check is that if there is a valve between the expansion tank and the boiler piping that it is open.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Gravityhelp
    Gravityhelp Member, Email Confirmation Posts: 2
    Thank you... I have an old fashioned air-cushion expansion tank with an airtrol fitting though.
  • Jamie Hall
    Jamie Hall Member Posts: 21,541
    Well -- that's not a problem (or shouldn't be). In fact, it might be easier -- you need to empty that tank (close the valve to it if there is one) and empty it (there should be a drain) and then close the drain and open it back up to the system. See if it holds its air cushion.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    JUGHNE Member Posts: 10,792
    Sounds like a compression tank.....different animal completely.
    Can you post some pictures of it showing the tank and fittings on the pipes going to it?
    EBEBRATT-Ed Member Posts: 13,810
    edited January 2020

    It's normal for the pressure to rise when the water is heated even if the expansion tank is working properly. Most systems would probably see 21-22 psi when hot. I would consider anything approaching 24 or over to be excessive if the system has a 30 psi rlief valve.
  • hot_rod
    hot_rod Member Posts: 19,921
    rap on the compression tank to tell where the water level is, if it doesn't have a site glass. They do become waterlogged from time to time.

    Make sure that the are not any auto air purgers, they will pull that air bubble out over time.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Gordy
    Gordy Member Posts: 9,537
    edited January 2020
    The compression tank should be about 2/3 full if the right airtrol fitting was installed.

    To make sure you have proper tank level open the brass fitting on the bottom of the airtrol. If no water runs out it’s low on water. If it is low, with brass fitting open fill system until a stream of water runs out freely then close the fitting, and shut off the supply water.

    If it has to much water let the airtrol drain until it sputters into a bucket, then close brass fitting.