Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Too much pressure!

Too often those older gravity systems, with large pipes and water volumes, were fitted with a #30 expansion tank because, "that is what new boilers of the size I put in use"....

I tend to install two or three such tanks, (two #30's being less money usually than a single #60) and have some redundancy too.

Regardless, you should attempt to calculate your water volume for a proper selection.


  • Kevin_67
    Kevin_67 Member Posts: 2
    help! too much pressure!

    This summer my wife and I bought a house that has a 70 year old hot water / boiler heating system. I have never owned a house with a boiler before, so my knowledge is very limited.

    When the boiler is not heating, the pressure is at 12 psi, but as soon as we turn it on, it does not stop rising. I understand it should ideally remain between 12 and 20 psi, but after an hour of heating, the pressure is past 30 psi and rising. The safety / relief valve kicks in and starts spewing boiling water on the floor.

    The boiler is gas powered, has a circulation pump, is a closed system and has an expansion tank which seems to be working (we drained it once to see if that was the problem, but no luck). I have tried turning off the feeder pipe, but that does not seem to make a difference either. All the radiators have also been bled. They do get hot when the boiler is running, and the thermostat seems to be working.

    Currently, we are only running the boiler for about 45 minutes at time, before we have to turn the thermostat down to switch the boiler off. I doubt that this temporary solution will be very practical during the quickly approaching Canadian winter. We live in Manitoba, and -30 temperatures are not to be toyed with...

    Any advice would be greatly appreciated! My knowledge of plumbing and heating is limited, but i'd rather not pay big bucks to have this thing fixed if there is a simple solution.

  • Timco
    Timco Member Posts: 3,040

    Your expansion tank is either undersized, or more likely just plain bad. Cheap enough to just replace it...

    Just a guy running some pipes.
  • Steamhead (in transit)
    Steamhead (in transit) Member Posts: 6,688
    Please Describe

    Exactly what you did and Exactly what happened when you "drained" your expansion tank.

    To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"
  • Craig_8
    Craig_8 Member Posts: 33

    are you sure you got all the water out of the tank when you drained it? If you don't let air in, all the water won't come out. It can seem empty evan when its completely full.
  • Kevin_67
    Kevin_67 Member Posts: 2

    Thanks for all the advice! Turns out the tank wasn't completely empty, as Craig suggested. I opened another valve and suddenly another 10 gallons of water came out.

    Now it seems to be running fine. Thanks for saving me a fist full of money!
  • ChasMan
    ChasMan Member Posts: 462
    Bigger Is Better?

    I calculated my water volume and have a #30. My pressure moves from 10 static to 20 hot. If I put a bigger one, will I get less of a rise as it heats?
  • Brad White_203
    Brad White_203 Member Posts: 506

    The larger the air cushion, the less expansion pressure increase you will have. Air being compressible, the more you have, the more you have to compress.

    More "cushion for the pushin" as some might say. :)

    How much of a rise you get, well, that is an individual thing.
  • ChasMan
    ChasMan Member Posts: 462

    Ha, Ok, it made sense to me but you know hydronics and science go together much better than hydronics and random thoughts. I think I may change my little 30 to one of those stand up jobs. I don’t like the pressure so high if it isn’t needed.
  • Brad White_203
    Brad White_203 Member Posts: 506
    Science, Hydronics and Random Thoughts

    Since when are ANY of these mutually exclusive :)

    May I reinforce my suggestion that instead of tossing the 30, you can add one or two more for less than the cost of a larger stand-up sized one and have redundancy to boot.

    That is my sort of "default" approach to these larger systems, not that there is anything wrong with a single large tank.

    In my commercial and institutional specification work, those TACO ASME-stamped tanks are things of beauty to behold. Just too many stickers all over them, that's all :)
  • Joannie_15
    Joannie_15 Member Posts: 115
    Yeah, but

    We NASCAR fans find beauty in all those stickers.
  • Brad White_203
    Brad White_203 Member Posts: 506
    \"Whom among us....

    "is not a NASCAR Fan?"

    John F. Kerry; D-France-2004

    When I see my Taco CBX making the final lap at 120 MPH, I too will be impressed.
This discussion has been closed.