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Never had such high pressure in my boiler system before. Any comments?

I have a 100 year old house w one pipe steam system. 13 radiators in total. One particular radiator I had not been able to get enough heat in it so I took off the vent and for 2 years I got enough heat out of it, never a problem.
But recently when it was 36 degrees outside and at the end of the 'big morning burn' from 66 to 70 degrees, I was halfway down the stairs I heard a rushing sound, so loud I could hear it through a closed door. I went to the radiator and there was a very strong stream of air coming out the vent hole. Like if you pursed your lips and tried to blow air as hard as you could. But even harder than that. There were no other reportable issues with the other radiators at this time.
So such pressure, it had to be systemic pressure right? The pressure control device must not be working, the one on the boiler right? I had a service guy come and replace the pressure loop and do a general maintenance.
I also converted the '4 degree big morning burn' into two separate 2 degree burns. There no longer seems to be an issue. I hope this issue is solved...


  • mattmia2
    mattmia2 Member Posts: 9,669
    edited November 2022
    Is one or more of the main vents clogged or stuck closed?

    Maybe whatever was clogging the pipe to the radiator broke loose or a pocket of water evaporated.

    Also, put a vent on the radiator, if the system builds pressure and it starts venting steam you will lose water and constantly adding fresh water will kill your boiler.
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,852
    like so many, if removing a vent solves a problem, then why try to find the real cause of the problem.

    Was the professional aware of your "removed vent" solution to your insufficient heating problem? If so, did they have any insight regarding that problem?

    Removing that vent will allow the air out so the steam can get in. (air and steam don't mix so air in the radiator = no steam and no steam = no heat). the problem is that when the steam gets to the vent location, there is nothing to keep the steam in. you see, the air vent only vents air, it closes when the steam gets there. If there is no air vent, just a hole, the the steam can get out.

    That room may have a higher humidity than the rest of the house as a result of letting the steam out of that radiator. I guess that is OK if the higher humidity is not too high that it causes problems. Time will tell.

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

  • dabrakeman
    dabrakeman Member Posts: 552
    Put a vent on the radiator. Slow down some of the radiators that are heating quicker than this one with smaller orifice vents (or use adjustable vents).
  • davevarga
    davevarga Member Posts: 44
    Thank you all for your feedback. There are three mains in this system. This radiator is on a main changed 2 years ago. Never the less I need to check it and the other two main vents.
    I will put in a varivalve on this radiator, open to the max. I will start with this. Again, thank you...
    EBEBRATT-Ed Member Posts: 15,543
    You should never run the boiler with an open vent especially up in the house. If the pressure control failed or got plugged or someone overfilled the system, you could have a mess on your hands. The safety valve would protect the boiler but not the water/steam in the house
  • random12345
    random12345 Member Posts: 469
    @davevarga You should read this document on how to balance steam systems:


    Simply putting a bigger vent on that radiator may not fix your problem. The mains get vented before the radiator runouts (pipes carrying steam to the rads) even start to fill. The goal is to vent the mains as fast as possible. That means putting big main vents on the ends of the mains. Your best bet is in my opinion a combination of B&J Big Mouth vents and Gorton #2.

    In order to balance the system as a whole, you need to consider how far each rad is from the boiler, the air volume of the radiator and the runouts, and the order of the runouts from the mains. For example, in my house there is only one main. The first rad to get filled is supplied by the first runout from the main following the direction of steam from boiler->main vent, but the 2nd, 3rd, and 4th runouts from the main actually supply the rads on the second floor. So when I was balancing my system, the second floor rads would always heat up before the first floor rads. The second floor rooms were too hot and the first floor too cold. So I had to put smaller vents on the second floor rads to slow down the venting. It's all covered in that document. Also, if you haven't changed your rad vents in a few years, that might be worth doing. I recommend Maid O' Mist 0220-5L to experiment. With the amount of pressure you're describing, it's also possible/likely your boiler is significantly oversized relative to the installed radiation.