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Hissing steam radiators

kl89 Member Posts: 2
Newbie here. I just moved into a new home built in 1920. Some of the radiators were making a loud hissing noise. After some research I replaced the air vent valves using plumbers tape on the threads. Also elevated the sides with the valves like I read to do. One radiator specifically is still very noisy with hissing sounds after changing the vent valve twice and shimming up one side. The steam comes out of the valve making the hissing and leaving water build up on the valve. Does anyone have any recommendations to help before shelling out money to have someone come look at it. Thanks in advance!


  • EzzyT
    EzzyT Member Posts: 1,290
    Start by checking and seeing what the pressuretrol is set at and turn the pressure down. Check and see if there is adequate main venting if there isn’t then add more. 
    You have pictures you can post of the near boiler piping?
  • Fred
    Fred Member Posts: 8,542
    Also make sure the pigtail (looped pipe) that the Pressuretrol is mounted on isn't clogged. If it is, the Pressuretrol can't see the system Pressure to manage it.
  • kl89
    kl89 Member Posts: 2
    How do I know if the pigtail is clogged? It’s warm to the touch but that’s all I can tell

  • BobC
    BobC Member Posts: 5,473
    What kind of radiator vents are you using and if they are adjustable what are they set to? What pressure does the boiler get to?

    If a vent is loud it usually means the pressure is high. Take the cover off that pressuretrol and you will find a white wheel, the wheel should read one or less. The curly pipe under that pressuretrol (the pigtail) should be cleaned out every year because they can sludge up and then the pressuretrol can't see the pressure inside the boiler.

    You have to take the pressuretrol off to check that pigtail. make sure the boiler is turned off at the breaker panel or fuse box. Unwire the pressuretrol (only 2 wires) and remove the cable so you can spin the pressuretrol off. Use a wrench to loosen the pressuretrol (at the hex bushing) and spin it off the pigtail. Try blowing int the open pigtail, you should be able to blow with just a little resistance. To clean out the piugtail it had to be removed from the boiler, you should be able to untwist it from the boiler but it will take some effort to get it started. Once it's off you can run water through in a sink to flush out and gunk. Also probe around the open tapping with a stiff wire or wire tie on the boiler to see if there is any gunk on the inside of that hole.

    Smith G8-3 with EZ Gas @ 90,000 BTU, Single pipe steam
    Vaporstat with a 12oz cut-out and 4oz cut-in
    3PSI gauge
  • Fred
    Fred Member Posts: 8,542
    kl89 said:

    How do I know if the pigtail is clogged? It’s warm to the touch but that’s all I can tell

    You have to take the Pressuretrol off of the pigtail and try to blow into it. If you can't, it is clogged and you need to either clean it out or replace it. From the dust I see on top of the unit, I suspect it hasn't been touched in a few years and I'd bet money that is your problem.