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hissing vents at low pressure

Hi, I am a homeowner with a single pipe steam system. The system seems to maintain a set temperature without too much trouble, but if it has to make up more than a degree, I was getting hissing/fizzing out of most of the air vents after I hear them close. I had Vent-Rite #1s in all the rads. First I replaced all the vents with Hoffman 1A vents, but no change and they fizzed as well. I was worried that it was not shutting down on pressure and the pressuretrol was pretty old (P47AA-25). I replaced the pigtail (the old one was clean though) and the pressuretrol with a PA404A1033 and added a low pressure gauge to find out what's going on. Now, I can see that when the gauge reads ~0.3 psi, the vents start to hiss and fizz. So, the pressuretrol isn't coming close to cutting out on pressure. If anyone has ideas as to what might be going on, I would appreciate it.


  • TylerH
    TylerH Member Posts: 1
    ...I forgot to mention that the rads are fully hot by the time they start fizzing.
  • AdmiralYoda
    AdmiralYoda Member Posts: 627
    Sounds like normal operation to me. Setbacks can be tricky with steam systems as the radiators will be full of steam and as hot as they can get even while the thermostat is still calling for the boiler to continue to run.

    This will build pressure and could lead to short cycling, or at least a slight increase in pressure if your boiler is properly sized.

    Is water coming out of the vents or just a little hissing?
  • Jamie Hall
    Jamie Hall Member Posts: 23,275
    The pressure gauge reads - 0.3 psi? Negative pressure? Something doesn't add up there.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • TylerH
    TylerH Member Posts: 1
    Thanks for responding. Sorry, I used a tilde (~). I meant to imply 'approximately 0.3 psi'. Pressure stays below 0.5 psi. Once the rads are hot, I do see some bubbling coming out of the top of at least one of the vents, but they sound like they are fizzing with moisture. To AdmiralYoda's point, yes the rads are hot and the thermostat continues to call for heat, but the pressure stays low and it doesn't cut off. It's not short cycling and it continues to fire until the thermostat is satisfied. I'm confused as to why the air vents don't seal so the system builds some pressure to cut out (if I understand things correctly)? With the Hoffman 1As, the 'clink' as they close is distinct, so I know they close, but I'm not sure why they wouldn't seat properly?
  • ethicalpaul
    ethicalpaul Member Posts: 5,701
    Makes me wonder how wet your steam is/what your near-boiler piping looks like
    NJ Steam Homeowner. See my sight glass boiler videos: https://bit.ly/3sZW1el
  • neilc
    neilc Member Posts: 2,703
    I had suggested to another poster, different thread, same issue, that boiling their vents in vinegar for 1/2 hour or so would clean them,
    and they had success,
    or you need to replace the fizzy ones,

    post a picture of your boiler, floor to ceiling, showing your header,
    known to beat dead horses