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Radiator squeals and vibrates after stem valve is shut

I was referred to this site by Reddit and this is my first post so hopefully I'm doing this right.

Anyway, I'm visiting some friends in Manhattan, and I'm staying in the living room of their 5th floor (it's also the top floor, probably built pre-war) walk-up apartment. It's atrociously hot in here, so we got permission from the superintendent to turn off the radiator by closing the stem valve. While the room has indeed become cooler, the radiator has occasionally stated making a horrifyingly loud sound from time to time akin to a loud, buzzing squeal of varying pitches that lasts for several minutes.

I've observed a few things:
1. The sound is accompanied by a relatively high frequency but fairly low amplitude vibration in the the radiator itself.

2. This vibration seems to be the worst around the trap in the lower-right corner of the radiator.

3. I was listening for the steam cycle to begin, felt around a noted: the return pipe & trap were cold, the radiator was cold, and the steam pipe was hot. About 90 seconds later the sound started and I observed that while the radiator was still cold and the steam pipe was still hot, the return line and the trap were both just as hot as the steam pipe.

4. The noise only started after we turned the radiator off and it had cooled down.


Trap & Return:

Steam pipe & Stem valve:

Everything I was able to dig up here or online has to do with knocking in the radiator or related pipes, so my search was fruitless. Does anyone know what's causing this or what can be done about it? My initial inkling was that it's some sort of thermal difference between the bulk of the radiator and the trap somehow related to the boiler's steam cycle and there is naught to be done. Is there such a thing as steam backflow? And considering how dangerous steam can be should I just re-open the stem-valve?

For good measure here is the response I got on Reddit, courtesy of user NorthernUrban
"Steam systems are built and balanced so that all radiators are running, when you take one out of the equation you un-balance it and you can get interesting results. On top of that, the valve you closed might not completely seal allowing some steam to get in and if it's traveling through a small hole into the radiator it might make the noise you describe. You mentioned this is the 5th floor rad, is this the top floor? Typically the top radiators in my house heat first since hot air rises, so turning this one off will give a more dramatic effect than closing one on a lower level. One thing you might want to do is check the pitch of the radiator, is it draining towards the lower pipe? Set a level on top of it and see. Water trapped inside the radiator mixing with steam will give you water hammer.
You can check out heatinghelp.com, they are pretty knowledgeable about steam but I'm not sure there's much to do here except open that radiator back up."

Thanks guys!


  • nicholas bonham-carter
    If you do open up the valve, what happens? As a temporary expedient, drape some blankets, or old rugs over the radiator to reduce its output.
    This looks like a system problem, caused by poor maintenance. Normally, there should not be back flow, unless some of the traps are inoperative, which is the case here. The pressure has been allowed to elevate beyond the mere ounces required, costing everyone in that building at least 50% more in fuel, and shortening the life of the boiler.
    Is this a rental, condo, or coop building?
    Talk to the management about this, and have them come here for advice, before it is too late.--NBC
  • Jamie Hall
    Jamie Hall Member Posts: 23,446
    Gotta love some of those old walk ups!

    Try opening the valve -- not a lot, but some. You may be able to find a setting which keeps just enough pressure in the radiator to make it shut up -- but not so much as to bake you out.

    It sounds to me as though there are two problems (at least) -- the system pressure is too high and there is at least one bad trap somewhere else on the same return as your noisy radiator.

    Not much that you can do about either one... except maybe persuade your super to also enquire here about the system. He may be wasting a lot of money with a system which needs some maintenance
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • els2155
    els2155 Member Posts: 2
    Thanks for your responses!

    I'll refer my friends to the super; I was reading up on steam traps and saw they should be replaced or at least inspected after 5-6 years and...well lets just say that the screws on the radiator cover had about 3 layers of paint on them, so it's safe to say they have not been maintained.
  • vaporvac
    vaporvac Member Posts: 1,520
    I would also point out that one can shut off rads in a two-pipe system such as this, but not a one-pipe.
    Two-pipe Trane vaporvacuum system; 1466 edr
    Twinned, staged Slantfin TR50s piped into 4" header with Riello G400 burners; 240K lead, 200K lag Btus. Controlled by Taco Relay and Honeywell RTH6580WF