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.

radiator knocking


I have recently moved into the top floor apartment in an old brownstone. My apartment has two radiators which I believe are steam powered. they have one pipe going into them and have outlet valves that hiss. They are tipped on the far end to angle towards the valve and heat the room well.

One of these radiators emits intermittent knocks, every few minutes, at the beginning of the heating cycle. It's as if someone is hitting it with a hammer. Once the hissing starts the knocks subside. It's a pain because it is hard to sleep through.

Does anyone have any ideas what can be done to resolve this? I also tried switching off the radiator while sleeping but the knocks still occur. It almost sounds like switching off the radiator doesn't work as I still hear stuff going on inside the radiator though don't get any heat from it during that time.

Is this something that could be resolved or does one just have to live with this sort of thing?




  • nicholas bonham-carter
    whole lotta knockin

    this is undoubtedly a symptom of deferred maintenance of the whole steam system. i would guess the pressure is too high, and the main [not radiator] vents are inadequate, but at least the fuel company is happy with the excess fuel being used!!

    until the system can have some service to rectify these problems, you can turn off the radiator by rotating the radiator air vent so that it points down. just make sure that you do not keep rotating it in the same lefty-loosey way so it becomes unscrewed!

    suggest to your building manager a trip to this site for advice on making this system quiet, comfortable, and economical.--nbc
  • gerry gill
    gerry gill Member Posts: 3,078
    Its possible

    that your hearing knocking that sounds like its coming from the radiator but really may be coming from a sag in the pipe leading to the radiator..but first make sure the radiator inlet valve is fully open.
    Serving Cleveland's eastern suburbs from Cleveland Heights down to Cuyahoga Falls.

  • Hap_Hazzard
    Hap_Hazzard Member Posts: 2,846
    Expansion noise

    What you're describing sounds like thermal expansion noise. Most radiators are held together by push nipples, which look like short, slightly tapered pieces of pipe that connect each radiator section to the next. Because they're made of mild steel and the radiator sections are cast iron, they expand at slightly different rates, and if they become loose, the expansion can force them out of their sockets, causing the sections to separate slightly, increasing the tension on the truss rods, causing them to vibrate.

    The key in your description is that, when the valves start hissing (which they shouldn't do, by the way) the radiator has reached its working temperature.

    When the radiator cools the process reverses, and you may hear creaking noises, but not the sharp "ping!" that you hear while it's heating.

    Unfortunately, there is nothing you can do to stop this short of taking the radiator apart and replacing all the loose, worn push nipples. I'm planning to rebuild a couple of my radiators over the summer, but this is not a job for the faint hearted. Cast iron radiators are very heavy, and moving them--especially negotiating stairways--can be dangerous. Then the radiator has to be disassembled, and the push nipples inspected, and any that need replacement need to be sent to the only company in the US that makes replacements.

    A stop-gap measure would be to swap your radiator for a quieter one of the same size, if you happen to have one in another room in your house.
    Just another DIYer | King of Prussia, PA
    1983(?) Peerless G-561-W-S | 3" drop header, CG400-1090, VXT-24
  • Charlie from wmass
    Charlie from wmass Member Posts: 4,322
    try plastic shims

    under the legs furthest from the radiator.
    Cost is what you spend , value is what you get.

    cell # 413-841-6726
  • cjyabraham
    cjyabraham Member Posts: 2

    Hi All,

    Thank you all for your advice. Here is an update on the situation.

    Last night I tried turning the outlet air valve upside down. It didn't have the effect of shutting off the radiator. In the morning it was hissing and the radiator was hot. It is this kind of valve: http://goo.gl/Aenkk and is brand new with no water in it.

    Previously when I tried turning off the inlet valve completely, the radiator would stay cold. However, I could still hear the knocks and a quiet sound that made it sound like steam was still getting into the radiator. Now I'm starting to think that steam wasn't in fact getting into the radiator but that just the sound from some other part of the system was travelling and made it seem so. Gerry Gill suggested this might be the issue. Does that sound like it may be the problem to anyone else?

    To the other suggestions, the radiator is already on shims, angled towards the inlet pipe. And since it doesn't actually heat up when the inlet valve is off, I don't think the knocks are coming from expansion noises in the radiator itself.

    Does anyone have any further thoughts?


This discussion has been closed.