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.

Radiators making a horrible knocking sound.


My building's heating system is making so much noise that I can barely get any sleep lately. I am not very knowledgable about heating systems, particularly old ones, and cannot figure out what needs to be done. I have called our management company and they just keep saying "it's an old building, noise is normal." but they refuse to tell me when the system was last maintained or even have someone come out and listen to the noise to see how obnoxious it truly gets.

If someone could help me figure out what to do I would greatly appreciate it!

Here is a full description of my situation:

I rent an apartment on the basement floor of a building in downtown Denver. The building was built ca.1910 and has roughly 25-30 units in it. The entire heating system is at the other end of the building, also on the basement level. We have what looks like 4in diameter pipes coming into the apartment and 3in diameter pipes going out. there are 3 radiators secured to the ceiling of the apartment in the dining room, living room, and bathroom. The radiators are approximately 4ft long by 1.5ft wide with a knob on one end and what looks like an overflow valve on the other. All of these "overflow valves" have been painted over, just like the radiator and pipes themselves.

Now, the problem. Now that it is growing cold, at night the heating system kicks on. When it does, our pipes make a loud knocking sound, it almost sounds like someone is hitting the metal pipes with a hammer. It sometimes lasts for about 10-15 minutes, but other times will go on for hours non stop. It is so loud when knocking that it wakes me and my girlfriend up in the middle of the night, and we both have a very hard time going back to sleep. We live next to a busy road, so we are by no means light sleepers, but this noise is just unimaginably annoying, especially at 3am.

I have tightened down the knobs on the radiators to turn them off, but this doesn't stop the noise, the noise happens in all rooms, and seems to originate from the pipes, not the radiators. Our landlord said that if we tightened down these knobs we would have very light knocking for a couple minutes, but this has not been the case. We talked to the previous tenant who said that they did not have much of a problem with it last winter, so I am assuming this problem has just recently gotten this bad (unless they are both hard of hearing or something)

I have heard that loud knocking is usually caused by condensation forming in the pipes, which needs to be drained to alleviate the noise. How would I do this myself? Would it be different for oil vs water vs steam heat? How much is the industry standard to have this looked at by a professional?

If someone could help us out with a little advice and knowledge I would greatly appreciate it, I haven't gotten a good nights sleep in over a week, and cannot stay here if this is going to be happening all winter.



  • Timco
    Timco Member Posts: 3,040

    You have water hammer. Steam passing water in the same pipe or radiator does this. Just like tapping on the pipe with a hammer. Needs new traps (opposite side from valves, not a overflow devise) and pipes / radiators sloped back to the return. When too many traps go bad, they crank up the pressure to get heat to all the rads. With working traps, pressure can be turned WAY down to like 2psi and that would stop banging AND save BIG $$$$$$.
    Just a guy running some pipes.
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853

    Where the heck is that? Oh yeah, it is east of Heeney, about an hour and a half :-)

    I live in Denver, and I haven't even turned my heating system on yet. Here in Heeney, I'm using radiant windows to know the chill off in the AM, but still haven't started my heating system up here, and it is a LOT colder up here than in Denver.

    Knocking and hammering on initial cold start up is common situation until the mains get heated up and stay heated up. If this hammering continues after the system has stabilized, then it indicated other possible issues. Having worked with and for most of the property management companies in the metro area, I can tell you that they do not have qualified people working on them. Not all contractors in Denver are good at steam. Some talk the talk, but can't walk the walk, and as TIm said, the first thing they do that is totally wrong is to "Turn It Up".

    Keep this site on your Bookmarked list and if the hammering continues, come back.


    There was an error rendering this rich post.

  • Brad White
    Brad White Member Posts: 2,398
    edited September 2010
    A few things to look for

    There can be several causes alone or conspiring. If upon start-up, and reflecting on what Mark said above, one of the most common is a lack of pipe insulation. Not, "less insulation than is ideal", but, "none at all". Seems too often asbestos pipe insulation would be abated but no one replaced it with fiberglass or anything else for that matter. 

    (The reason I go to this firstly and sight unseen is that it is so common yet relatively easy to address at least in part. It also pays dividends in other ways.)

    What happens then is that steam, say at about 2 psi and with a specific volume of about 24 cubic feet per pound, collapses into water when it encounters a section of bare, cold piping. Water has a specific volume at 212F, the immediate change of state, of 0.01672 cubic feet per pound. (Yes, I looked that up in my tables, not off the top of my head!).

    So imagine the vacuum created by 24 cubic feet, -a volume comparable to a very large refrigerator or Shaquille O'Neal's right sneaker, collapsing to a volume of about 29 cubic inches, about the size of a Rubik's Cube. A virtual black hole in your piping system.

     Once created, all matter within range is drawn toward the vacuum, be it water, steam, air, curious household pets,  to collide with some force indeed. A pure vacuum is just under 15 psi below atmosphere at sea level and about 12 psi below your atmosphere. So imagine a one pound hammer dropped from a foot, times twelve.

    Even Blue Oyster Cult does not have that much cowbell.

    Poorly functioning traps, steam passing over water, dead-end water pockets, clogged returns, all of those are potential causes, but each depends on when in the cycle the banging occurs, beginning, middle or end.

    Let's hope one of the able steam practitioners on this site are nearby.
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
This discussion has been closed.