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Loud banging in 20's radiators

I just moved into a 1924 home with a hot water radiator system of the era. I have this occasional (every 20 minutes or so) loud bang in my cast-iron radiators since I started up the boiler a few weeks back. Water temp is normal (set at 180 with hot temp averaging around 150) and pressure is 12 at cold to 22 at hot. The radiators bang (sounds like someone hitting the radiator, hard, with a metal mallet) when the radiators heat up and cool down. Occasionally hear it several hours after cool down as well.

I've bled all the radiators in the system to no avail (it not being a steam system, I didn't think this would do much anyway). I discovered some binding between the risers and the floorboards and cut out around the risers to make space to no avail either. I don't think it is a flow issue because it happens on cool-down and well after..not just on heat up. The radiators that make the noise are original 4-to-8 footers and are well embedded in the original hard wood. Could this be an expansion/contraction issue because the radiator feet can't move? Would putting something under the feet (steel shims?) help with this?

I have the boiler/radiator folks coming out in a few days to replace my expansion tank and T&P valve (for unrelated reasons). I would like them to help me with this issue as well, but am not sure how to guide them. Any advice is welcome.



  • Jamie Hall
    Jamie Hall Member Posts: 18,972
    Expansion and contraction. That will make quite a spectacular bang in some installations -- and it tends to be just a single bang, sometimes with another a little later. Sometimes just one.

    It is probably related to those nice big radiators -- and the place to start (no guarantees here, mind you) is really a two-fold fix: first, make sure the feet of the radiators really aren't "embedded" in the floor -- you may have to put a bit of a shim to get the floor smooth -- and then to allow the feet to slide. A lot of folks -- including myself -- have had wonderful luck with a shim under each foot made of a piece of ordinary plastic milk jug. Nice and slippery, but tough, which is what you need.

    Your expansion tank does need help, judging by the pressure variation you quote -- I'm glad someone is coming to look at it, and I do hope that he or she fixes it right...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • cvanaver
    cvanaver Member Posts: 2
    I made a sandwich consisting of a thin flat piece of metal to cover the indentation the radiator had made in the floor, topped with a piece of milk jug for low-friction sliding and that was topped with a one-inch metal washer to give something firm for the radiator foot to sit on. Slipped it under all four feet and it worked like a charm! Maybe over-engineered it a bit but I know that should last forever. Thanks for the help!
  • Jamie Hall
    Jamie Hall Member Posts: 18,972
    My that is fancy -- mine are much more primitive! But I'm glad it worked for you and, as you say, it should last forever!
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England