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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.