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Banging hot water radiator

Can anyone tell me what would cause a loud bang noise once the thermostat has been set so the heat kicks in? It is such a loud bang that is scares us and the dogs. Anyone know how to fix this problem? I have already bled the air out of the radiators and the problem still lingers. We only have this problem with one particular radiator.

Thank you to anyone with advice,

Re: Banging hot water radiator

Is this a steam boiler or a hot water boiler??

Some steam boilers allow one or two radiators to heat with the small amount of hot water tapped out of the bottom of the steam boiler & if the boiler water happens to be low, you will get a banging; water on the sight glass of the boiler should read "half full".

Check to see if you have a "sight glass" on the boiler which would indicate that you have a steam boiler.

If you have a HOT WATER boiler, the banging is much less likely as long as you get the air out of the radiator; the most likely place is the "riser" pipes coming out of the floor that often rub against the little circular hole in the floorboards.

This is because metal piping always expands when it heats up.

If you determine that this is the cause, try squeezing a dab of high heat caulking compound in to each of the riser holes of the rad.

High heat caulking can be obtained at HD/Lowe's in their fireplace dept., or any plumbing supply house.

Try to locate exactly where the noise is coming from (the radiator body or the piping leading into the floorboards.

While the rad is in the beginning of the heating process, there are several minutes when you can hold your hand on different parts of the rad before it gets too hot, to see if you can "feel" where the noise is coming from.

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