lound noise when funace shuts off

[wen]

I have a gas fired hot water heat system that a few seconds after it shuts off running there is a loud banging noise. I have bled the pipes and still cannot figure this problem out. Any advise would be appreciated Thankyou

[KField]

In order for me to help, you would need to give more details. Like where does the sound seem to come from, inside the boiler or upstairs? Is it a bang or a boom? Bang might be water, boom might be flame. Does it do it every time the boiler runs? I hope you see what I'm trying to get at. I know the components of your system, but need to narrow down the search before pointing the finger.

Ken

[wen]

the noise is a loud bang that happens about thirtyto sixty seconds after the furnace shuts off. the noise occurs in the furnace area at the pressure tank not upstairs. someone suggested the pressure might be too high but it was at 20 and I thought for two floors each being zoned the pressure should be higher. should we adjust the pressure? my husband says it sounds like it is oil canning inside the tank. thanks for any help

[KField]

Don't mess with the pressure. All the ststic pressure does is to fill the radiators. If your top floor radiators are full, you have enough pressure. The circulating pump moves the water by causing a pressure differential between the supply piping and the return piping. The circulator does not care if you have 2 psi or 20, it can still do its job.

The oilcanning was a good suggestion and I don't thing you would have it in the water system. Does your boiler have a metal flue collector box on the top of it? If so, that may be the culpret. It would cool off in about the time frame you described and could easily warp and oilcan. If you have one, it would be under the top jacket cover and may have the smoke pipe coming out of it. Let us know so we can continue the hunt.

Ken

[wen]

yes the boiler does have a metal box with a pipe sticking out of it. my husband lifted the top off and listened for the noise. he is not sure if it is coming from that section or the lower heating chamber. they are right on top of each other. he thinks it could be the lower section. so now what do we do please?

[KField]

The lower section is backed by water and would not cool off that fast and would sound different. I suspect it is the flue collector but can't tell you how to stop it. The best thing I can think of would be to lift up the jacket top and give the collector box a whack with a hammer to put a dent in it. That may change the stress on it and stop it from oilcanning. Sorry I can't help more than that.

Ken

[arkayassoc]

It might be good to use the round end of a ball pein hammer, to reduce the possibility of cracking the sheet metal.