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Beren 10-12-2007 01:34 AM

Gas furnace "bangs" when stopping
Got woken up this morning by a "bang" and had no idea what it was. The next time the furnace cycled off, I found out. The gas furnace seems to be coming on every couple minutes, and when it shuts down, it does so with a BANG. I guess it could be a CLANK or a THANK!

Any suggestions? It is blowing warm air, at least, it's just hard for me to sleep.

redline 10-12-2007 11:02 AM

How old is the unit?

Beren 10-12-2007 11:08 AM

I'll check tonight and see.

Also, I realized last night that the banging is not coming from the furnace itself. I think it may be coming from the ducts.

redline 10-12-2007 11:17 AM

What may be happening is that when the blower shuts off the ducts "relax" from the pressure drop. You may be able to find the location of the duct that is flexing and secure it to reduce the amount it flexes.

Beren 10-12-2007 11:22 AM

Understood. I'll check for loose ductwork. What confuses me is that this did not happen during the summer when the A/C would shut off. Same blower, same ductwork.

redline 10-14-2007 08:24 PM

The duct may have just loosened up?

scorrpio 10-15-2007 07:37 AM

It sounds like you got a damper someplace that is opens/closes when air pressure comes on. There will usually be a rubber bumper to cushion the damper, but if the bumber has worn off, the damper's door would bang directly against the housing.

HVACDave 10-15-2007 08:37 PM

Have you done any reno's or anything where you may have changed the amount of airflow the unit is moving. Have you pushed any furniture up against the return air ducts or blocked any of them off by accident.

Ussually cooling is run at a higher speed than heating if your furnace has a multi speed fan. It could be possible that the wires could be reversed and you are now on a higher speed.

You could check by switching your fan setting on your tstat to "fan on" and shut back off and listen for the noise. Of course check and make sure the filter is clean(I would supose that was your first thought, but I had to ask)

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