What's that thumping sound when my gas furnace turns off?

Answered by Brendan ~ January 17, 2012 ~ No Comments

My newly installed Carrier furnace makes a thumping sound immediately after it shuts off. The sound only occurs if the furnace blower has been operating. I hear the sound near the unit's vent pipes, and it is loud enough to hear throughout my condo. Any ideas?

Jon ~ Flagstaff, AZ

Brendan Fowler

This is a very interesting and annoying problem that you are experiencing. I am assuming that this only happens when the heat has been used not the AC. It sounds to me like expanding and contracting metal and plastic. As metal or plastic cools it can make all sorts of strange noises. If the new ductwork is all metal and it is pressed up against the new framing it can make noise as it changes shape. Because you mentioned that it only occurs after the blower has engaged, I would suspect that it is the duct work fed by the blower. Most of the noise you will hear is going to occur on the cool down cycle (during the contraction of the ducts or pipes). I have also had metal roofs make strange noises as they cool down. If there is a metal roof surface that could be getting warmed up from your furnace running it could make noise. If the furnace is sitting in a large sheet-metal pan with pvc drains running from it could be making noise as well. Identifying the source of the noise can be very difficult, but I think you are experiencing cool down, contraction noise. Good Luck!

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