thud sounds from furnace

Old 01-18-10, 07:19 AM
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thud sounds from furnace

We have a forced air gas furnace- every now and again- we hear a low, boom sound that vibrates the duct work and makes the house vibrate- it happens it seems right before the furnace ignites- It's not gas- it almost sounds like pressure build up in the ducts (cool vs hot?) and then works normally- Anyone ever hear of this?
Old 01-18-10, 07:39 AM
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Probably delayed ignition. You need to clean the burners and inspect the heat exchanger.
Old 01-18-10, 05:47 PM
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Originally Posted by skinut2234
- it happens it seems right before the furnace ignites- It's not gas

My bold emphasis. It is doing it BEFORE it ignites. At least that is what skinut says.That IS one for further thought. He already told us it is not the gas doing it, due to the fact it happens BEFORE it ignites.


Have you actually gone into the big panel of the furnace to observe when the sound is heard in relationship to when the burner fire occurs? Otherwise how do you NOT know that it is not delayed ignition?

If it is not the gas, as you claim, - maybe it's from when the furnace cycles repititiously, what you are hearing is a sudden contraction of some duct work as it is cooling down from the cycle before - which coincidently occurs just before a new call for heat. To rule THAT one out, you would have to intentionally turn down the stat for a while, then turn up the stat and see if it still does it right before ignition, then.

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