Noisy furnace fan


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Old 10-12-12, 07:17 AM
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Noisy furnace fan

Hello everyone. Maybe you can help.

My gas furnace fan makes a loud bang sound only when it shuts off, even if no heat is on, just the fan.

It has done this for the past 3 years and it's getting louder.

Any idea will be appreciated.
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Old 10-12-12, 07:36 AM
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When the fan switches on, it pressurizes the duct system like a balloon, causing it to physically expand in size a bit.

When the motor shut off, the balloon deflates, and in some duct system you get a noise called "oil canning" as the sheet metal returns to it's normal size. Well designed duct systems reinforce the duct in various ways to avoid that noise.


Does that sound like what you are noticing?
 
 

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