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October 2011
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December 2011

Banging noise when furnace turns off

Recently my furnace starting making a loud "boom" sound at the end of a heating cycle. It's a relatively new furnace, and this is the first sign of any trouble I've had with it. I turned to Google and learned that a common source of that kind of noise is an incorrectly closed baffle, or a dirty filter.

Unlike my previous home, this one does not have automated baffles. So I checked to make sure that I had remembered to turn it to the "heat" position, and that it had actually moved correctly. Both things checked out fine. So, I called the fine folks at Althoff Industries to come take a look at what I thought must be a gas burner problem. (That's what it sounded like me.)

Much to my chagrin, the nice technician quickly diagnosed the source. It was, indeed, a dirty filter. I dismissed that as "unpossible" when I read it on Google and didn't even check my filter. Well, here's what I learned from the HVAC tech:

A dirty filter can cause the furnace's fan to speed up in an attempt to compensate for the low air flow it is experiencing. This causes increased pressure in the duct work--more pressure than it normally has--and then when fan turns off at the end of a long cycle, a duct can "pop" back into it's low-pressure place. Hence, "boom" or "bang".

Had the results I found in my search explained how a dirty filter could cause this problem, I surely would have checked mine. But without an explanation I dismissed it as illogical. My bad. And now, hopefully, someone else will find this and save a service call.