Duct banging/Returns in the basement


Old 10-30-11, 12:18 PM
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Duct banging/Returns in the basement

A little over a year I had new duct and furnace installed. The HVAC guys said it was necessary to have a return and supply vents in my unfinished basement, so they put in the vents for both. This summer I had a whole house weatherization done. Everything was sealed in the basement and the attic. The weather guys said it's a huge mistake to have a return vent in the basement due to air quality and colder temperature. I told them my basement temperature is usually around 70 degrees all year around. However, we agreed to tape the return with a metal tape and close the supply shutters, and see how it all works out. I could always peel away the tape.

It seemed to make sense at the time and I wouldn't have any issues, untill I started hearing a loud bang when the furnace kicks in full blast. It has a varying speed fan, but I only hear it when it's operating on full.

So my question is: Who would be in the right here, the HVAC or the Weather guys? Is the bang caused due to not having enough returns? The sheet metal is creased and I have 3 returns remaining on the main floor of our 1100 sq feet of our little house.
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Old 10-30-11, 09:31 PM
Join Date: Mar 2006
Location: Wet side of Washington state.
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The duct is "oilcanning". What is happening is that the pressure inside the duct (negative because of blower operation) is causing the normal atmospheric pressure on the outside of the duct to partially collapse the duct. Heavier sheet metal may have been prudent when the duct was made. You may be able to install some external stiffeners to reduce the effect.

That the oilcanning is taking place also shows that the duct is too small for the airflow. I would concentrate on air sealing the perimeter of the basement (doors, windows, sills etc.) and making the basement itself a partially conditioned space. That would include having the supply and return ducts operable.
Old 10-31-11, 04:01 AM
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I agree that the return is to small. I also agree with your HVAC company. Having air in the basement is good for IAQ.
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