# Air duct pulsating



## Pulsating (Jun 4, 2007)

OK, I'm looking for some advice on Heating and Cooling duct work. I built a new hose and when the AC unit was started up the furnace duct work started pulsating. Sounded like there was a 50 year old furnace installed in the basement that was on it's last leg. You could actually see the cold air return duct moving in and out like a giant bellows. The contractor told me it was caused by a damper that was not open all the way. After they fixed that the AC evaporator froze up several time flooding the basement. Finally they got that solved but the house wouldn't cool on a hot day. 80 degrees inside, 93 degrees outside. So now they installed a larger AC unit. Originaly had a 3.5 ton, and now has a 4 ton. I have a 3700 sq ft house, 2 story with 960 on second floor, plus a full basement that is not finished yet. There is a zone damper to control the air to the first and second floor. But now the duct work is back to pulsating again, expecally when the upstairs calls for air. Any comments on what is causing this would great.


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## JohnWill (Oct 19, 2002)

Well, I suspect four tons of A/C isn't enough for 4700 sq/ft. I have three heat pumps, totalling 8.5 tons of A/C for a 5000 sq/ft house.

A far better way to do a house of this size is to use separate A/C for each floor. I have the 3rd one for my 3 room office suite, and one for each floor of the main house.


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## Wino (Dec 1, 2001)

Two things I can think of:

1. Blower Fan rotation incorrect.
2. Insufficient return air causing fan to cavitate (starved for air). This can be caused by inadequately sized return air registers.

Other culprits to check: dirty filters; poorly designed duct system; dampers used to balance system incorrectly set..........all these can cause fan blowers to 'puff'.


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## Tstright (May 19, 2007)

Wino said:


> Two things I can think of:
> 
> 1. Blower Fan rotation incorrect.
> *2. Insufficient return air causing fan to cavitate (starved for air). This can be caused by inadequately sized return air registers.*
> ...


I'll vote for #2, After being in the Mechanical business for 22 yrs I've seen that more than a few times.


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