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return air supply bangs

I cannot stop a loud banging sound coming from my retrun air supply when the fan turns on or when the fan stops. It does not matter if the hear or air conditioning is on. The bang takes place on the start and stop to the fan. I have isolated where it take place. I just cannot stop the sound. The return supply line in the basement is the culpret. I can recreate the sound by pressing on the sheet metal and releasing it. I can also feel the movement when the fan turns on and off. The supply line is exposed at this time so I do have some access to it. Can you please help. The bang is very nerve racking to all of us.

Re: return air supply bangs

perhaps gluing some hard insulation to it while it's flexed out would eliminate the sound. I would also suggest sound dampening insulation between that area and the rest of the house.

The pressure is probably causing it, make sure your filters are all clean and the returns are clean as well.

Eventally you'll get used to it.

Re: return air supply bangs

I will try the insulation board glued to the return. That makes sense to me. Then the metal will always be in a pushed in position. Filters are new and always cleaned or replaced monthly. I will try the board first.

Anyone else with similar experiences out there???

Re: return air supply bangs

It's not uncommon for ducts to make the noise your describing .... usually with forced air heating. As the metal ducting heats it will expand and cooling causes it to contract usually at weak or distorted points.... with the cold air return sounds as though it's trying to collapse in your case.

This does bring up some questions ... is this a forced air heating/cooling system and if so does this noise occur during heating?

Has anything been changed with the air handler prior to when the noise first appeared?

Re: return air supply bangs

Is this a brand new total system (new house)......or perhaps a new unit installed to an older existing system of ductwork?

Or is this an older system that just recently developed the problem?

Whichever the case might be.........me thinks there's a good chance that the total number of grills on this return branch are too few....or the combined size of all the involved grills is too small..........for the output of the blower (running at its current speed). IOW, it can't freely pull all the air it would like to have and so a certain amount of vacuum/negative pressure is being pulled on this return duct. That is causes this duct to flex in response because it isn't stiff enough to adequately resist. No news there, right?

One potential answer to the noise problem could be as simple as stiffening the current flexing area of the duct....or somehow fastening it to a nearby floor joist,etc. However, doing so *might* cause that flexing/noise problem to tranfer to the next most vulnerable/flexible area of the duct. Impossible to say with certainty.

A better solution might be to increase the number of return grills .......or the size of the existing ones. Less restriction equals less vacuum/negative pressure on the suction/return side ducts....and therefore better air flow. A bit better/greater air flow will also likely result in a bit better efficiency overall..........although there are practical limits to this equation.

Re: return air supply bangs

Thank you for the input.
The bang can occur at any time the fan is turned on. It does not even need hear or cool air conditioner air. Just the fan on this forced air unit. It has been happening since it was installed. Although I have recently changed it to cause a larger noise. I found some leaks in the return side and used sealant to seal the open areas. Made it tighter. Now I have more noise. I wonder....

Thanks everyone.

Re: return air supply bangs

Although I have recently changed it to cause a larger noise. I found some leaks in the return side and used sealant to seal the open areas. Made it tighter. Now I have more noise. I wonder....

I'm along the same thinking as goldhiller.

The ducting may be distorted at that point where it makes the noise. The return duct has a negative pressure to begin with .... with a restriction to air flow from things like an inadequate amount of intake vents would cause the duct to cavitate ... especially at it's weakest point.

By the sounds of it you changed the fan motor?

If this is the case the replacement may be running at a faster RPM which will cause the blower to move more air and creating more negative pressure in the air return ducting.

Depending where the weak point of the ducting is located you may be able to attach something rigid like wood or metal as a strong back to re-enforce the weak area.

You might also consider having an HVAC contractor to do a balance test on the ducting as well.

Just a thought. :)

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