Air conditioner bangs loudly (furnace, heating, installation, leaks)

[therewego]

when our a/c unit turns off, it always ends with a loud bang. I have never heard that before at our old house, this is a 2 yr old house we have now. Any ideas what this is??
Thanks.

[texdav]

Sounds like the trap door in the return is closing to me and is noisy.

[studedude]

Can you tell if the noise is coming from furnace/air handler inside, compressor outside or duct work? Long flat pieces of sheet metal in duct work, if not scored to make them more rigid, can bang when fan kicks on or off.

[jimboburnsy]

If it's coming from inside the house maybe you have a damper in the ducts that slams shut when airflow stops?

[Cosmic]

I got the same thing on heating. The duct work plenum on the return air pops in and out and makes a bang when the blower turns off. Big area of duct work, it has a stiffening pressed groved area but it doesn't work the way it is supposed too.

The duct work flexes because I sealed up the areas around the filters to get them to work like they should, very tight metal ductwork all taped at the joints, no leaks in the system. Pressure changes causes the ductwork to flex in that large area just at the furnace. Sucks in when the blower is running, pops back out when it stops. Can hear it all over the house. Only happens on one side of the furnace, the other side, the stiffening features in the plenum ducts work as they should. Part of it is probably the installation was a lil quick and did not strengthen that big area enough by some other method. Probably could fix it if I took the duct work apart and re-enforced at that point.

Reason #47 I hate damn forced hot air.

[BigJon3475]

Like the others asked does your A/C have return duct work or is it a wall mounted unit that has a filter grille? Also are you using pleated filters? I've heard loud bangs from systems that have return duct work that is way too small. In those cases the return duct work was sucking in and flexing and when it cut off and lost the negative pressure pulling it inwards it would pop back out making a loud banging type noise. Think about when you take a large piece of metal and flex it to a concave or convex position...

[BigJon3475]

Originally Posted by Cosmic

I got the same thing on heating. The duct work plenum on the return air pops in and out and makes a bang when the blower turns off. Big area of duct work, it has a stiffening pressed groved area but it doesn't work the way it is supposed too.

The duct work flexes because I sealed up the areas around the filters to get them to work like they should, very tight metal ductwork all taped at the joints, no leaks in the system. Pressure changes causes the ductwork to flex in that large area just at the furnace. Sucks in when the blower is running, pops back out when it stops. Can hear it all over the house. Only happens on one side of the furnace, the other side, the stiffening features in the plenum ducts work as they should. Part of it is probably the installation was a lil quick and did not strengthen that big area enough by some other method. Probably could fix it if I took the duct work apart and re-enforced at that point.

Reason #47 I hate damn forced hot air.

It happens more often with the more restrictive returns. The pressure differentials in properly sized returns compared to atmospheric pressure in the home are not usually enough to make loud bangs but sometimes they will make slight flexing noises. You appear to already know what you need to do if you wanted it fixed....

[therewego]

sorry hadn't responded, had computer issues. Well, the noise comes from the outside unit every time it reaches the set temp. I will get it checked. Thanks all!

[jtess]

I recently had my 2 stage furnace changed from 1 zone to 3 using 3 dampers. Now when the a/c goes on there is a bang near the duck work that the zone is calling for and a bang when it shuts off. can this be corrected and how?