HVAC Question

Heating and air folks,

We’ve been in our house for 5+ years and just this winter we noticed our duct working making noises. After the furnace shuts off we here a decent sized “bang”, it sounds almost like someone is hitting the duct work with a wooden spoon. Then a minute or so later we hear it again in another part of the house, then again somewhere else. It probably happens 5 or 6 times between heating cycles. I have no idea what this could be, but it’s starting to cause sleep problems at night and drive me out of my mind during the day.

Any ideas? Thanks.


Do you have a home warranty… why not call in the HVAC person and let him have a look. for around $50.00 they can at least tell you what the problem is.

I have the exact same problem. My home is fairly new built in 2003. Whenever the furnace is running every once in a while a loud bang. No idea where it is coming from.


     5 yrs Heating/Air when I was younger.. 

On a Forced Air unit there is a supply and return. Depending on your installation, a Horizontal unit in the attic, a vertical stand up in the closet or a package unit on the roof. Usually this banging sound that you hear is more than likely from the return air plenum. This is the big metal box that has a flex duct connected to it. When the box (plenum) is made on a break (big metal bench which allows you to fold etc.) the installer would create an X from corner to corner so it would strenghten the box. When the unit is cycling it is sucking the return air from inside the house filter-grill back into the unit to recylce the air and make more effecient. The air suction is so strong that it is causing your box to bend inward and when the return air stops it pops back out making this loud sound. If you suck on a straw and cover the opposite end of the straw it is the same concept. I hope that made sense… There are two ways to help this. One is to see how far the distance is from the return air plenum (on unit) and the return air grill in hallway. If they are within 10’ and the installer only put 10’ of ducting you will more than likely have a really noisy and powerful return air. I would add more of the ducting to create more distance for the air to travel which would lower the cfm the air is traveling. We would put a entire box of 25’ in the attic to create a quite return air and it would still be very efficient. If there are no cross bends in the plenum it will be very hard to solve the problem without taking it off and replacing. The cross bends (like and X) are there to strengthen the box to avoid this from happening. Another possibility is if the installer ran too small of return air duct 12" or 14" instead of what it needs to work properly. This would cause a very strong return air duct and the unit would turn off and on constantly due to HIGH LIMIT SWITCH going off. The unit gets to hot and cannot breathe properly. Basically like us running around the block breathing through a straw. We would not get very far. Hope this info helps…


That’s a great explanation. I don’t understand entirely, but the analogies you used helped greatly. I’ll place a call to our HVAC guy and have him come out. I don’t entirely trust him so having this post available to me will make me a bit more comfortable with what he proposes.

Thanks for the help.