Banging noise in ducts when furnace fan turns off


  #1  
Old 06-15-18, 12:51 PM
E
Member
Thread Starter
Join Date: Dec 2011
Location: Canada
Posts: 281
Received 0 Votes on 0 Posts
Banging noise in ducts when furnace fan turns off

Came across a problem I didn't have before replacing old furnace. Nor did i have this problem when it was first installed.

I've noticed the ducts banging when the furnace stops the fan cycle I have set. We are in ac mode and I let the fan come on every 10 minutes just to keep the air circulating (new thermostats allow some of the neatest things). I noticed that when the fan shuts down I hear a very distinctive pop from the return plenum (vertical section going into furnace filter then furnace blower). I then noticed the supply plenum shrink (for want of a better term) back in, like it is deflating, followed by another pop but much lighter than the first.

My guess is that it is a pressure question and not a heat/cold thing as only the fan is running. I do however, notice the same kind of banging when the ac is running and stops. I use the more expensive brand air filters and the filter is clean. I checked the supply at the furnace and there isn't anything blocking the way. I imagine it could be something further up the supply line. Returns are not blocked and mostly all registers are open. I keep basement ones closed during ac season.


Anyway, I've taken up way too much of your time asking you to read all this but I was wondering if anyone has any ideas as to what could be going on. Newly installed furnace that didn't have a problem then, all of a sudden, it starts popping.

Thanks
 
  #2  
Old 06-15-18, 01:40 PM
airman.1994's Avatar
Member
Join Date: Dec 2005
Location: VA
Posts: 5,795
Received 8 Votes on 8 Posts
Common issue in residential systems. Duct is to light of a gauge/no cross breaks/under sized. Its one or all three of those. Hopefully the company that did the install made sure the duct was the correct size. So the fix if the duct is the correct size is to stiffen the duct up. This can be done with a thicker peace of metal screwed to the duct at the location of the noise.
 
  #3  
Old 06-15-18, 05:14 PM
E
Member
Thread Starter
Join Date: Dec 2011
Location: Canada
Posts: 281
Received 0 Votes on 0 Posts
I understand that Airman but that sheet metal has been up for over 30 years without popping. Why now?
 
  #4  
Old 06-15-18, 05:22 PM
F
Member
Join Date: Mar 2006
Location: Wet side of Washington state.
Posts: 18,495
Received 36 Votes on 28 Posts
Your NEW furnace may have a more powerful blower than the old furnace. You MAY be using a more restrictive filter that causes a greater pressure differential between the supply and return ducts.
 
  #5  
Old 06-16-18, 04:51 AM
E
Member
Thread Starter
Join Date: Dec 2011
Location: Canada
Posts: 281
Received 0 Votes on 0 Posts
Thanks Furd. The blower may be more powerful but wouldn't that mean the popping would have started immediately after install? Does it take time for the sheet metal to react to the new pressure created by the stronger blower? There were no issues when the furnace was first installed, about two months ago.

Same with the filter. Why did it take so long for the popping to show up? I will try a less restrictive filter to see what happens.
 
  #6  
Old 06-16-18, 07:44 AM
Houston204's Avatar
Forum Topic Moderator
Join Date: Oct 2009
Location: Texas
Posts: 7,425
Received 17 Votes on 14 Posts
The blower speed is often higher in cool made than heat mode .

I would try the less restrictive filter but I would also try to see if I could see the source of the noise by standing next to the furnace when it cycles off.

I would screw some standing esses to the outside of the duct after proving that it is the duct moving in or out.
 
  #7  
Old 06-16-18, 02:49 PM
E
Member
Thread Starter
Join Date: Dec 2011
Location: Canada
Posts: 281
Received 0 Votes on 0 Posts
Thanks for your thoughts Houston204. The sounds are coming from, in order, the vertical return right near/below the humidifier before the air filter. This is about a foot away from the furnace. After that pop there is a pop in the evaporator coil area sheet metal (left side). I can actually see the sheet metal bulge out when the fan starts and then pop back into place when the fan is off.


The standing esses you are referring to, how exactly do they work best? What length, where, etc.
 
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: