We have a 20 year old home that has a finished basement with 2 main trunks running directly through the middle area of our home basement. During the initial heating stage, within 20 minutes, there is a loud bang that sounds like the metal flexing and hitting the floor joists, etc. After the heating cycle, as the duct cools, the same bang happens again. Every time the heating comes on or off, this same event takes place.
Is there anything we can do about this? At 3am it is quite loud and wakes the whole house!
If you can narrow down were in the main trunk line it's coming from, you can take down the ceiling there to fix the problem.
One of two things will cause it, the main trunk line is binding on something somewere along the run, or one section is binding or wasn't made right with the proper stiffener bends in it.
What's happening is when it heats up it expands, gets caught on something and pops when it has enough force to breal free. Reversed when it cools down.
This sounds more like an air flow problem. the first thing you need to look at is how many return air inlets you have as well as the sizing of you furnace. if your furnace is over sized or there is not enough inlets this will move a high volume of air through the duct system increasing the static pressure in the return air duct. once this happens the duct work will draw inward when the furnace kicks on and expand once it shuts off. this will make a banging sound when the furnace shuts down.
To solve this problem you will need to determine if its a ducting issue or a over sized furnace. Count the number of return air inlets you have. an example will be if your furnace is 70 000 to 80 000 btu's you should have one 24x6 and two 14x6 inlets. on the main floor. and one in the basement 14x6. your return air main trunk line should be 16x8 to 18x8. this will handle and air flow of 1200 cfm.
If your return air is lacking add a return air off the main drop duct this will be the duct work that drops from the ceiling then connects to the furnace. install a 7 inch pipe drawing air 10 ft away from the furnace or on an adjacent room not drawing air from the mechanical room.
Thank you guys,
Trouble with these is one, all the areas are drywalled, two, the furnace is multi-stage so air volume would not be great enough to flex the ducts. I can hear when it changes speeds and when high speed kicks in, it makes no difference... oh yeah, when I cool the house, the ducts make no noise, only heating, so that tells me the ducts, joints, supports, etc are expanding and moving around...
Seems as though the only option is forget it or open the ceiling spaces around the ducts.
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