Monday, 1 February 2021

Loud thud when hvac shuts off - flexing ductwork

Hello,

I have an issue caused by the flexible ductwork leading to my hvac fan/blower. When the HVAC kicks on the box-shaped ductwork pulls into itself (like sucking in your belly), and when the hvac shuts off, the negative pressure is released and the metal duct box pops back into it's original shape. The result is a very loud thud every time it shuts off...sounds like someone hit a drum in the middle of the night.

FYI - Have the same issue in two locations. In one location (pic A) I was able to resolve the issue by using a ratchet strap and scrap wood to hold the duct from popping out. In the second location (pic B), I cannot really do the same because of how everything was built. I'll include pictures of both locations.

Any suggestions on keeping it from doing this?
Attached Images
  


source https://hvac-talk.com/vbb/showthread.php?2226249-Loud-thud-when-hvac-shuts-off-flexing-ductwork&goto=newpost

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