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Joined: 1/12/01 Posts: 62251
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Either metal expansion when the duct got hot,

or else you have a damper in one or more trunk lines that moved (due to a loose wingnut on its positioner) when the circulating fan started, though it seems the damper would tend to move to the open position and not make too much noise.

... or for a real stretch, the metal grate at a return filter wasn't fully closed/latched and the start of air movement was enough to close it (with a bang)

[Post edited by Trigon at 02/12/2020 11:34AM]

(In response to this post by NJHoo)

Posted: 02/12/2020 at 11:30AM


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Current Thread:
HVAC puzzle for the DIY group -- NJHoo 02/12/2020 11:10AM
  You were all close -- NJHoo 02/12/2020 12:27PM
  The other return was just 12 x 12" -- NJHoo 02/12/2020 1:44PM
  Re: The other return was just 12 x 12 -- Trigon 02/13/2020 03:41AM
  The hi-tech filters do not let enough air-flow. ** -- Guy Caballero 02/12/2020 1:00PM
  Either metal expansion when the duct got hot, -- Trigon 02/12/2020 11:30AM
  Bang from the furnace or bang from a duct somewhere? ** -- MaizeAndBlueWahoo 02/12/2020 11:29AM
  Duct expansion? ** -- JohnHoo2 02/12/2020 11:15AM

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