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I cut a hole in a basement ceiling drywall and found an open duct at the end of the span between the joists (see photo). I'm sure this isn't good for the efficiency of the hvac system. What are the best practices to handle an open duct that is unused? Or how should I close it up?

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Sometimes the Joist bays and stud bays are used as conduits for the air, being in between the living spaces it may not affect the efficiency and be needed for moving the conditioned air or return air. If there is no connection to other areas it would not hurt to seal it up. A piece of sheet metal some self piercing screws and some silicone sealant will seal most duct openings put a bead around the opening screw a piece of sheet metal in place and screw ~every 4” let the sealant cure prior to running the system or it may blow your seal and a leak like that can be very loud like blowing on a blade of grass between your thumbs loud , but if it cures and holds there will be no problem with noise.

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  • There is no vent in that bay... I know you said sealing it won't hurt but is there any way to figure out if it is needed or was intentional before sealing it up? – mikeLdub Jul 22 '20 at 14:32
  • Also, I'm pretty sure this is for supply, not return. – mikeLdub Jul 22 '20 at 14:43
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    I would just verify it is not being used , turn your fan on and see if it is blowing or sucking air at the opening you cut. Next go up stairs and check the vents or return air grill , if you have no or very little air movement because of the opening you made it is being used and should not be sealed. But it could have been a vent for the basement that was closed off when they installed the Sheetrock. Once you verify the upper area is not using a stud bay going up from that location that’s about all you can check. – Ed Beal Jul 22 '20 at 14:44

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