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I have duct work in a house that I recently bought were it looks like some ducts were removed and then just sealed over with a metal sheet. The vent seems to be leaking air and in my opinion should be better sealed. I am not sure why this was done maybe because it was in a closet.

How can I seal it?

I thought that air duct sealant would be the best, so I bought a tube of Red Devil F-seal 181.

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    If it is not leaking why? They make duct tape designed for this but fails after time. I have had better luck with metal foil tapes. The sheet of metal is the easiest way of closing an opening in a flat duct.
    – Gil
    Commented Feb 2, 2023 at 23:03
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    Note that cutting holes in ductwork and even in indoor units (air handlers) is not all that uncommon in order to effect repairs.
    – SteveSh
    Commented Feb 3, 2023 at 0:11
  • @SteveSh while it may not be uncommon, I think the way they left it sucks. It leaks air and should have been sealed better in my opinion. I don't care so much why but more the most effective way to seal it. Commented Feb 3, 2023 at 0:56
  • @Gil I get that sheet metal is the easiest way to close a duct but it leaks air. The question is how to close and seal better? I think duct sealant Commented Feb 3, 2023 at 0:57
  • There are good quality metal foil tapes made specifically to air seal ducts. I find tape easier to use.
    – Armand
    Commented Feb 3, 2023 at 3:03

2 Answers 2

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Combining the suggestions made in the comments and some additional thoughts into an actual answer:

  • Plugging a hole in metal duct work with a sheet of metal is common and accepted in the HVAC industry.
  • Apply duct sealing mastic between the existing duct work and any flaps/flanges/etc on the patch to help seal between the pieces of metal.
  • Apply proper duct tape around all the edges of the patch, covering the patch and the existing metal that was being patched.
  • Ensure the patch is well screwed (with self-drilling sheet metal screws).
    • Note that this is the Mylar and metal based stuff that's sold in the HVAC aisle of the big-box store and is relatively expensive.
    • Note that this is not cloth based "duck" tape sold at the deep-discount store in 10,000 colors and patterns and used for decorative purposes for next to nothing per roll.
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You can use an epoxy adhesive to attach the two pieces of metal. You can use that as a pseudo gasket and hold in place with screws. There are also polyurethane and cyanoacrylate based adhesives that will do the job. You could place about a 1" or larger band of adhesive around the hole, you do not have to cover all of the area.

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  • Most cyanoacrylate adhesives don't fill gaps very well, so would not be effective for this purpose. And these adhesives may be more permanent than you want if you ever expect to need access to the duct again.
    – keshlam
    Commented Mar 30 at 14:14

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