Repairing an air grill hole that is cut too big is easy with the right tools and techniques. When it comes to replacing your air vent, the issue isn’t that you’ve cut too much of anything off, but you may need to face the challenge of contending with an air vent that doesn’t fit — and, unfortunately, there are only a few solutions to this issue. You can use what is called 20 minute mud to fix You mix it like peanut butter and apply Once dry it will be good and hard and then sand, prime and paint. Plasterboard in residential applications is usually either 10 or 13mm thick, and as the flange on the air duct looks to be reasonably thick, i'd suggest the screws you use are at least 40mm long
Something like these 8g x 40mm white button head screws will do a great job and not stand out too much. If you are ok with leaving the vents in place, get some 1/8 hardboard and cut it to fit into the inside of the grille Use some weather stripping to seal the perimeter like a gasket Reinstall the grille with the hardboard inside sealed against the drywall. A/c latch can easily be attached to your grille restoring the ability to open and close with ease. If the small weak metal latches on your ceiling mounted or wall mounted hvac system return air grille are broken or difficult to use, they can easily be replaced with strong rare earth magnets.
The instructions in the video also show how the flame reacts to too.
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