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My house has a centralized gas heater, that pumps the hot air through metal ducts to vents in each room. I discovered that the past owner had previously had a greenhouse, and so there was a duct that went outside. It isn't very well sealed, so I'm likely losing considerable efficiency on the system. The exit to the outside is about 10" diameter and 25 feet from the furnace via horizontal pipes. Is it safe to fill the exit of this full of expanding foam?

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    Foam can "outgas", i.e., release volatile substances. Putting just ordinary expaning foam in the heating ducts could spread noxious fumes in the HVAC system and so throughout the living space. Don't do that! In addition, expanding foam will exert outward force as it expands. This could rip apart the ducts from the inside. Commented Oct 1, 2023 at 10:23

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Use a 10" duct cap to seal the disused duct opening correctly. Since you mention diameter, I've shown a round one.

Duct cap

image source https://images.homedepot-static.com/productImages/0dd0da23-e656-4356-8460-643a9d976b0d/svn/master-flow-caps-dc10-64_1000.jpg no endorsement implied

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  • Attach it with a couple of sheet metal screws, then seal it with duct tape (the aluminum & plastic stuff found in the same aisle as the duct cap), not duck tape (the cloth backed stuff that comes in 10000 colors & patterns from your local craft store). Duct tape will seal and last, duck tape will dry out and fall off in a year or two.
    – FreeMan
    Commented Oct 1, 2023 at 11:56
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    This, or simply jam it with some fiberglass batting, which would be a lot easier to reverse if the duct ends up being needed someday.
    – Huesmann
    Commented Oct 1, 2023 at 13:07

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