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