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Curious what the purpose of this duct is and whether I can close it off. It goes directly outside.


I was planning to convert this room to an office. The furnace and hot water heater are a few feet away from this opening. Today is our first cold day (26 degrees and snowing in MT) and I can feel the cold air beneath the opening.

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up vote 5 down vote accepted

furnace and hot water heater are a few feet away

Combustion air intake which is necessary on an insulated and sealed buiding.

Otherwise negative pressure prevents combustion gasses from going up the stack. In the case of any carbon monoxide content, this condition can be deadly.

Partition the furnace and water heater off so they're not part of the office.

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Got it, thank you! – Trevor Hartman Oct 28 '13 at 23:56
You're welcome! – Fiasco Labs Oct 29 '13 at 1:50
... And stick a carbon monoxide detector down there. Last thing you want is your wife finding you dead at the computer with a My Little Pony video playing. Awkward! – Chris Cudmore Oct 29 '13 at 14:12
@ChrisCudmore LOL – Trevor Hartman Oct 29 '13 at 15:31
Good Article, it all adds up and if you've sealed things up, you need to think about fresh air intake. As a huh? Wow! Consider this. My living room has three decorative wood beams spanning 14 ft. On each side, there was a 1/4" slot between the gypsum board that was only covered by the insulation vapor barrier. Not much til you run the numbers. 14' = 168", 168" * 0.25" = 42 sq.in. which multiplied by 6 gives you an opening of 252 sq.in. Ok, square root of 252 gives us a hole in the ceiling 15.87" on a side. That's a lot of hot air to lose. Caulk is the wallet's best friend. – Fiasco Labs Oct 29 '13 at 22:49

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