Richard Trethewey looks up at a forced-air heating duct
Photo: Keller & Keller
Q: We live in northern Vermont, where it gets kind of chilly in January. Would it be okay to enclose my forced-air furnace and feed outside air into the enclosure so that the furnace won’t use air from the house?
Bob Linde, Wolcott, VT.

A: Richard Trethewey replies: Enclosing your furnace and supplying it with outside "combustion air" will probably improve your comfort and save some energy. But you should coordinate your work with a local building inspector, who can make sure that it meets local codes and is safe. A sealed enclosure without enough combustion air will actually reduce furnace efficiency and increase the chance of "back-drafting," which can poison the air with carbon monoxide and other gases.

First, you want the enclosure to be large enough for the furnace to be serviced easily. I'd leave at least 3 feet of clearance at the sides and back, and some more at the front of the unit. Check with the folks who maintain your furnace and ask them what they'd prefer. And don't forget to make the doorway big enough to remove the furnace if it ever has to be replaced.

Next, size the duct supplying the combustion air so that there's 1 square inch of opening for every 1,000 Btu/hour capacity of the furnace; the manufacturer or the owner's manual has this information. Code requires the duct's intake to be at least 10 feet from any appliance vents, plumbing vents, or exhaust-fan outlets, unless those outlets are at least 3 feet above the intake. It can't be located in a garage, close to a driveway, or in an unheated crawl space; it has to be at least 1 foot above grade; and it must be covered with a screen, to keep out animals.

Position the outlet end of the duct near the floor in the vicinity of the furnace's combustion-air inlet. Don't connect it directly to the return-air vent or the combustion vent; that could be bad for your furnace and increase your fuel bills. And keep this duct away from any water pipes—you don't want all that "kind of chilly" Vermont air causing them to freeze.
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