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I have a newer split level house (2006) with unfinished basement. The natural gas furnace is on the lower level in a utility room that is also laundry and bathroom. There's an enormous 8 or 10" pipe that serves as combustion air intake just over the furnace, which goes outdoors through a screened hole.

Furnace and Combustion Air Intake

Now that it's become very cold outside (0°F/-17°C tonight) the intake seems to just be cooling off the entire lower level.

The furnace is not in its own enclosed space; it's open to the unfinished laundry/bath room, and the door to the room has a gap of about 1" to the concrete floor. If I stand just in front of the furnace, which is the same location to do laundry (just the opposite direction), the cold draft is noticeable even when the furnace isn't running.

Do I need to enclose the furnace in its own space to avoid this problem, or something else? I understand its purpose, but it seems to counter efficiency.

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Don't forget the combustion air is for all fuel burning appliances, not just the furnace. The water heater and possibly your clothes dryer also need combustion air. Yes, it is counter efficient, but not providing combustion air risks carbon monoxide poisoning. Not good!

One viable alternative would be to adequately heat the space, but that is certainly not efficient. It appears you could easily enclose the water heater and furnace in a utility closet. Be sure to determine the needed clearances and working access for each appliance and ensure they are maintained. AFAIK, the mechanical code often applicable in the US still requires two combustion air openings, one high and one low. You should verify your local requirements to ensure everything is correct. This is not the area to cut corners.

Even with the closet containing the cold air, if you have a gas clothes dryer, it still needs adequate combustion air. You would probably need some vents out of the closet to maintain this, but they can be relatively small, so the enclosure would still largely contain the cold air. Again, check your local requirements to determine the minimum vent area required for a dryer.

You would want to insulate the closet walls, and maybe even consider using an exterior door for access, as it will provide better insulation and weatherstripping to limit infiltration. OTOH, an undercut interior door might be adequate for dryer combustion air provision. You have a little research to do.

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Water heater is gas, and right next to furnace, so those could be enclosed together. Clothes dryer is electric. Gas range (upstairs) I wouldn't think relies on a combustion air that's so far away? –  JYelton Dec 30 '12 at 21:23
    
Gas ranges are not as strictly regulated because they are normally installed in areas that communicate with a large part of the house. The concern is mainly for appliances often placed in small closets. With an electric dryer, putting a closet around the water heater and furnace is the ideal solution for you. –  bcworkz Dec 31 '12 at 22:48

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