I live in a 3 story townhouse with a garage in the basement. Off the side of the garage is our furnace room. The ventilation and exhaust for the furnace goes to the roof. For ventilation (air coming in) I have a straight which opens in the far corner of the Furnace Room, which the furnace sits in the middle of. Currently there is about 4ft of distance between the furnace and the ventilation pipe opening. The furnace room is almost airtight to the garage.

For another project,(to gain room in a closet on the third floor) I would like to move the ventilation pipe so that it opens into the furnace room directly over the furnace. I'd like to do this without cutting a new hole in my roof. So would I be creating any airflow issues if I put a bend into the pipe, and have it open within a foot of the foot print of the furnace. It would have 3 ft of vertical distance between the furnace and the intake.

If the distance is an issue. Could I put another bend in the pipe, in the furnace room, so that it would exit the ceiling directly above the furnace, but continue the pipe to near the same location as it currently exits the ceiling?

If I am over thinking this please tell me.

2 Answers 2


Your best bet is to refer to the owners manual of the furnace. The manual will tell you how long the vent can be and the maximum number of bends.

When it comes to C02 I get really nervous and venting is important. Call A pro. It should not be a hard job.

  • You are absolutely correct regarding furnace exhaust venting, but the OP is questioning the combustion air intake duct configuration. Carbon monoxide (CO, not CO2) escaping into the house IS a legitimate concern but not much of a risk related to modifications of combustion air intakes.
    – bcworkz
    Mar 14, 2013 at 22:22

In the US, the mechanical code is not very specific about allowance for combustion air. So many square inches of free area are required for every 1000 BTU capacity of the furnace. 1/2 of this should come in near the bottom of the enclosure and the other 1/2 near the ceiling, from independent ducts. The code is mute about horizontal arrangement or allowable bends, only that the duct configuration not be restrictive. If your air duct is generously sized, what you propose should be OK.

It may be that some other code I'm unfamiliar with only requires a single duct, or it's an often overlooked shortcut, because I've seen single duct combustion air provisions more than once. The important thing is to ensure the furnace has a copious supply of fresh air available that is highly unlikely to become obstructed.

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