Rumford Technical Discussion
Exterior Air
Sometimes called "outside air" or "combustion air"

Here is the International Residential Code on exterior air:
R1006.1 Exterior air. Factory-built or masonry fireplaces covered in this chapter shall be equipped with an exterior air supply to assure proper fuel combustion unless the room is mechanically ventilated and controlled so that the indoor pressure is neutral or positive.
Combustion air ducted from outside directly into the firebox is inadequate at best, does not improve efficiency, can lead to bad smells and blowing ashes, can cause the fireplace to smoke and, in some cases, if it can back draft, it can be a fire hazard. For the last reason, it's a controversial code issue.

So the best solution is to make sure the room is "mechanically ventilated and controlled so that the indoor pressure is neutral or positive" and then don't provide exterior air specifically for the fireplace at all, as code permits. A make-up air system installed in the mechanical room is often the best way to do this.

We recommend not installing combustion air directly into the firebox unless you are required to. Section R1006.5 allows the vent to be within 24" of the firebox opening but some officials insist on the vent in the firebox if glass doors are installed. If you are forced to install exterior air, the most unobtrusive way to do it is with the Outside Air - Ash Dump Kit set in the inner hearth. Then keep the vent closed to avoid the bad smells, blowing ashes, smoky fireplace and fire hazard.

Now, that doesn't absolve you of providing a balanced ventilation system. In fact the code implies as much when it exempts exterior air if you have neutral or positive indoor air pressure. Ideally you should provide as much clean, tempered outside ventilation air as you exhaust through various appliances, fans and leaks. See Balancing the Ventilation System.

Related discussions at Exterior air code issues and smoky fireplaces and Balancing the Ventilation System

Exterior Air Caused Fireplace to Smoke
Exterior Air Set the Siding on Fire
Wolf head Air intakes
Why is such a bad idea required by code?

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