combustion air intakes sounded like a good idea on paper so they were written into the codes in response to a widespread rise in smoking fireplaces. Afterall, if you have a tight house, then introducing outside air should cure the problem, right? Here is some basic info. to ponder:
The typical 36"x29" open hearth exhausts about 400-600 cfm, which includes combustion air( for the chemical oxidation of fuel) and excess air (goes along for the ride). You typical 4' air intake kit with one 90 degree ell and short run with no kinked hose at a 5 Pascal pressure gradient can flow about 12-15 cfm on a good day. On a bad day, when the inlet is under negative pressure, say as when on the leeward side of the home, it can actually backdraft. Dale has a video of one with flames shooting out with wind gusts! Now you know why the IRC required a metallic hose with a 1" clearance to combustibles on an air intake....

These air kits can be configured several ways. Some draw air from the ash pit, which may be totally sealed waiting on Geraldo to open it, have an iron door to the inside, and iron door to the outside, or one of these kits that replaces the outside door with a mesh/ louver device for admitting outdoor air. You'll see other brands on air kits that are nothing more than a 3" iron pipe with a very sloppy sliding door. These are either mortared in during construction or installed using a core drill. They introduct cold air right into the fire cooling and distorting the flames. One gas log mfr. I spoke with is thinking about recommending sealing these kits when their logs are installed because it blows out the pilot and causes sooting. At standby, they allow gobs of cold air to infiltrate.

Here is my recommendation on air kits: While the code requires their installation, the code cannot and does not require their use. Fix the house and the fireplace will work. Balance the house and it will draw sufficient warm air from the room without all these negatives. Better yet, replace the fireplace with a direct vent or power vented appliance such as a gas or pellet insert.

HTH

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disgusted with some people on this forum. Out of here!