Why burn outside combustion air?

Q: I can understand how a [vent free] gas stove vented into the house would reduce the oxygen level, as it would consume the oxygen, then vent the exhaust into the house. I can’t see how a wood stove could reduce the oxygen level, as it takes in air from the house then vents it out of the house. The only way it could reduce the oxygen level would be if the house was so tightly sealed that it actually reduced the air pressure in the house. I am not sure if this is even possible. Let me know your comments. Maybe I am missing something.

A: A wood fire consumes a VAST amount of oxygen, and the chimney updraft simultaneously vacuums a HUGE amount of both burned and unburned air out of the house. Meanwhile, return air must squeeze in through tiny openings around doors, windows, etc.. If the house doesn’t present enough openings to the outside atmosphere to allow static pressure stabilization, the atmosphere inside the house will remain at a lower pressure (and a lower oxygen level) than the atmosphere outside the house as long as the fire is burning. The tighter the house, the more pronounced the effect.

We have witnessed a variety of examples of household depressurization over the years. It is very common to hear complaints of a cold draft whenever there’s a fire in the wood, gas or oil stove, or that the furnace kicks on every time the fireplace is lit.

Several years ago, a local contractor built one of the first “super energy efficient” house in our area. The house was of block construction, and incorporated plastic vapor barriers, gasketed windows and doors, etc. He put a woodstove in that house (not vented to outside combustion air), and the eventual buyers found that, unless they opened a window, the vacuum effect produced by the wood fire would actually overcome the chimney updraft, pulling woodsmoke backward into the house through the draft controls on the stove!

While it is true that most older homes aren’t nearly as tightly constructed as that one was, it is equally true that most homes aren’t leaky enough to create neutral pressurization when a fire is going.

There is another advantage to burning outside combustion air; an increase in heating efficiency. While the fire is consuming room air in Winter months, the replacement air being drawn in through cracks around doors, windows, etc. is COLD.

If outside combustion air is provided for the fire, the opposite happens: heated air expands, so the house becomes positively pressurized, which tends to carry heated air TOWARD all the cracks, helping to distribute the heat throughout the house.