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.