Should woodstoves and fireplaces have an outdoor air supply, or should they draw on indoor air? I've heard both.
Back in the 1980s, I was one of those people who promoted outdoor combustion air, but that was before anyone had seriously studied the issue. Since then, testing done under controlled conditions has shown no advantage, and has revealed some disadvantages. Nevertheless, some jurisdictions require outdoor combustion air supplies.
Three reasons are often given for supplying outdoor air:
• To prevent the wood heater from “starving for air” in a tight house
• To prevent depressurization of the dwelling that might affect other fuelburning devices
• To save energy by not using warmed room air for combustion
However, as several studies have demonstrated, none of these reasons stands up to scrutiny.
The main issue is that the air consumption of a woodstove is only a small part of a much larger exchange of air between the house and the outdoors. The average air consumption of a modern wood heater is about 10 to 25 cubic feet per minute (cfm). Even a tightly sealed, 1,500-squarefoot home would have an absolute minimum air change rate of 66 cfm, and the rate for most houses would be far higher.
On the question of saving energy, remember that there’s no free lunch. In other words, outdoor air will have to be heated one way or another, either by the fire, or by seeping in through leaks and being warmed by your space-heating system. Regardless, if your local building code requires an outdoor air supply, it must be installed.
However, I have seen a few cases in which wind pressure around a house caused hot exhaust to be sucked back outside from the stove or fireplace through the combustion air duct. If your wood-heating appliance malfunctions in windy weather, disconnecting the outdoor air supply may fix the problem.
— John Gulland, www.woodheat.org