Including furnace flue leakage in a simple air infiltration model.

Although there are many simple infiltration models already available none of them have an appropriate method of dealing with what is often the single largest leak in a building; a furnace or fireplace flue. Flues are different from the distributed leakage used in simple in­ filtration models. Flues represent 10% to 30% of the total building leakage all of which is concentrated at one location above the ceiling height. The flue top is often unsheltered when the rest of the house is shel­ tered from wind. Because the flue is filled with room air most of the time this leads to an increased stack effect. The pressure-flow exponent, n, for a furnace flue is about 0.5 rather than the value 0.6 to 0.8 typical of the rest of the building leaks.