Simplified, physical models for calculating infiltration in a single zone, usually calculate the air flows from the natural driving forces separately and then combine them. For most purposes-especially minimum ventilation or energy considerations-the stack effect dominates and total ventilation can be calculated by treating other effects (i.e. wind and small fans) as perturbations, using superposition techniques. The stack effect is caused by differences in density between indoor and outdoor air, normally attributable to the indoor-outdoor temperature difference.
Mechanical devices such as exhaust fans and air handlers interact strongly with natural infiltration. In the past, the effects of mechanical systems have either been treated separately from those of natural infiltration or have been combined using simple models. This paper extends the theory of the interaction of unbalanced mechanical systems with stack-driven infiltration. The effects of leakage distribution and the flow exponent on fan efficiency are discussed. A simple model for combining the two effects is presented and compared with two previously proposed models.
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