Field measurements of interaction of mechanical systems and natural infiltration.

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. The induced infilmtion is one-half of the unbalanced fan flow if the unbalanced flow is less than twice the natural infiltration rate. Otherwise, the induced flow is the difference between the fan flow and the natural infiltration. The model is supported by detailed flow and pressure measurements made on a home in the Pacific Northwest during winter conditions. The home had a whole-house multiport exhaust ventilation system as well as several other exhaust fans. The measured flow through the multiport exhaust system was 35 L/s. When running, this system induced 17 L/s, in good agreement with the fan-model predictions.