Single-Zone Stack-Dominated Infiltration Modeling

Abstract

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. This report derives an exact, but practical, expression for calculating the stack effect from the air densities and leakage distribution using the power law formulation of envelope leakage. The neutral height -- the height at which there is no stack-related indoor-outdoor pressure difference -- is a key intermediate in stack modeling. This report defines a computable parameter called stack height, which contains all of the leakage distribution information necessary for estimating stack flows, thus freeing the model from specific assumptions (e.g., that the leakage is separable into evenly distributed floor, wall, and ceiling components). Example calculations including comparisons with other models, as well as validations using measured data from dwellings, are also presented. The dimensionless neutral level, which ismore » related to the neutral height, is often used as an indicator of leakage distribution and in superposition. Its definition and role in these regards are discussed in detail. The more exact formulation is then used to analyze the simple box cases normally assumed in infiltration modeling and other approximations. Measured ventilation data will be used to infer leakage distributions and neutral levels as well as for example calculations.« less