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Air infiltration modelling (June 2016)

Air infiltration modelling

Air infiltration is the unintended flow of air into a building through the cracks in the structure due to pressure differences across the building envelop caused by wind and stack effects resulting from the inside-outside temperature difference, and internal mechanical ventilation systems. With the improvements of window products and building insulation, air infiltration is becoming an increasingly important issue for building energy efficiency. Now it is widely accepted that it is necessary to determine the thermal load resulting from air leakage into the building, especially for residential buildings as the vast majority of them worldwide are ventilated through natural ventilation, infiltration and exhaust fans.

To improve modelling infiltration, a multizone infiltration model was recently developed. In the multizone air flow model (Ren and Chen 2014), a building was treated as a series of well mixed zones that were interconnected through air flow paths, which include open or closed windows/doors, ducts, leakage through walls, joints or other cracks. Air infiltration rate through a crack was represented by a power law equation: Q = k(Δp)n  where Δp is the pressure difference across the crack, k and n are the air leakage coefficient (m3/s at 1 Pa) and the pressure exponent of the crack respectively.

Mass balance equations were applied to each zone of the building to solve for interior static pressures in the building and then the air flow rates through the flow links.

The multizone air flow model has been validated against theoretic analysis and blower-door test results (see Fig. 1 below). The sensitivity analysis of k and n was also conducted. The comparisons between the multizone model and the two current used simple models show the new model can improve modelling accuracy for energy performance simulation. The model and case study are published in two journal papers (Ren and Chen 2014; Ren and Chen 2015). Further improvement in simulation speed is required for the new infiltration model to be implemented in future formal release.

Chart
Comparison of infiltration rate at 50 Pa between the model and blower-door test (ACH: air changes per hour)

References

Ren Z. and Chen D. (2014) Estimation of air infiltration for Australian housing energy analysis. Journal of Building Physics, 39: 69-96.

Ren Z. and Chen D. (2015) Simulation of air infiltration of Australian housing and its impact on energy consumption. Energy Procedia, 78:2717-2723.