The majority of residential buildings worldwide are ventilated through natural ventilation, infiltration and exhaust fans. Up to 30% of space heating and cooling load are due to air infiltration. Air infiltration is a nonlinear phenomenon which is dependent on the air leakage of the building envelope and the weather-driving forces (wind and indoor-outdoor temperature difference). An exact calculation of infiltration for a real building is very difficult (if not impossible) as it requires a great many details of the building, its environment, the driving forces and occupant behaviour. Approximate techniques from simple to complex have been developed. Most of the widely used whole building energy simulation tools employ simplified approaches in considering the impacts of wind on infiltration and some do not consider wind impacts at all.
The infiltration model currently used in AccuRate employs a simple approach. The infiltration rate, in air changes per hour (ach) for each zone, is specified as A+B·U, where the two constants A and B are used to account for the stack and wind infiltration factors respectively, and U the local wind speed. Two issues with this simple infiltration model are:
To potentially improve the simple model, the single-zone model, which considers both wind and stack effect with a superposition approach described in ASHRAE 2009, was adopted to adjust the constants A and B using blower-door test results. The recently developed multizone infiltration model, in which air infiltration rate through a crack is defined by a power law (Ren and Chen, 2014), was also implemented to evaluate a potential improvement to the infiltration model in the Chenath engine.
The differences of space heating and cooling loads between the multizone model and the two currently used simple models are illustrated in the figure below. It shows the predicted cooling load variations between the multizone model, the simple approach and the modified simple approach in Cairns, a cooling dominated climate.
The multizone, integrated model simulating airflows through large openings and cracks simultaneously is the state-of-the-art method for simulating indoor air flow. For interested readers, two papers are referenced, which include detailed conclusions. This model has been implemented in the research version of Chenath for evaluation for future updates to the software.
Figure 1: Comparisons of the annual space cooling loads in Cairns predicted by the three models