Comparison of semi-quantitative and formal metrics for multi-zone airflow model quality assessment

Multi-zone air and contaminant flow models of actual buildings have many practical and scientific uses, such as estimating air leakage rates, predicting exposure to contaminants, and studying ventilation controls to name only a few. A multi-zone model is an abstraction that may predict performance that differs from observed behavior of an actual building due to inaccuracy or uncertainty in inputs, (e.g., envelope leakiness, and mechanical ventilation rates) or to non-uniform airflow and contaminant distributions in spaces. To ensure close agreement between model and actual building, a procedure for calibrating (equivalently, tuning) the model by adjusting model parameters based on measurements in the actual building is needed. A method for calibrating multi-zone airflow models using a semi-empirical metric (defined as the fraction of correct interzonal airflow directions) has been subjected to a number of field tests with encouraging results, i.e., the metric value increased as a result of calibration. Given the ad hoc nature of this metric, it is important to investigate whether its improvement is corroborated by a more rigorous standard of quality. At present, ASTM Standard D5157 (Standard Guide for Statistical Evaluation of Indoor Air Quality Models), which defines model quality on the basis of predicted and measured concentrations, is available for that purpose. In the present study, model calibration using the interzonal airflow direction metric and ASTM Standard D5157 evaluations of CO ₂ tracer gas releases at several measurement locations were conducted in two test buildings on a university campus. ASTM Standard D5157 results showed measureable improvement in model quality, which suggests that the semi-empirical metric is useful for its intended purpose. A secondary result of the study was the recognition that ASTM Standard D5157 in its current form is difficult to interpret when applied to whole buildings with multiple spaces. Consideration should be given to making ASTM Standard D5157 more suitable for evaluating complex buildings.