This paper describes how a multi-zone air flow simulation program has been used for the evaluation of the performance of a hybrid ventilation system in a Swedish school. The idea was also to determine whether it is feasible for an HVAC consultant to apply a multi-zone air flow simulation program in a straight forward way. Before the analysis the tool was used to simulate the present state i.e. checked against measured data. First the checked model was used to evaluate the sensitivity of the system to wind fluctuations, then to study air flows as a function of temperature and finally to evaluate the performance of the solar chimney. Multi-zone air flow simulations can be very useful for the design of hybrid ventilation and the chosen program can most likely be used by competent HVAC designers.