A hybrid ventilation system, composed of a natural supply inlet and mechanical exhaust, was predicted numerically for an apartment. Analysis was performed using Computational Fluid Dynamics (CFD) for ventilating flow rate of $60m^3/h$ for the living room-kitchen region and $180m^3/h$ for the whole apartment region. The heating period chosen in this study reflects how residents are usually exposed to poorer indoor environments in winter. An effort was made to create acceptable residential comforts regarding air current, temperature and $CO_2$ concentration distributions. The results show that an ACH of 0.7 or greater seems to be a reasonable value for the permissible minimum ventilation flow rate in occupied zones. The results also show that for a ventilating flow rate of $60m^3/h$, some acceptable criteria are satisfied and residents achieve comfort. In case of the whole region with $180m^3/h$ and 288K, however, residents no longer feel as comfortable with regard to thermal conditions and energy losses occur.