Improving HVAC performance is of paramount importance to sustain passenger comfort in a car. Numerical analysis of a three-dimensional model predicts a detailed description of fluid flow and temperature distribution in the passenger compartment and on the inside surface of the windshield. During the winter, the highest temperature is observed on the lower part of the windshield and in the vicinity of the defroster nozzle. Defrosting performance was excellent when the injection angle of the defrost nozzle was between 15 to 25 degrees with a standard distance ratio of one. During the cooling period, it was found that the temperature and humidity decrease to a comfortable range almost linearly at the initial stage. The numerical predictions are in good agreement with the experimental results.