The objective of this work is to evaluate the possibility of ensuring indoor temperature and relative humidity in an acceptable range by controlling heating and ventilation associated with minimising the energy consumption or cost by the use of a numerical simulation program. An acceptable indoor range means that indoor temperature is 20 degreesC with small fluctuation and relative humidity is in the range of 20-70%. A simple heating and ventilation control regulation is proposed. Simulation examples are conducted for different types of buildings based on energy consumption or cost factor. Simulations demonstrate that the indoor condition is much more improved and significant energy consumption or cost can be reduced by using the control regulation developed in this paper.