The accurate techniques of heating loads calculations are essential pace for equipment selection, system sizing and system design. With the help of getting the accurate data of heating loads and seasonal heating demands, the energy sources design for buildings become more effective. The present study highlights the dependence of heating loads values on the thermal properties of buildings envelopes; hence, a modified method of calculating the heating loads values and seasonal heating demands of residential buildings is developed mathematically. The present results are compared with ASHRAE standards. The results show that the data obtained from the present method are more accurate and effective than compared results. Moreover, it proves that the duration of heating seasons for each building even in the same climatic conditions are different. The modified method will open a new horizon in the field of heating system to provide accurate calculations of heating loads for many applications.