Study on the Effectiveness Rule-of-Thumb Method in Sizing of Air-Conditioning Systems

The sizing of air-conditioning systems for a building is usually performed by consultants and contractors in order to counter the heat gain within the occupied areas so that thermal comfort can be achieved and at the same time optimizing the related capital and operational costs. The tasks of system sizing mainly involve tedious steps requiring solving of simplified heat transfer equations using database of local weather conditions. It is not uncommon that the Rule-of-Thumb approach is used, which involves simple proportioning between space area and the cooling energy. Nevertheless, such method may seem to be very crude and not reliable in ensuring acceptable thermal comfort and costs. The objective of this paper is to study the effectiveness of using Rule-of-Thumb in the sizing air conditioning systems. The study mainly involves comparison of the equipment load resulted from the Rule-of-Thumb method and that using cooling load temperature difference (CLTD) method for several types of occupied areas, namely a mosque, and academic, residential and office buildings. It is found that the Rule-of-Thumb method is not always appropriate in determining cooling loads of air-conditioning system. Based on the present study, most of the buildings are slightly over designed by an average of 44%, except for mosque which is under designed by 16% in comparison to the CLTD calculation method.