Energy and Buildings 37 (2005) 49–54 Space temperature difference, cooling coil and fan—energy and indoor air quality issues revisited S.C. Sekhar ∗ Department of Building National University of Singapore, 4 Architecture Drive, Singapore 117566, Singapore Received 10 March 2004; received in revised form 26 April 2004; accepted 6 May 2004 Abstract In designing an energy-efficient air-conditioning system that also simultaneously addresses the needs of adequate ventilation and acceptable indoor air quality, several factors begin to play an important role. Among several others, the cooling coil, the fan and the temperature difference between the space and the supply air (commonly known as the Space T) can be considered to be crucial. For a given space cooling load, the choice of a particular Space T has an implication on the amount of supply air required, which further has an impact on the performance of the cooling and dehumidifying coil as well as the fan. Inherent in these implications are issues related to energy, ventilation and indoor air quality. This paper investigates these implications and quantifies them by considering a hypothetical building in a tropical climate and subjecting the design to several parametric variations involving different Space Ts for a given space temperature and humidity condition. The total power requirements, comprising additional cooling, reheating and higher fan power, for a design involving a Space T of 5 ◦ C can be as high as a factor of 2.2 of the total power for a design with a Space T of 8 ◦ C. The implication of higher supply air flow rates on duct design is qualitatively discussed. For a given space cooling load and a given Space T, the implication of increased design ventilation rates to address part-load ventilation requirements can lead to an additional installed cooling capacity of 17.5%. Finally, emerging technologies that are aimed at addressing both energy efficiency and IAQ are discussed. © 2004 Elsevier B.V. All rights reserved. Keywords: Cooling coil; Supply air temperature; HVAC system; Energy; Ventilation; Indoor air quality 1. Introduction The optimal design of an air-conditioning system and its successful implementation and operation in a building is obviously a great challenge, especially when both energy and indoor air quality (IAQ) issues are of equal importance. It is, indeed, the designer’s knowledge and understand- ing of the dynamically varying system characteristics as a dynamic response to the inevitably varying cooling load characteristics that will ultimately prove to be crucial in the design of an optimal and successful system. Often, some of the fundamental principles in psychrometrics are likely to be either overlooked at design or accepted as limiting constraints in current state-of-the-art technology that lead to designs, which may be considered as the best compromise. Among several factors concerning energy consumption and IAQ that are intertwined in conventional design practice, it is often economics, and particularly, the capital cost, that become the governing criterion in the selection of a design. ∗ Tel.: +65-874-3479; fax: +65-775-5502. E-mail address: bdgscs@nus.edu.sg (S.C. Sekhar). In recent times, the need for a life cycle costing approach towards the selection of air-conditioning and air-distribution systems is strongly advocated, considering that the eco- nomic life span of such systems are reasonably long and are, typically, in the range of 10–15 years [1–3]. This pa- per revisits some of the fundamental thinking in the design considerations of two of the most important and critical components in an air-conditioning system: • Cooling and dehumidifying coil • Fan Having estimated the cooling loads in a building, the volume flow rate of air supplied to the various zones is then a function of the space dry bulb temperature (DBT) and the supply air DBT. The difference in these temperatures, called the Space T, is a critical design parameter as it has implications in terms of both the cooling coil capacity and its performance and the amount of supply air necessary for handling the space cooling loads. The implication includes both energy and IAQ and is prevalent throughout the entire operating range of the air-conditioning system. For exam- ple, a small Space T for a given space DBT would not 0378-7788/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2004.05.003