Why 24°C

Restricting air-conditioning in government premises to no higher than 24°C should on paper be a no brainer and serves as an initial step towards achieving energy efficiency (and reducing CO2 emission). However, the fact that indoor thermal comfort is not solely dictated by the dry bulb temperature lends credence to lay questions and opinions expressed in the media since the announcement. Therefore, queries on why not 23°C or 26°C or even 28°C will need to be answered.

Let us start by understanding that a principal purpose of air-conditioning is to provide conditions for human thermal comfort, “that condition of mind that expresses satisfaction with the thermal environment” (ASHRAE Standard 55). The conscious mind appears to reach conclusions about thermal comfort and discomfort from direct temperature and moisture sensations from the skin, deep body temperatures, and the efforts necessary to regulate body temperatures. In general, comfort occurs when body temperatures are held within narrow ranges, skin moisture is low, and the physiological effort of regulation is minimised. Comfort also depends on behaviours that are initiated consciously or unconsciously and guided by thermal and moisture sensations to reduce comfort. Some examples are altering clothing, altering activity, changing posture or location, changing the thermostat setting, opening a window, complaining, or leaving the space. Surprisingly, although climates, living conditions, and cultures differ widely throughout the world, the temperature that people choose for comfort under similar conditions of clothing, activity, humidity, and air movement has been found to be very similar (Busch 1992; de Dear et al. 1991; Fanger 1972). 

In simpler terms, indoor (room) thermal comfort condition is dependent on various factors including air temperature, mean radiant temperature, humidity, clothing, metabolic rate and air movement preference of the occupant. Obviously, in this instance it would relate to government office premises with normal (sedentary) activities and light clothing as expected of office occupants. 

Since clothing and metabolic rate of occupants are not ‘controllable per se’ by mechanised air-conditioning, the three (3) main factors affecting room thermal comfort will be the dry bulb temperature, relative humidity and air movement (air velocity). 

In general, an individual feels comfortable when metabolic heat is dissipated at the rate at which it is produced. The human body temperature needs to be maintained at a constant 37 ± 0.5°C regardless of the prevailing ambient condition. The higher the space relative humidity, the lower the amount of heat the human body will be able to transfer by means of perspiration/ evaporation. If the indoor air temperature is high and the relative humidity is high (above around 11.5 g vapour per kg dry air), the human body will feel uncomfortable. 

Generally, the relative humidity for indoor comfort condition should not exceed 70%. 

Air movement (or air velocity) is essential for bodily comfort as it enhances heat transfer between air and the human body and accelerates cooling of the human body. Air movement in an occupied space gives a feeling of freshness by lowering the skin temperature, and the more varied the air currents in velocity and direction, the better the effect. A draught is created when the temperature of the moving air is too low and/or the velocity is too high. At normal comfort room temperature of 23 to 26°C, the acceptable air velocity would be in the region of 0.15 to 0.50 m/s. It is important to bear in mind that in a hot and humid climate, a higher air velocity is needed as compared with a temperate climate locality with low humidity. For the latter, the acceptable air velocity is in the region of 0.15 to 0.25 m/s which will elicit endless complaints if applied in Malaysia.