Air temperature thresholds for indoor comfort and perceived air quality

Air temperature thresholds are investigated and proposed for acceptable comfort in air-conditioned buildings. Using the ASHRAE database of field studies in which acceptability votes were obtained from occupants, it is shown that within the thresholds the acceptability is indistinguishable. Therefore, there is little gain from conditioning spaces to an 'optimum' air temperature, a practice that involves significant energy cost. However, beyond the thresholds there is a significant drop-off in acceptability. Ideally, air-conditioning would be used only when the environmental conditions are beyond the thresholds. The use of ceiling fans or personal environmental control systems broadens the threshold range. Thresholds are determined for both air-conditioned and ventilation-cooled buildings in the database. The equally acceptable range between the thresholds is 8-10 K in both types of buildings. It is possible that a perception of reduced air quality in warm environments could impose an upper temperature threshold. Perceived air quality is examined in two laboratory studies with air temperatures ranging from 18 to 30°C. Perceived air quality is seen to be closely correlated to thermal comfort rather than to temperature; as long as thermal comfort is maintained by air movement, perceived air quality will be acceptable. Relationships between temperature thresholds and productivity, operating set-points, and energy use are also discussed.