Center for the Built Environment

Many experimental studies demonstrated that elevating indoor temperatures to 28 or 30°C is comfortable if fans are used, while this elevation often is not applied in practice. This study aims to investigate AC thermostat use behaviors when elevated air movement is available, as well as to provide insights at guiding occupants to select warmer environments without reducing their comfort. 20 subjects participated in a series of tests at three initial ambient temperatures (26, 28, and 30°C) with different settings of an AC thermostat and a ceiling fan: (1) a free-controlled thermostat without a fan, (2) a free-controlled thermostat with a free-controlled fan, (3) a free-controlled thermostat with an always-on fan (controllable but not allowed to turn off), (4) a limited thermostat (no lower than 28°C) with a free-controlled fan, and (5) a limited thermostat with an always-on fan. The results show that without the ceiling fan, subjects used AC to lower indoor temperatures to 25.7°C. When the fan was available (free-controlled), they did not only rely on lowering the AC set-point alone but also turned on the fan immediately when starting using AC. This behavior ended a final ambient temperature up to 27.4°C. Under always-on fan conditions, the fan was running at a high speed at the beginning of using AC. Although subjects turned down the fan speed, the final fan speeds were about doubled the speeds as compared with free-controlled fan conditions. This higher initial speed by the always-on fan made subjects select higher indoor temperatures: at the 28°C initial temperature, there was no further lowering of ambient temperatures; at the 30°C initial temperature, the final temperatures were 28.6 and 28.9°C for the free-controlled and limited AC test conditions, about a 3-K set-points extension compared with the no-fan condition. Further, despite different conditions, more than 90% of subjects had comfortable feelings, which indicates that people with adjustable AC prefer a constant thermal perception rather than a constant ambient temperature. Therefore, the strategy of using fans before AC should be adopted to guide people actively to select higher AC set-point temperatures.