The time-scale of thermal comfort adaptation in heated and unheated buildings

Air conditioning technologies (HVAC) have dramatically shaped today's building environment. However, the mutually dependent relationship between the indoor climate experience and occupants’ thermal comfort adaptation rarely been given attention. To investigate the dynamic processes of adapting to different indoor climates, two comparative thermal comfort field studies were conducted in China, where the northern cities such as Beijing (with district heating) have much warmer indoor temperatures than the southern cities such as Shanghai (without district heating) during the winter. The results indicate that occupants’ thermal comfort adaptation exhibits asymmetric trajectories. Building occupants can raise their expectations much faster to accept a neutral indoor climate than they can lower their expectations and acclimate to underconditioned environments. The northern groups took 3 years to adapt to cold indoor temperatures while the southern groups accepted neutral and warm indoor temperatures in less than 1 year. These dynamic processes are well reflected by the ‘demand factor’ metrics. Furthermore, we discussed the practical implications of the asymmetric nature of building occupants’ thermal adaptation. It can not only help elucidate the underlying principles behind adaptive comfort phenomena but also help guide building design and operation.