Effects of constant and fluctuating temperature modes of foot heating on human thermal responses in cold environments

Heating demand has increased dramatically in winter in southern China. Foot heating pad, as a personal comfort system device enables to improve occupants' thermal comfort in winter while consuming less energy. However, the research on the heating modes of these devices remains insufficient. The effects of fluctuating foot heating on thermal responses were compared to those of traditional constant heating in this study. A climate chamber experiment involving 16 subjects was conducted to assess physiological and psychological thermal responses under constant and fluctuating foot heating modes with equal total heating energy. Skin temperatures and thermal perception votes at three indoor temperature conditions of 8, 11, and 14 °C were collected and analyzed. The results indicate that the investigated parameters were significantly improved by foot heating, with foot skin temperature rose by 7 °C, foot thermal sensation rose by 2.7 levels, and overall thermal comfort rose by 0.4 level at least. Constant heating can improve skin temperature and thermal sensation to a higher extent than fluctuating heating in cold environments, especially when ambient temperature was lower than 12 °C. However, heating the foot alone cannot effectively improve overall thermal comfort in a cold environment below 12 °C. With ambient temperature between 13 and 17 °C, fluctuating heating can better avoid local overheating and ensure thermal acceptance rate larger than 90%, and fluctuating heating with high frequency and small amplitude is better than that with low frequency and large amplitude. Thus, the optimal fluctuating foot heating mode was recommended to be applied in cold environments higher than 13 °C.