Abstract: Slow heating up during the start-up phase due to the insufficient heating capacity of the heat pump is one of the main causes of indoor thermal discomfort. Determining the most unfavorable start-up conditions for air conditioners is the critical to ensuring their rapid heating capacity and enhancing thermal comfort during the start-up phase. Given that, a numerical thermal model of typical urban residential buildings in Beijing and Nanjing is established in this study. Subsequently, the most unfavorable start-up conditions of heat pumps for bedrooms and living rooms under periodic heating conditions as well as long-term unheated conditions in two typical cites are analyzed. The results suggest that periodic heating has obvious alleviating effect on the most unfavorable indoor start-up condition compared with the long-term unheated condition.

Key words: Heat pump air conditioner, Starting condition, Periodic heating, Numerical simulation