## 1 Introduction  
Residential heating issues in the hot summer and cold winter (HSCW) climate zone of China have attracted increasing attention for the low outdoor temperature and absence of district heating [1; 2; 3]. The HSCW climate zone of China, located near the lower reaches of the Yangtze River, is one of the most economically developed regions, and has the highest population density in China. The mean temperature of January is between 2 \({}^{\circ}\)C and 7 \({}^{\circ}\)C, which is about 8 \({}^{\circ}\)C lower than other places of the same latitude in the world [4]. According to Chinese heating policy [5], central heating systems are not provided in the HSCW zone. To improve the poor indoor thermal environment, more than 90% of households have been equipped with individual air conditioners for heating. The survey performed by [PERSON] [6] and [PERSON] [7] showed that the operation of heating devices in the HSCW zone is intermittent, and occupants run heating devices only in the room they are using[PERSON]  
For intermittent heating mode, envelopes release heat during the heating cessation period. Therefore[PERSON] more heat is absorbed a stored by building walls during the heating period, while it is not required for continuous heating mode. For this reason, the heating load of the unit floor area per unit time for intermittent heating mode is larger than that of continuous heating mode. The situation where daily heating load of intermittent heating is larger than that of continuous heating may occur, which leads to a threshold value of daily operation hours[PERSON] [PERSON] and [PERSON] [8] investigated the effect of heating duration on energy consumption under two residential buildings with different levels of insulation. The results showed that the amount of energy reached a peak value for heating durations longer than 18 h. Slightly larger threshold values of heating duration of about 20 h were obtained by [PERSON] [9] for a local residential building in Jordan with several levels of insulation thickness. However, the main objective of those studies [8; 9] was only to investigate the dynamic thermal performance of external walls. In the HSCW zone, usually just one room in the house is heated at a given time, which leads to the temperature difference between indoor air and the adjoining room air being slightly lower than that between indoor air and outdoor air. Moreover, the inner surface area of the internal walls is approximately 5 times larger than that of the external wall. Therefore, the effect of dynamic thermal performance of internal walls on the threshold value of daily operation hours need to be analyzed. Much research has been performed on the dynamic thermal behavior of walls for intermittent heating mode. [PERSON] [10] investigated the effect of thermal resistance and heat capacity as well as thermal constant on the heat exchange through the building envelope by implicit finite-difference method. [PERSON] [11] and [PERSON] [12] analyzed the temperature response rate and the heat flow of different wall insulation forms under intermittent and continuous operation in summer by simulative and experimental method, respectively. [PERSON] et al. [13] analyzed the effects of insulation and thermal resistance on dynamic heat transfer of building walls by combining mathematical models and numerical solutions. Their results showed an evident process of heat storage and release of walls was found for intermittent heating mode, while the key points of those studies are still the heat transfer of external walls. The thermal performance of internal walls plays a vital role in affecting the threshold value of daily operation hours, but little research has been carried out. According to the above problems, the aim of this study is to explore the effect of internal wall configurations on the threshold value of operation hours for intermittent heating mode.