## 4 Conclusions  
The relatively low outdoor temperature and absence of district heating leads to the intermittent heating operation mode in the HSCW climate zone of China. More heat is absorbed by building walls during the heating period due to the heat release and storage of the internal wall, while it is not required for continuous heating mode. Therefore, the heating load of the unit floor area per unit time for intermittent heating mode is larger than that of continuous heating mode. This means that the daily heating load of intermittent heating will be larger than that of continuous heating, and a threshold value of daily operation hours occur for intermittent heating.  
In the present work, a three-dimensional simulation method is established to study the heating load of buildings with different intermittent heating duration and configuration of internal walls. The threshold value of daily operation hours is analyzed under four configurations of internal walls by comparing the heating load of continuous and intermittent heating. Thorough experiments on temperature distributions of thermal mass are carried out to validate the simulation model.  
In the intermittent heating room, the temperature of building envelope increases by storing energy during heating period, and the inside layer cools to a lower temperature during heating cessation period. As a result, more energy is used to heat the inside layer of the walls at the starting time of the  
Figure 11: Variations of daily heating load (\(Q\)) with operation hour.  
heating cycle. This leads to a lager heating load index for intermittent heating system than continuous heating system with increasing rate ranging from 31.58% to 152.63%.  
By comparing the daily heating load with operation hours, it is found that the daily heating load increases to a peak value as the operation hours increase, and then starts to decrease. This indicates that the threshold value of operation hours is 18.04 h, 15.80 h, 14.59 h and 13.46 h for four internal wall configurations, respectively. Moreover, with the increase of insulation level of internal walls, the optimum heating duration decreases. In addition, the results indicate that it is more economical to use continuous heating when the daily operation hours are more than the threshold values.  
It should be noted that the threshold value of daily operation hours given in this paper is obtained according to the present situation of heating in the HSCW zone. However, the heating duration per day and the amount of heated rooms are increasing with the rapid growth of the economy and continuous increase of disposable personal income. This leads to a higher average air temperature of adjoining rooms; thus, a smaller threshold value of daily operation hours is obtained, which will be discussed in future works. Moreover, just four configurations of internal wall commonly used in the HSCW zone are investigated in this paper, and the threshold value of daily operating hours varies for other configurations of the internal wall.  
Conceptualization, S.W. and K.Z.; Formal analysis, S.W.; Funding acquisition, K.Z.; Methodology, S.W. and K.Z.; Writing--original draft, S.W. and K.Z.; Writing--review & editing, S.W.  
This research was funded by National Natural Science Foundation of China (Grant No. 51478098) and the Fundamental Research Funds for the Central Universities (Grant No. CUSF-DH-D-2017098).  
The authors declare no conflict of interest.