## 3 Results and Discussion
### Threshold Value of Daily Operation Hours Under Four Internal Wall Configurations  
It is obvious that the extent of building walls heated by indoor air is different under various heating durations, which will lead to different heating loads of the unit floor area per unit time. Therefore, the heating load index \(Q_{\rm i}\), as expressed in Equation (17), is introduced to analyze the characteristics of the heating load during the heating period. Also, the increasing rate \(\eta\), as expressed in Equation (18), is employed to compare the heating load index between intermittent heating mode and continuous heating mode. \[Q_{\rm i}=\int\limits_{0}^{\tau_{\rm o}}pd\tau_{\rm o}/(F\times\tau_{\rm o}) \tag{17}\]  
\[\eta=\frac{Q_{\rm i,m}-Q_{\rm i,c}}{Q_{\rm i,c}}\times 100\% \tag{18}\]  
Figure 8: Variation of inner surface heat flow \(q_{\rm i}\) of four internal walls with time: (a) \(C_{\rm r=1}\), (b) \(C_{\rm r=8}\). Figure 9: Variation of outer surface heat flow \(q_{\rm o}\) of four internal walls with time: (a) \(C_{\rm r=1}\), (b) \(C_{\rm r=8}\). where \(p\) is the running power of heating device. \(\tau_{\rm o}\) is the heating duration. \(F\) is the floor area of the investigated room. \(Q_{\rm i,m}\) is the heating load index for intermittent heating. \(Q_{\rm i,c}\) is the heating load index for 24 h continuous heating (\(C_{\rm con}\)). Figure 10 demonstrates the effect of intermittent heating duration on the heating load index \(Q_{\rm i}\) for four internal wall configurations, and the case of 24 h (\(C_{\rm con}\)) heating duration is also given. Figure 10 shows that \(Q_{\rm i}\) of intermittent heating is significantly larger than that of \(C_{\rm con}\). This is because the inner layer of walls release heat during heating cessation time, which leads to more energy being required to heat the inside layer of the walls at the next heating cycle for intermittent heating. Compared with Figure 10a-d, it can be found that the heating load index with the same heating duration drops in turn under the Wall 1, Wall 2, Wall 3, and Wall 4. The heating load index depends on the heat convection between the inner surface of walls and indoor air. Wall 1 gives the largest temperature difference between the inner surface of walls and indoor air. Therefore, the largest \(Q_{\rm i}\) is obtained by Wall 1. It can be seen in Figure 10a that the increasing rate \(\eta\) ranges from 31.58% to 76.35% for five heating durations. Moreover, the heating cessation time ratio of \(C_{\rm\tau=8}\) is 20%, while the increasing rate \(\eta\) is 31.58%, which is larger than 20%. This indicates that the daily heating load of heating time ratio 19.2/24 is larger than that of 24 h continuous heating and a threshold value of daily operation hour exists for intermittent heating mode. By comparing Figure 10a-d, it also can be seen that with various internal wall configurations, the increasing rate \(\eta\) is different. This will lead to various threshold values of daily operation hours. The daily heating load is calculated by Equation (19):  
\[Q=Q_{i}\times\frac{24\tau_{\rm o}}{\tau} \tag{19}\]  
where \(\tau\) is the hours contained in a heating cycle (see Table 3). Figure 10: Variations of \(Q_{\rm i}\) and \(\eta\) with operation cases: (**a**) Wall 1, (**b**) Wall 2, (**c**) Wall 3, (**d**) Wall 4. The variation of daily heating load (\(Q\)) with operation hour for internal wall configurations is shown in Figure 11.  
As illustrated in Figure 11, the daily heating load (\(Q\)) is increasing continuously at first and then decreasing linearly with the operation hour per day. Comparing the heating load of intermittent (\(Q_{\text{m}}\)) and 24 h continuous heating (\(Q_{\text{c}}\)), \(Q_{m}\) is larger than \(Q_{\text{c}}\) when the heating time ratio is 19.2/24 and \(Q_{\text{m}}\) is smaller than \(Q_{\text{c}}\) when the heating time ratio is 12/24. The threshold value of operation hours 18.04 h, 15.80 h, 14.59 h and 13.46 h, respectively, for Wall 1, Wall 2, Wall 3, and Wall 4.