## 4 Results and Discussion
### Comparison between Reversely Variable and Reversely Fixed FTHXs  
Then, the reversely variable FTHX was tested in the ASHP and compared with a reversely fixed FTHX. Figure 6 shows that the reversely variable FTHX exhibited four circuits in the evaporator mode but 1.5 in the condenser mode. Hence, two types of reversely fixed FTHXs were taken into consideration, i.e., the four-circuit one and the 1.5-circuit one.  
For an easy execution of the comparative tests, the single-way valves in Figure 6 were actually replaced with check valves, and the three circuitries could be obtained with different on/off schemes of the two check valves, as reported in Table 5. The four-circuit configuration of the FTHX was realized with both check valve always open, while the 1.5-circuit was built with the two check valves always closed. The variable circuitry, however, was obtained with the two check valves open in the evaporator mode but closed in the condenser mode. Moreover, for easy installation and uniform feeding, the four parallel circuits of the FTHX in the reversely variable circuitry were modified slightly, as shown in Figure 6. The top two inlets in the evaporator mode were close to each other, and the bottom two outlets were connected together with a T-junction. Then, the FTHXs in the three schemes were tested in both heating and cooling conditions.  
Figure 8 illustrates the energy outputs of the ASHP with the above three FTHXs. Note that the four-circuit reversely fixed configuration was obtained with both check valves always open in the prototype; the heating outputs of the four -circuit reversely fixed FTHX were actually the same as that of the reversely variable one in Figure 7. Similarly, the cooling outputs of the 1.5-circuit reversely fixed FTHX were also the same as those of the reversely variable one.  
Figure 8 indicates that the reversely variable circuitry had better overall performance compared to the other two. In the rated cooling conditions, the reversely variable FTHX resulted in 6.1% higher capacity and 7.6% higher COP of the ASHP than the four-circuit reversely fixed one. In the rated heating conditions, by contrast, the reversely variable FTHX resulted in about a 3.9% larger capacity and a 1.6% larger COP of the ASHP than the 1.5-circuit reversely fixed FTHX. That is to say, the reversely variable FTHX enhanced both the heating and the cooling performance of the ASHP.  
Figure 7: Conceptual diagram of the reversely variable circuitry.  
In short, the reversely variable circuitry allowed different flowpaths for the refrigerant in evaporator and condenser modes, thus better matching their respective requirements are met.  
Figure 8: Energy outputs for the AHSP with three types of FTHXs.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Nodes** & **Evaporator** & **Condenser** \\ \hline Prototype & & \\ \hline Variable & & \\ \hline
4-circuit fixed & & \\ \hline
1.5-circuit fixed & & \\ \hline \hline \end{tabular}
\end{table}
Table 5: On/off scheme of the check valves in different modes of the FTHX.  
by varying the circuit number. The condenser with fewer circuits enhanced the overall heat transfer coefficient and promoted the capacity increase of the outdoor FTHX. The evaporator with more circuits reached a balance between the overall heat transfer coefficient and the temperature difference, finally increasing the capacity of the outdoor FTHX on the basis of this trade-off. Consequently, the overall energy performance of the ASHP was enhanced in both heating and cooling conditions.