## 4 Discussions  
For space heating air source heat pump, since the \(COP_{h}\) is significantly affected by the supply water temperature, low temperature heating terminals such as radiant floor heating, fan-coil warm air heating or expanded radiators are normally adopted. Thus, in the designed cases of this paper, the highest supply water temperature was set as 50 \({}^{\circ}\)C. For space heating, the refrigerants that are commonly used in existing heat pumps are R22, R134a, R410A and R407C. Through the comparisons above, the proposed RHR-1 has higher \(COP_{h}\) than others in most of the design cases, especially when supply heating water are 50 \({}^{\circ}\)C and 45 \({}^{\circ}\)C. R134a has higher \(COP_{h}\) compared with R407C in this study, which accords with a previous study [23] that also reported R134a has longer frost-free operation time than R407C and R22. The compression ratio of RHR-1 is lower than R134a but higher than R22. The discharging temperature of RHR-1 is higher than R134a, but lower than R22. Considering the performance of refrigerants including \(COP_{h}\), CR and DT, RHR-1 has shown relatively strong comprehensive properties. Besides energy performance, the global warming potential of RHR-1 is 279 which is much lower than R134a (GWP 1370), R407C (GWP 2100), R410A (GWP 1700) and R22 (GWP 1790). As an alternative for R22, RHR-1 has not only no ozone depletion potential, but also low global warming potential, which strengthen the environment friendliness of ASHP refrigerants. With the above characteristics, RHR-1 might be the proper choice for space heating in rural residential buildings of North China or similar climate zones. In addition to the comparison of different refrigerants, the operation cases such as supply water temperature and outdoor air temperature could largely affect the performance of ASHP. The \(COP_{h}\) of ASHP using RHR-1 as refrigerant for space heating under the deigned cases were analyzed, as shown in Figure 12.  
In Figure 12, when the outdoor air temperature increases 5 \({}^{\circ}\)C, the \(COP_{h}\) will increase 0.17-0.46. If the supply water temperature decreases 5 \({}^{\circ}\)C, the \(COP_{h}\) will increase 0.15-0.55. Regression analysis was then conducted to get the relationship between \(COP_{h}\) of ASHP using RHR-1 and the temperature differences between supply water and outdoor air. The regression results are shown in Figure 13.  
Figure 12: The \(COP_{h}\) of space heating ASHP using RHR-1 under varied thermal cases. Using a logarithmic equation to regress the relationship of \(COP_{h}\) with temperature difference between supply water and outdoor air, it could be written as Equation (9)  
\[COP_{h}=-2.437\ln(t_{s,w}-t_{o,a})+12.418 \tag{9}\]  
From the study of [PERSON] et al. [6], the COP of heat pump should be above 3 to achieve lower cost and less environment impact than other heating systems. To get the reasonable \(COP_{h}\), the temperature difference between outdoor air and supply water of ASHP using RHR-1 is suggested to be controlled within 47.5 \({}^{\circ}\)C. Considering the temperature difference between supply water and indoor air, this result accords with the study by [PERSON] et al. [8] that indicated that the application of ASHP should control indoor and outdoor air temperature be within 41 \({}^{\circ}\)C to achieve an acceptable COP. In this study, the outdoor meteorological parameters of Tianjin were used as the typical thermal climate in North China, and the average outdoor air temperature in a heating season (15 November-15 March) of Tianjin is \(-0.16\)\({}^{\circ}\)C. Thus, to get the reasonable \(COP_{h}\) that was reported by [PERSON] et al. [6], the average supply water temperature of ASHP using RHR-1 in the heating season is suggested to be controlled below 47 \({}^{\circ}\)C.  
The influence of supply water temperature to \(COP_{h}\) determines that the control strategy of ASHP should be different from gas or coal boiler whose efficiency changes less under varied heating load or outdoor environment.