The experimental flat is located in Chengdu City, Sichuan Province. It is situated on the 11th floor of an 18-story residential building, and the area is 140 m² (Figure 1). The experimental tested system is mounted in the user’s home. To satisfy the experimental requirements, the installation position of the ASHP outdoor unit, the domestic hot water tank, the piping system, and the terminal should be reasonably designed to maximize the utilization of the space resources. The outdoor unit and water tank are installed on the balcony. The balcony communicates with the atmosphere, and the guardrail of it is decorated with grilles. Besides, the installation position of the ASHP outdoor unit and hallway are partitioned by installing sliding doors, and the outlet of the outdoor unit faces the outdoor atmosphere, and the inlet faces the sliding doors with 12 cm. The appearance of the ASHP and the test site is shown in Figure 2. The experimental site plan and the installation position of the ASHP outdoor unit are shown in Figure 3.

In this experiment, the heat recovery type ASHP unit, HSLR-T-13 (D) E, is selected as the cold and heat source of the system. It has five working modes, namely: summer refrigeration, refrigeration + hot water, hot water, winter heating, heating + hot water. The size of the host is 1300 (L) × 460 (W) × 1783 (H) mm and the compressor is a scroll compressor. The rated voltage is 220 V and the winter heat rating is 15 KW while its rated heating power is 4.65 KW. The axial flow fan air volume is 5000 m³/h and the air conditioning side pressure loss is 35 KPa.

In this experiment, JTRG-Ⅱ Automatic Test System for Building Thermal Temperature and Heat Flow was adopted to record temperature data, including JTRG—SHENG host for building thermal temperature and heat flow, copper thermocouple and data management software (as shown in Figure 4). To ensure the accuracy of the experimental data, the thermocouples used in the test are compared with each other. The verified thermocouple line temperatures were all within 1°C, which met the experimental test requirements.

An ASHP is selected as the heat source and fan coils are selected as the terminal to adjust the indoor thermal environment (Figure 5). In winter, the ASHP produces hot water which the fan coils use to heat the indoor air through convection heat transfer.

COP is used to evaluate the heating performance of the system and is defined by the following equation. The inlet and outlet water temperatures were measured by a thermometer (with the accuracy of ± 0.3°C), the flow rate was measured by a water meter (with the accuracy of ± 2%), and the power consumption was measured by the electricity sensor (with the accuracy of ± 2%). The thermometers are installed at the water inlet and outlet of the main engine, and the water meter is installed at the water outlet of the main engine. CO P h = Q h W = C × L × Δ T W where: Q _h = heating capacity of the unit (kW); W = machine input power (kW); C = specific heat capacity of water [4.2 kJ/(kg·K)]; L = water flow (m³/h); ΔT = unit supply water temperature difference (°C).

The fan coils in each room operated during the test period. To study the difference of the heating performance under different conditions, the outdoor unit ventilation conditions of the partitioned outdoor space were changed during measurement under the same operating condition of the internal unit. In the experiment, seven conditions were selected by switching the state of the sliding doors and the security door in Figure 2. The seven conditions are listed in Table 1. In the table, √ indicates the open state while × indicates the close state. For the first condition, the sliding doors were removed and the security door of the home was fully opened. The test period for each condition is 50–70 min to avoid excessive changes in the outdoor temperature variations and to better compare the effects of different ventilation conditions.

The COP under 7 conditions was studied experimentally in this paper. Under the results of the 7 conditions, Figures 6–12 respectively show the variations of the COP of the heating system and the outdoor air temperature variations under conditions 1 to 7. The ventilation efficiency of the outdoor unit in condition 1 was the best because the sliding doors were completely removed and the security door was fully opened. The COP varied between 2.67 and 2.97 (average 2.86) during the test period and the outdoor temperature was stable between 7.10 and 7.60°C (average 7.40°C). For condition 7, the sliding door was pushed to the left (the right side is open) and the security door was closed. The COP varied from 2.19 to 2.74 (average 2.49) and the outdoor air temperature during the test was between 8.00 and 8.40°C (average 8.20°C). The COP of condition 7 with higher outdoor temperature was significantly lower than condition 1, which indicated that the unfavorable ventilation conditions caused a negative impact on the heating performance of the heating system.

From condition 2 to condition 6, the time starts and ends from 11:36 to 13:00, 13:00 to 13:55, 13:55 to 14:40, 14:40 to 15:45, and 15:45 to 17:50, respectively. The outdoor air temperature rises first and then decreases. The lowest temperature is 7.4°C at 11:36, and the highest temperature is 13.2°C at 15:02 in the afternoon. The heating coefficient of the main engine is different under different working conditions.

At condition 2, the average outdoor temperature was 8.5°C, and the COP varied between 1.10 and 2.74 (average 2.57). In condition 3, the average outdoor temperature was 10.4°C, and the COP varied between 2.44 and 2.90 (average 2.79). In condition 4, the average outdoor temperature was 12°C, and the COP varied between 2.36 and 2.83 (average 2.61). In condition 5, the average outdoor temperature was 12.4°C, and the COP varied between 2.30 and 2.83 (average 2.52). In condition 6, the test time is longer, and there is a large difference between COP values before and after. The COP value of the heating performance coefficient of the main engine is small from 15:45 to 16:20 with an average value of 2.43. The average COP value reaches the lowest at 16:20 to 16:40, which is only 1.83. Between 16:40 and 17:50, the COP value increased significantly, and the COP varied between 2.59 and 2.83 (average 2.75).

The outdoor temperature of condition 5 was 0.40°C higher than condition 7. Since the heating performance of opening the left-side sliding door was preferable to the right side of the sliding door, the heating system COP increased by 3.20%. Similarly, the outdoor air temperature in condition 6 was 0.40°C higher than in condition 2, but the COP under condition 6 was reduced by 11.00% due to the poor ventilation environment of condition 6. It can be seen that the COP changes in the ASHP heating system were different when the outdoor temperature changed uniformly so that the influence of the ventilation environment on its heating performance cannot be ignored.

The heating performance coefficient of the main engine has a certain relationship with outdoor air temperature and generally increases with the increase of air temperature. In this experiment, the COP value did not rise with the rise of the outdoor air. It is due to the experimental factors affecting the main COP value as host evaporator heat fin ventilation effect. The ventilation airflow organization has differences under different conditions which directly affect the heating performance of the host. At the beginning of working condition 6, the reason for the low COP value is that the airflow in condition 5 is not well organized, which worsens the environment of heat exchange fin and leads to the frost of the main engine. The main engine automatically enters the defrost working condition and the COP value is the lowest at this time, even lower than 0. The reason for this result is that the heat produced by the compressor enters the evaporator first when the defrosting is forced into the refrigeration condition. Then it enters the plate heat exchanger to exchange heat with the circulating water at the end. The return water temperature may be higher than the supply water temperature, and the temperature difference is less than 0.

Figure 13 shows the comparison of the average COP of the measured heating system under different conditions and the average outdoor air temperature. It can be seen that under the different ventilation conditions of the outdoor unit, the average COP of the measured heating system varied between 2.49 and 2.86 and the difference was clear. The test data showed that the ventilation conditions of the outdoor unit had a qualitative effect on the heating system. However, due to the limitations of experimental conditions, it is difficult to maintain the same ambient temperature of the altered ventilation conditions, so there is a certain difference in the outdoor air temperature for each condition.

Air temperature is one of the most important factors affecting the COP of the outdoor unit of the ASHP, but the outdoor air temperature under different conditions is different. Therefore, the factors of the diverse outdoor temperatures must be excluded in order to analyze the influence of the outdoor unit ventilation conditions on the heating performance comprehensively. Figure 14 shows the variation curve of COP of an outdoor unit with the outdoor temperature at different outflow temperatures. COP increases by 0.02 while the outdoor air temperature increases by 1.00°C.

The COP of the outdoor unit in the test was revised with the coefficient correction in Figure 11. Figure 15 presents the comparison of the COP before and after outdoor temperature correction under different conditions. It can be seen that: (1) When the security door was opened and the sliding doors were removed, the COP was 2.84, which is higher by 20.80% than when the sliding doors were closed. (2) When the security door was opened and the right fan and left fan sliding door were opened, respectively, the COP was 2.55 and 2.60, respectively, which was 13.50 and 11.80% higher than the condition of the closed sliding door respectively. (3) The security door was closed, and the sliding doors were removed, and the COP of the heating system was 2.51, which is 10.40% higher than when the sliding doors were closed. (4) The security doors were closed and the right and left fan sliding doors were opened, respectively, and the COP of the heating system was 2.44 and 2.49. The COP was increased by 0.19 and 0.24, respectively, compared to the condition of closing the sliding doors, nearly 7.80 and 9.60%. (5) When the security doors and sliding doors were closed, the COP of the heating system with the value of 2.25 was the lowest. The ventilation conditions of the outdoor unit have a significant influence on the COP of the heating system of the household ASHP system.