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Air source heat pumps (ASHPs) have gained popularity in cold regions without central heating owing to their higher energy efficiency when compared with fossil energy sources. Although studies have shown that the short-term performance of ASHPs may meet the heating demand at an ambient temperature of −20 °C, long-term performance under actual variable meteorological conditions needs further investigation. In this study, to comprehensively evaluate the long-term performance of an ASHP system in cold climates, a three-month field test was conducted in Daqing city, which is the coldest region in China. The ambient temperature fluctuated from −24.7 °C to 15 °C during the test. The test results showed that the system coefficient of performance (COP) was related to the outdoor temperature (r = 0.8955), and when the supply water temperature increased by approximately 6 °C under an outdoor temperature of −5 ± 1 °C, the corresponding COP decreased by 14.2%. Apart from the frost temperature of −5 ± 1 °C, the relative humidity (RH) had a slight effect on the system COP. Moreover, during the three coldest days when the ambient temperature ranged from −24.7 °C to −15.0 °C, the heat produced met the heating demand, and the system ran smoothly. Compared with other common heating methods, ASHPs have the advantages of high energy conversion efficiency and low pollutant emissions. From the field test results, it can be inferred that the low-temperature ASHP system is a clean and energy-efficient heating method that is well suited for severely cold regions. • The field test was investigated to evaluate long-term performance of ASHP. • The system coefficient of performance was agreeable with the outdoor temperature. • The performance was decrease with an increase of supply water temperature. • The COP was 1.63–2.17 for ambient temperature of −24.7 °C to −15.0 °C. • ASHPs have the high energy conversion efficiency and less pollutant emissions. [ABSTRACT FROM AUTHOR]

Among the factors that affect the energy efficiency of the air source heat pump system, water volume influences efficiency by determining the unit's ON OFF time under partial load, especially in the low thermal inertia heating systems in the applications of northern China. The water volume in relation to cycling losses is extremely important. In order to strip out the multiple influence factors and clarify the impact of water volume on system efficiency through the actual heating system, an analytical model, presenting of the effect of system water volume on energy efficiency is established firstly. It is identified total water volume, including water volume in the buffer tank, terminals and pipelines by means of the proposed analytical model and the actual monitoring data of 6 fixed speed air source heat pump systems. Considering buffer tank heat loss and partial load loss caused by overmatching household air source heat pump systems, and combining with the efficiency and economy optimization, a recommended allocation index of total water volume 30~40 L/kW under nominal heating capacity for rural houses in northern China is put forward. [ABSTRACT FROM AUTHOR]