Air-source heat pump systems with vapor injection have promising potential applications in cold areas in China owing to their high efficiency at low temperatures. This study investigates the effect of the refrigerant charge on the energy efficiency and operating parameters of air-source heat pump systems with vapor injection at low temperatures. The results demonstrate that as the charging amount increased from 18 kg to 25 kg

the suction temperature and pressure of the compressor slowly decreased from ?11 to 14 °C and from 0.295 to 0.32 MPa

respectively; the discharge pressure slowly increased by 5.6%; the heating capacity and compressor power steadily increased; the energy efficiency gradually increased. When the refrigerant charge exceeded the critical charging amount of 25 kg

the operation of the compressor degraded; the temperature of discharge gas increased by more than 11%; the discharge pressure increased by more than 13.6%. At the optimal refrigerant charging amount of 26 kg

the maximum energy efficiency of the unit was 2.12

which demonstrated a 14.6% increase compared with the case of 19 kg refrigerant charge. As the charging amount increased from 26 to 28 kg

the coefficient of performance slowly decreased.