When an air source heat pump (ASHP) unit is used for space heating at low ambient temperatures in winter, frost may form on its outdoor coil surface. Since the accumulated frost adversely affects its performance and energy efficiency, periodic defrosting of the outdoor coil is necessary. Currently, the reverse-cycle defrosting (RCD) method is widely used for the defrosting of ASHP. However, this operation interrupts space heating during the defrosting process. A time lag occurs to resume heating at end of the defrosting cycle. Moreover, frequent reversing of the 4-way valve may cause mass leakage of the refrigerant, even make the system unsafe. Furthermore, some amount of heat is dissipated to the atmosphere through the compressor casing. To improve the defrosting process and use this waste heat, a novel ASHP unit is developed. The space is heated during the defrosting process using the heat dissipated by the compressor. Experiments using both the RCD method and the novel reverse cycle defrosting (NRCD) method developed in this study are conducted on an ASHP unit of 8.9 kW nominal heating capacity. The experimental results indicated that in the NRCD method, the discharge and suction pressures are increased by 0.33 MPa and 0.14 MPa, respectively, the defrosting time is shortened by 65% while the resuming heating period vanished with the NRCD method, and that the total energy consumption in comparison to RCD method is reduced by 27.9% during the period which is composed of defrosting period and resuming heating period. Moreover, the NRCD method ensured continuous heating during defrosting. The mean temperature difference between the air entering and leaving the indoor coil reaches 4.1 degrees C during defrosting. Over a test period of 125 min, compared to RCD method, the total heating capacity and input power are increased by 14.2% and 12.6%, respectively. The increase in the system COP is 1.4%. (C) 2014 Elsevier Ltd. All rights reserved.