An investigation is reported on the performance of a two-stage variable capacity air source heat pump (ASHP) system, recently installed at the Archetype Sustainable House (ASH) in Woodbridge, Ontario, Canada. The controlled testing on the system was carried out in both summer and winter seasons for 4 to 6 weeks. TRNSYS simulation was performed and validated to model the performance of ASHP. The experimental data were extrapolated to predict the annual performance of ASHP. It was noticed for the cooling mode, the coefficient of performance (COP) varied between 4.7 to 5.7 as the outdoor air temperature changed from 33 C to 16 degrees C, respectively. While in the heating mode, the COP varied between 1.79 and 5.0 as the temperature of outdoor air changed from -19 degrees C to 9 degrees C, respectively. The performance result has indicated that the use of variable speed ASHP provides an improvement in the efficiency as compared to other available options. These enhancements are due to the reduction in the operating cyclic time and relatively lower operating speeds of the device. (C) 2015 Elsevier B.V. All rights reserved.