The thermal performance of a new two-stage variable capacity air source heat pump (ASHP) and a horizontal ground loop ground source heat pump (GSHP) was investigated side-by-side at the Archetype Sustainable Twin Houses located in Toronto, Canada. The heat pumps were tested in cooling mode, as well as heating mode under extreme winter conditions. In cooling mode, the ASHP COP ranged from 4.7 to 5.7 at an outdoor temperature of 33 degrees C and 16 degrees C respectively, while the GSHP COP ranged from 4.9 (at an ELT of 8.5 degrees C and EST of 19.2 degrees C) to 5.6 (at an ELT of 12. 4 degrees C and EST of 17.8 degrees C). In heating mode, the ASHP COP ranged from 1.79 to 5.0 at an outdoor temperature of -19 degrees C and 9 degrees C respectively, while the GSHP COP ranged from 3.05 (at an ELT of 44.4 degrees C and an EST of 2.7 degrees C) to 3.44 (at an ELT of 41.5 degrees C and an EST of 5.48 degrees C) during the earlier winter test period. Data extrapolation and energy simulation was also performed to predict annual heat pump performance in Toronto as well as other Canadian regions.