Over the past decade, improvements have been made in air-source heat pump units; however, performance during periods of extreme high and low temperatures remains poor. In order to increase the COP and thermal capacity of the heat pump during these extreme periods, the unit was coupled to a short-term underground rock-bed thermal store which enables tempering ambient air that is drawn to the outdoor heat exchanger. The system was designed, built, and commissioned at the Urbandale Centre for Home Energy Research at Carleton University. Experimentally during the cooling season, it was found that the rock bed is suitable to moderate the outdoor temperature. However, the overall system performance did not improve as a result of increased flow resistance through the ductwork, significant thermal losses and air leaks within the outdoor system.