A reversible compression type refrigeration system is commonly known as a heat pump for conditioning a space having a conditioning side and a service side. The conditioning side is that area or environment where it is intended to achieve the benefit of the alternate heating and cooling effects produced by the reversible system. The service side is that side from which heat is drawn or to which heat is rejected when the opposite effect is required on the conditioned side. The system includes a conditioning coil connected to the conditioning side for absorbing heat from the space when cooling is required (cooling cycle) and rejecting heat to the space when heating is required (heating cycle); a first "service" coil connected to the service side adapted to reject heat to the outdoor air during system cooling cycles and absorb heat from the outdoor air on heating cycles; a second "service" coil connected to the service side adapted to withdraw direct or stored heat from a liquid stream heated by a solar collector, and reversible means to allow the first service coil to withdraw heat from the outside air under conditions where the supply of direct or stored solar heat intended to be delivered to the second service coil has been exhausted.