Abstract: a heat pump system selectively operable in cooling and heating modes of operation and having an internal combustion engine prime mover which produces refrigerant vapor compression, and which produces excess heat for rejection, is provided with a first heat exchanger that evaporates compressed refrigerant in the system heating mode of operation, with a second heat exchanger which uses prime mover rejected heat to heat a working fluid, and with a fluid distribution arrangement which is selectively operable to flow the heated working fluid in heat exchange relation to said first heat exchanger also, in the system heating mode of operation to provide a defrost capabilities and to improve heat pumping heating capacity at low ambient temperatures.