A heat-actuated space conditioning system comprising a sub-atmospheric natural-gas-fired Brayton cycle engine driving a Rankine cycle heat pump. A centrifugal Freon compressor is driven directly from the Brayton engine rotating group through a permanent magnet coupling. The system utilizes an in-line combustor which is operated to burn natural gas at atmospheric pressure by virtue of the associated sub-atmospheric Brayton cycle engine. Ambient stoichiometric air is drawn through an associated recuperator where it is preheated before being introduced into the combustor. Compressor discharge gas is also cycled through the recuperator and used as diluent to provide added flow and the desired turbine inlet temperature. Waste heat is used to power a boiler for the Freon in the Rankine cycle side, and this converted energy is used to drive a second turbine providing added power to the Freon compressor. A boiler feed pump is included which also serves as a starting mechanism for the rotating assembly.