A heat pump system including a heat engine having a valve assembly positioned to divide the heat engine chamber into a first chamber and a second chamber. A piston is positioned within the heat engine and includes a first piston head in the first chamber, a second piston head in the second chamber, and a connecting rod connecting the first and second piston heads and extending through a bore in the valve assembly. The valve assembly includes a one-way inlet valve and a one-way outlet valve in fluid communication with each of the first and second heat engine chambers. The valve assembly is also in fluid communication with a condenser and evaporator of the heat pump system. The system may also include improved one-way valves, a condensate pump, and a novel shifting valve.