Abstract: A compressor is connected with an evaporator, a condenser, and an electrically controlled valve for circulating a working fluid in a system for recovering waste heat to provide heated water for human use. A suction superheat temperature is determined from a measured compressor suction temperature and a suction saturation temperature. The electrically controlled valve is adjusted to maintain the suction superheat temperature at a suction superheat set point. The electrically controlled valve may be incrementally closed when a compressor suction pressure exceeds a maximum suction pressure. A discharge superheat temperature can be determined from a measured compressor discharge temperature and a discharge saturation temperature. The electrically controlled valve may be incrementally closed when the discharge superheat temperature falls below a minimum discharge superheat temperature.