An air-to-water heat pump has been designed to operate with a variable-speed compressor, an electrically driven motorised expansion valve and variable-input air mass flow rate; all of these are supported by a microcomputer-based control and instrumentation system. The paper develops the control algorithms which provide for independent control of the valve, compressor speed and air mass flow rate so that the refrigerant pressure across the evaporator can be maintained to ensure maximum heat transfer occurs in the evaporator. Experimental results demonstrate that the efficiency of the heat pump can be improved by the on-line control system.