A solar thermal water pump using a flat-plate collector with n-pentane as the working fluid is described. A heat-transfer analysis has been carried out for this system with account for all heat losses. Variations in temperature and pressure of pentane in the collection system with time are predicted. Two different situations are considered: one with pentane getting heated without its vapour being used for operation of the pump and the other with pentane vapour used for pump operation during the heating period. Variations in collector efficiency with time are discussed.