A heat pump water heater for bathing (HPBWH) was designed, which was assisted by shower waste water and could supply hot water in a few minutes of waiting. The system employed a regenerator and an evaporator that can recover the waste heat from shower water. R134a was chosen as working fluid. Thermodynamic models were developed for the system including the condenser, the evaporator, the regenerator, the expansion valve, the compressor, and the water tank. For static simulation, the effects of the main input parameters, including the condenser inlet water conditions, the condensing temperature and the evaporating temperature, on six output parameters were studied. It was indicated that the performance of the HPBWH was stable and good in any season and weather conditions. The condensing and evaporating temperatures were optimized at 51.50 and 11.68 degrees C, respectively, when the optimal COP of the whole HPBWH system reached 4.97. For dynamic simulation, the dynamic responses of seven output parameters at the system start-up and with the operating condition variation were investigated. The simulation results provided a good instruction for the further design and optimization of the HPBWH. (C) 2013 Published by Elsevier B.V.