To recover waste heat from household bath wastewater and improve energy efficiency

a heat pump water heater with waste heat recovery was designed. Experimental benches were set up for experimental studies. The experiment was carried out with an inlet water temperature of 16 °C and an outlet water temperature of 36–46 °C to study the effects of changing parameters

such as the refrigerant charge amount

water output

heating capacity

and heating coefficient of performance (COP). The experimental results show that increasing the refrigerant charge benefits the heating and heating COP stability. However

the maximum heating capacity is 7.9 kW

and the heating COP is 7.7 at the minimum refrigerant charge. For the outlet water temperature of 36–46 °C

the device can effectively recover the heat in the waste water

the heating output is stable at approximately 7.5 kW

and the heating COP is above 6.0.