The performance of CO2 heat pump water heater was studied experimentally in this paper by comparing a loop heating and a direct heating hot water. In the loop heating mode

with an increase of the water temperature

the compressor inlet and exhaust pressure increased gradually

the power consumption increased and the gas cooler inlet and outlet temperature increased

too. The temperature difference between carbon dioxide and cooling water and heat transfer decreased

the COP of CO2 heat pump system decreased. The operating parameters of the system were stable and the COP remained unchanged in direct heated condition. When the water temperature is 17℃

the COP of the system is about 3.5

much higher than that in loop heating. The results show that the lower inlet temperature can be obtained by modifying the water tank structure and it is a very effective way to improve system performance.