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An instantaneous-heating air source heat pump water heater with a temperature stratified water tank was put forward. The instantaneous-heating air source heat pump water heater was designed to produce hot water instantly by three modes, namely condenser heating, water tank heating and condenser-water tank joint heating. The heating mode is chosen as condenser heating when the ambient temperature is above 20°C (including 20°C) and is chosen as condenserwater tank joint heating below 20°C. A 3HP instantaneous-heating air source heat pump water heater was built and tested in an Enthalpy Lab. Influences of refrigerant charge, throttle step, compressor frequency on heat capacity, and heat coefficient of performance were analyzed, and the effect of thermal stratification of the buffer tank and the operation performances in various conditions (according to China's National Standard GB-23137) were tested. The results showed that the instantaneous-heating air source heat pump water heater can generate 42°C and 6 L/min hot water at ambient temperatures of 20°C and 43°C with heat coefficients of performance (of the instantaneous-heating air-source heat pump water heater) of 4.5 and 12.7 respectively, and at ambient temperature 27°C with a heat coefficient of performance (of the heat pump module) of 2.76. It also showed that the advantage of the system is the compressor's suction and exhaust pressures in certain working conditions are steady and the effect of the temperature stratification of the buffer tank is significant. In conclusion, the instantaneous-heating air source heat pump water heater has great application value for household.

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