For the cascade air-source heat pump operated under different environment temperature

the proper control of intermediate pressure is important. In this paper the principle that two stage compression ratio is approximately equal is used to determine the intermediate pressure at the optimal middle temperature. The proposed method and control strategy were implemented by highly applicable Proportion Integration Differentiation (PID) algorithm and Programmable Logic Controller(PLC).A test was conducted

and it showed that the control method was sensitive

stable and accurate with various outside conditions and indoor loads.