The function of a check valve is to allow the refrigerant to circulate in only one direction. It mainly consists of the main body, block, and valve needle. When the pressure on side A is higher than on side B, the valve needle moves towards side B, opening the refrigerant circulation channel, allowing the refrigerant to flow from side A to side B at this time; conversely, when the pressure on side B is higher than on side A, the valve needle moves towards side A, blocking the refrigerant circulation channel, thereby cutting off the refrigerant circulation channel. During system cooling, the check valve parallel to the capillary tube opens while the check valve parallel to the expansion valve closes. At this point, the system uses the expansion valve for throttling, achieving the purpose of controlling the refrigerant flow according to the room thermal load changes and regulating the cooling capacity. During system heating, the check valve parallel to the capillary tube closes while the check valve parallel to the expansion valve opens. At this point, the system uses the capillary tube for throttling, keeping the refrigerant flow constant, thus maximizing the system's heating capacity. Beijing Midea Air Conditioning After-sales Service Center edited and organized on September 23, 2011. Midea Air Conditioning Repair Response: The role of a check valve in the air conditioning system.