The Carrier long line set guidelines call for the installation of a liquid line solenoid valve to prevent refrigerant migration during the off cycle. Refrigerant migration may cause refrigerant to condense at the bottom of the compressor, causing a flooded start. Condensed liquid refrigerant in the suction line may also cause flooded starts and damage the compressor. Crankcase heaters and liquid line solenoid valves can help prevent the refrigerant migration that causes premature failure.

Crankcase heaters keep the oil warm in the compressor, which prevents refrigerant from settling in there and condensing. Liquid line solenoid valves shut off refrigerant flow to the evaporator, and it only stops flow in one direction; some heat pump versions are bi-flow. Liquid line solenoid valves can be wired to Y and common in the normally closed position.

However, if you must use a liquid line solenoid on single-stage equipment, you will also likely need to add an OEM hard start kit.