In a split system heat pump using thermostatic expansion valves, what might cause the compressor to pump down in cooling mode?

When a split system heat pump is in cooling mode and the compressor pumps down, it indicates a situation where the refrigerant flow is restricted or reduced, causing high-side pressure to drop. The scenario where the indoor expansion valve power element has lost its charge is relevant because this component is critical for modulating the flow of refrigerant into the evaporator. If the power element has lost its charge, it cannot effectively control the expansion valve's opening, potentially leading to insufficient refrigerant entering the evaporator. As a result, the compressor may detect low suction pressure and activate a pump-down mode to protect itself from potential damage due to low refrigerant flow.

This condition highlights the importance of the expansion valve's functionality in maintaining proper refrigerant flow and system balance during cooling operation. Understanding the role of each component helps in diagnosing issues effectively in heat pump systems.