During which mode is the reversing valve most critical for proper heat pump operation?

The reversing valve in a heat pump system is crucial for its operation in heat mode because it is responsible for changing the direction of the refrigerant flow. This change allows the system to extract heat from the outside air and transfer it indoors, effectively heating the space. In heat mode, the proper functioning of the reversing valve ensures that the refrigerant absorbs heat from the outside environment and releases it inside, which is essential for maintaining the intended heating performance.

In contrast, while the reversing valve is also involved in other modes of operation, such as cool mode and defrost mode, its primary function of reversing refrigerant flow is most critical during heating. In fan mode, the system does not require refrigerant flow direction changes, making the reversing valve's role less significant.