During the off cycle, which pressure is sufficient to force the TXV firmly into its seat and stop refrigerant flow?

A TXV, or thermostatic expansion valve, relies on differential pressure to control the flow of refrigerant into the evaporator. During the off cycle, the valve must close tightly to prevent refrigerant from flowing improperly, which could lead to issues like flooding the evaporator or increased energy consumption.

The correct choice relates to the spring pressure within the TXV itself. The spring within the valve helps to close off the refrigerant flow when the system is not in operation. This spring provides a necessary force that, when combined with the residual pressures in the system, ensures that the TXV's pin closes securely against its seat, effectively preventing flow.

The other pressures mentioned—line (which refers to the pressure on either side of the TXV), high side (which is the discharge pressure from the compressor), and pump (typically not applicable in this context)—do not specifically provide the closing force needed from a standby position like the spring does. They may influence the system's operation during active cycles but do not play a critical role in forcing the TXV to its seat during the off cycle. Thus, it is the spring's action that is essential for proper closure.