What moves the main valve in a pilot-operated switch over valve?

The main valve in a pilot-operated switch over valve is moved by the pressure difference between the low side and the high side of the system. This type of valve utilizes the principle of pressure differentials to function effectively. When the pressures on either side of the valve are unequal, the resultant pressure difference creates a force that actuates the valve, allowing for the proper routing of the refrigerant through the heat pump system.

This mechanism is crucial for the operation of air-to-air heat pumps, as it determines the reversing of the cycle between heating and cooling modes. The design typically incorporates a pilot signal that is influenced by the existing system pressures, making pressure difference the primary driving force behind the valve's movement.

In contrast, other options do not accurately describe the functioning of a pilot-operated switch over valve. For example, temperature resistance or gravity force does not play a role in the direct operation of the main valve, and while electrical impulses can be related to various HVAC components, they are not the driving force in this specific type of valve.