The valve body and travel switch of the danfoss differential pressure switch are assembled on a single base plate. Lubricating grease enters the right chamber of the differential pressure switch valve body piston from main pipeline B under pressure, and main pipeline A is unloaded. Once the pressure difference between the two main pipelines reaches the set value, the piston overcomes the spring force in the left chamber and moves to the left, pushing the travel switch to close contacts 1 and 2, sending a pulse signal to the system electrical control box to order the reversing valve to change direction. At this time, main pipeline A is compressed, B is unloaded, and the piston is centered under the action of the spring in the two end chambers. Travel switch contacts 1 and 2 are disconnected, and the contact bridge is in the middle position. The system begins its second cycle of operation. Once the pressure difference between main pipelines A and B reaches the set value again, the piston moves to the right, and stroke switch contacts 3 and 4 close. The pulse signal causes the directional valve in the system to change direction again, starting the next cycle of operation. The application of differential pressure switches in HVAC systems is mainly controlled based on the resistance and flow curves of HVAC equipment. Water side heat exchangers (sleeve type, shell and tube type, tube plate type, and commonly used plate heat exchangers), water filters, valves, and water pumps in HVAC all have their pressure drop and flow performance curves. As long as the measured pressure difference on both sides of the differential pressure switch is compared with the pre-set value, the flow can be accurately controlled.