| Vacuum switches are installed on the inlet line to the pump. They are composed of two parts, the diaphragm, which responds to changes in system pressure, expanding toward its system side when pressure in the system falls below outside or atmospheric pressure, and the electrical switch which is held open by spring tension when there is no pressure differential and the diaphragm is in its normal position.
When the pump draws water into the system, the suction causes pressure on the inlet side of the pump to decrease, the diaphragm falls and the vacuum switch contacts close, completing the circuit to the burner system. When flow ceases, the diaphragm rises and a spring helps return the switch to the open position. With the switch open, the circuit to the burner system is broken, the solenoid valve closes, and fuel flow to the burner ceases, stopping burner firing.
NOTE: A potential problem with a vac switch can be created when the inlet filter or float tank becomes clogged. The pump will continue to run, creating a pressure drop on the inlet. When this happens, the vac switch will continue to close and start the burner.
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