Vaillant part is located around the flow switch section. The flow switch acts as a safety mechanism. It is a switch with an in-built sensor that sits in the pipework for both the heating and domestic hot water systems. When the pump is activated, the flow switches sense this movement and feed the information back to the printed circuit board, allowing the ignition sequence to progress. The function of the flow switch is to make sure that the burner cannot switch on if the system is low on water

Brand Name	Vaillant