The flow switch acts as a safety device. 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 powered up, the flow switches senses this movement and feeds the information back to the PCB, allowing the ignition sequence to progress. The flow switches function is to make sure that the burner cannot switch on whilst the system is low on water or dry.
We recommend that all parts are fitted by a fully qualified gas safe certified plumber.