Also known as a membrane, it is a boiler part that is normally located within the diverter valve or hydraulic section of the Baxi boiler. Its main purpose is to move a pin that then operates a micro switch which in turn sends a signal to the printed circuit board. The signal is normally to indicate the pump is running or there is a demand for hot water. They are sometimes referred to as the pump proving ? flow diaphragm or the domestic hot water DHW diaphragm.