Primary water flow switches Reasons a primary water flow switch will fail to start: Punctured / perished rubber diaphragm Seized bearing plate spindle / out of alignment Damaged / loose wiring harness Switch contact faulty A demand for central heating will activate the pump and a rubber diaphragm / spindle arrangement communicates to a micro switch assembly above, transmitting a signal to the printed circuit board for correct operation. To test a primary water flow switch (on a demand): Pressure differential type: Visually assess whether bearing plate spindle is rising through gland nut, activating the micro switch assembly above. (No spindle movement may be due to a perished rubber diaphragm and/or seized bearing plate spindle)….