have an indirect hw/ch system (with potterton suprima boiler) that intermittantly trips the main rcd when using the programmer to turn the heating on at the set times. The only way i can consistantly turn on the hw or ch without it tripping is when i manually start up the hot water by turning on the power with the cylinder stat on its lowest setting and then turning the stat temp setting up. This is the way to tuern the CH on as well. I presume this rules out any wiring problem and i have already replaced the programmer. One CH engineer and multimeter has already given up on this one. Appreciate any help

Had a couple of installations where the valve is directly below the pump and those isolator valves with a slot have been used - these valves leak and make the valve wet - RCD trips.