Glowworm balanced flue boiler - priority hot water with two-port valve, grundfos pump and manual clock type (British Gas EMD2) programmer.

When slide on programmer switched to Hot Water the mcb trips, could the programmer be faulty, or could perhaps the pump be jammed?

RSD, October 2009

I spent ages trying to find what was causing my Glowworm domestic boiler to trip the main circuit breaker intermittently. Eventually, I discovered the "fault": the flue exhaust above the flat-topped inner boiler housing was leaking condensation, which gathered on the flat surface, eventually creeping up to the small cut-out switch mounted there and causing it to trip. The water eventually evaporated, leaving no clue as to why this was happening. Quick fix - shield the cut-out with a ring of waterproof putty or similar but remember to CHECK FOR WATER BUILDING UP. Full fix - replace the exhaust gasket.

Arnold J. Bilgepump, December 2009

Purely a guess but its more likely to be the pump or 2 port valve, rather than the programmer. You can disconnect each item one at a time from your heating wiring centre, but do make a note of where the wires go.....!

John, October 2009