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
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