Gas boiler tripping out electrics when igniting

[Francis]

Conventional fully pumped gas CH & DHW system. Motorized valves to both. Honeywell valves & programmer. Cyl stat on hot water tank.

CH has been off for some while but DHW still on gas by programmer.

This morning the main circuit breaker in consumer unit clicked off a couple of times. Have narrowed this down to the boiler system. With programmer at HW only, I've tried firing the boiler from the DHW, this trips out the main CU breaker. With programmer at CH only, I've tried the CH from room stat, this also trips the main CU breaker.

System has been working faultlessly since installed 6-7 years ago

This is leading me to think that the CH & DHW circuits are ok and that the problem lies in the programmer, in the boiler or in the wiring connecting all.

Any help please on how to check further, find the problem and fix it ?

Many thanks,

Francis.

[Heliotrope Smith]

[John Rumm]

Where is the pump? i.e. in the boiler or a separate one...

Is it controlled by the boiler, or via the stat/valve wiring?

or the pump...

[Graham.]

When I had that problem it was a chain of ants between live and earth terminals in the wiring centre. They had literally welded themselves together and formed a conductive U link.

[Roger Mills]

Is it an over-current trip - or earth leakage (RCD)? If the latter, my first port of call - as suggested by others - would be the pump. This is the one component in the system where water and electricity are closest to each other, and where an earth leakage is most likely to occur.

You can check the pump by disconnecting its cable, fitting a 13A plug and plugging it into a normal socket (using an extension lead if necessary). If the breaker trips as soon as you switch on, you have your culprit. A slightly cruder experiment would be simply to disconnect the pump and then start the system and confirm that it *didn't* trip - but you mustn't run the boiler for more than a few seconds without the pump running.

[Francis]

Thanks to all for the knowledgeable replies. Disconnected electric supply and fed it from a mains circuit ~ circuit breaker tripped. With pump disconnected from junction box fired boiler ~ no problem. Thus, I'm quite convinced the pump is at fault. Now have another problem with a drain c*ck but will make that another post. Thanks again, Francis