CH makes RCD trip

Guys I need some help please!
Quietly sitting at home this morning and suddenly all the electrics went, checked the consumer unit and the RCD had been tripped. Done some basic fault finding and I've managed to identify that the problem is something to do with the central heating. Everything is fine with just the hot water switched on but as soon as the CH kicks in, it trips the RCD.
Can I have some pointers as to where the fault might lie.
If it's any help the boiler is about 30years old ! and is a Concord WCF the controller is a Potterton Ep2001
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Bet its earth leakage in the central heating pump.
You could try connecting the pump directly to the mains if you know how and see if this trips. Isolate the heating circuit while your doing this. If the trip goes the moment the heating comes on then you could, if your careful, disconnect the pump and then fire up the boiler and see if it trips or not. Shouldn't hurt to run it for a very short while with the pump electrically disconnected....
--
Tony Sayer


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Humm...just noticed that I'd assumed somewhere that the hot water was some sort of gravity fed system. Suggest that the motorised valves come under suspicion. Should be able to hold them open whilst disconnected and in most systems their fed via the room stat, and a small motor in them drives the valve part then there is a microswitch that feeds the boiler depending on your boiler and system. Either way the most likely culprit is a motor winding leaking to earth and this might not happen until the unit has warmed up a bit...
--
Tony Sayer


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In an earlier contribution to this discussion,

I made the same assumption - rightly or wrongly. It seems *quite* likely - though by no means certain - that a 30-year-old boiler will have gravity hot water and pumped central heating. If this is the case, the pump is the prime suspect - since it is turned on when CH is required but not when HW-only is selected.
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wrote:

.. there may be a clue there :-)

An RCD trip can be a live to earth or neutral to earth fault. It usually means a break down in insulation of some kind or wires touching.
The simplest way to diagnose without the use of a lot of test gear is to disconnect each item in the system one by one.
Before doing that, take the appropriate panels off of the boiler and see whether there are any signs of cable insulation having melted by touching something hot, or poorly cut when installed etc.
I would begin furthest from the boiler by disconnecting the pump, both live and neutral wires. Check again.
If you have a motorised valve or valves disconnect all wires at the terminal block, noting where they go and check again.
Check which type of control system you have by referring to the Honeywell documentation.
http://content.honeywell.com/uk/homes/systems.htm
Check thermostats, although these are less likely.
Remove the programmer from its plate and check again. This is not necessarily rigorous because wiring to other things goes through it, but it might help in the elimination process.
If you haven't found it by this stage then the fault probably lies in the boiler wiring or one of the internal components.
However, since that you said it's CH only, if the system is fully pumped with a motorised valve then the motorised valve would be suspect. If it's a gravity type HW with no valve, then the evidence points to the pump (or the attendant thermostats and wiring.
My money would be on the pump or motorised valves first because the insulation in the motor windings can fail.
.andy
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to
the
Something leaking around the CH boiler, pump, motorised valve(s) or getting into the wiring centre or other electrics, perhaps? Or damaged cable - frayed, gone rigid and getting cracked, heat damage?
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