help needed with fault finding

Gaz Bryan · Oct 7, 2010

Ok here we go, I changed a CCU and after getting rid of a few borrowed neutrals everything seemed fine. I carried out all the tests required. No problems there. Then came the crunch.
When the central heating switches off via the digital timer it trips the RCD . This doesn't happen every time but on average about 1 in 4 times. It doesn't happen if the central heating is switched off via the thermostat and it doesn't happen when the hot water goes on and off.
So far, I have changed the central heating supply to the other side of the ccu with exactly the same results. I have changed the digital timer with no effect. I tried switching of all the mcb's except for the one supplying the central heating, with no effect.
I'm starting to run out of ideas so any suggestions will be welcome. I am starting to suspect the central heating system itself now but I don't know how to prove it.
Any ideas folks?

Darkwood · Oct 7, 2010

The fault seems to be within the boiler as you,ve already eliminated anything else, at a guess i would suggest maybe a valve on the heating system which changes between hot water and heating, if you put heating on and turn the stat up so its calling for heat then let the boiler fire up you can then run hot water which should operate the valve as h/w has priority, if running the hot water trips the rcd only when the central heating is running then id be looking at a possible valve fault.

Gaz Bryan · Oct 8, 2010

cheers, I must admit I am scratching my head with this one.I like fault finding but I hate intermittant faults. The fact that this one is so particular to the timer and the heating only is quite frustrating. Also,every time the rcd trips and I reset it the answer machine informs me that it will answer after six rings. For now at least. Soon it will be blazing a trail across the night sky. On fire with my boot behind it.

Deleted member 9648 · Oct 8, 2010

Try operating the heating controls with the boiler itself disconnected....on some boilers there are capacitors which can fail and cause intermittent and odd tripping of RCD's.....We had many return visits to a tripping problem which was eventually traced to a tiny capacitor which had disintigrated,when the boiler heated up the capacitor would move just enough to short out on the side of it's enclosure!!...Some boilers have a supression capacitor which is also a common cause of tripping.