WORCESTER BOSCH BOILER 19/24 CBi Irratic runnig problem

[barrywat]

This has been in the house 6 years from new.
The problem is that sometimes it starts and stops as requested by the timer.Other times the boiler pumps runs on for hours(it should stop after about 15 minutes from shut down).Other times the boiler wont strike up when the timer asks it to.But you can activate it by turning the dial on the boiler to zero and then back up then it strikes up. The boiler has been regularly serviced and the stat changed 18 months ago when a similar problem occured.Everything possible has been checked out and we cant find anything wrong,it should work properly.Flu ok,pcb seems fine as far as we can tell.Water test proves no possible blockages. Its been servied and sorted by a reputable company.
Any ideas what else might be worth looking at. Could it be the stat could be faulty again.

 

[bengasman]

If all else is tested and proven ok, it is the pcb. Sadly, there is not way of testing them.

 

[barrywat]

If all else is tested and proven ok, it is the pcb. Sadly, there is not way of testing them.

thanks for this on the pcb not even if i send it back to worcester bosch surely it can be tested by the manufacturer on some sort of test bed or am i wishfull thinking here.

 

[numpty1]

wb 19/24 cbi's have a couple of issues in my experience
flame rectification is the main one,burners have a tendency to split on the rear rhs side,early boards can give similar faults to yours,personally i would do the unapproved method of splaying the probes.worc may of got the probes sorted by now.All the above are all common faults experiened in the early years of this models life.

 

[bengasman]

...thanks for this on the pcb not even if i send it back to worcester bosch surely it can be tested by the manufacturer on some sort of test bed or am i wishfull thinking here.

Yes, of course the manufacturer can test them, and so can specialist companies. I meant it literally when I said: "YOU can't test them". Nor can I or any other RGI that I know off. Not on site anyway.
It is rather pointless to take a pcb out, send it off, pay to have it tested, and then stick it back in if it turns out to be something else that wasn't tested correctly after all.

 

[barrywat]

...thanks for this on the pcb not even if i send it back to worcester bosch surely it can be tested by the manufacturer on some sort of test bed or am i wishfull thinking here.

Yes, of course the manufacturer can test them, and so can specialist companies. I meant it literally when I said: "YOU can't test them". Nor can I or any other RGI that I know off. Not on site anyway.
It is rather pointless to take a pcb out, send it off, pay to have it tested, and then stick it back in if it turns out to be something else that wasn't tested correctly after all.

fair comment and many thanks its good to have someone who can see the wood from the trees on things like this otherwise you go of in all sorts of directions and end up a blind alley.i have been monitorng the system for the last 48 hours,its times to do an hour on am and an hour late pm for water only,, blow me if all isnt working to schedule again,time on off ok,boiler pump out after 15 mins.i would have thought if the cb was duff then the system would be playing up al the time not correcting itself for a couple of days. it did this a few weeks ago and played up then suddenly corrected itself and ran ok for a short while then played up again.is this what faulty cb,s do,work ok and then dont

 

[bengasman]

Sounds like a loose/corroded electrical connection which could be anywhere in or outside the boiler. Unlikely, but not impossible, poor earth.