intermittent fault with central heating and hot water

29 Jan 2004
Reaction score
United Kingdom
We have an intermittent fault on our heating and hot water - it sometimes fails to come on in the morning. The timer is switched on and there is a humming type noise in the airing cupboard (where the timer is) but no heating or hot water. If I turn the timer off for both heating and hot water and then turn just the heating on and open the motorised valve by pushing the lever across, the heating will come on and if I then turn the water back on, it comes on too. This morning the heating came on when it was not programmed to.

I have had an engineer out 3 times in the past 2 months as this fault is occuring once a week. I have had the boiler serviced (Ideal Minimiser SE), the motorised valve for the hot water changed and a new timer (Honeywell ST6400c). The timer is set so the heating and water come on twice a day and strangely enough, the fault has never occurred when the heating comes on in the afternoon. The timer is set for the heating and water to come on at 6.10am and off at 8.30am and then the heating is on from 3pm until 9pm and the water from 4-8pm.

Any ideas? I have an engineer coming out again tomorrow but am beginning to lose hope .

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The Ideal Minimiser can suffer from lockout in the mornings, usually caused by damp probes. Get your engineer to try the following;

Adjust spark gap to 5mm
ensure detection probe is aligned with burner ports.
Increase burner pressure by 1mb
Remove the pcb 37 and fit a pcb 41

Ideal now provide a new pcb (41) which gives the boiler 5 ignition attempts and a longer pre purge which normally cures lockout.
Thanks for this advice. The heating engineer came out today and I now have to do a couple of checks the next time the intermittent fault occurs just to be sure it is the boiler at fault.

I have sourced a PCB 41 on line at £55 + VAT which he says is very cheap and would be prepared to fit a PCB I have bought if this should prove to be necessary.

Thanks once again. I feel there is now light at the end of the tunnel!

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