Ravenheat combi boiler starting problems

[One for the boiler experts, I suspect.]
I have a Ravenheat RSF 20 (or 25) combi boiler which is more or less working ok, but exhibits an intermittent fault when re-lighting while it is hot.
When the CH stat is calling for heat, it seems to sometimes take a couple of minutes to light. While failing, I can hear the fan starting and stopping and a clicking sound (like a relay or solenoid). The clicking is irregular. It will eventually fire up, but I fear something is on its way to total failure (especially as winter is upon us).
I started going through the installation manual troubleshooting guide, suspecting a fault in the fan or pressure/flow switch, when I heard water running and the boiler started immediately. A little experimenting confirmed it will always light immediately for HW. So, there is something amiss in the CH side. I wonder if it could be related to the modulation function - might it be wavering about whether heat is needed or not ? This problem does seem to happen more often when the system is up to heat. (Another way of saying I'm not awake when it comes on in the morning...)
The CH is controlled by a wireless digital stat, but the starting problem does not go away when I override that using its relay. Fiddling with the boiler thermostat does not appear to make a difference, although I have not been very rigorous in testing that.
All ideas welcomed. I fully expect it will be an expensive component (everything on this boiler is, by the look of it), but like to be fairly sure I am replacing the right thing when it comes to parting with cash.
--
"The road to success is always under construction."


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This is a bit like my current problem with a Halstead Finest (Wickes 90). DHW, no problem. CH, lights fine when cold, doesn't always re-light when warm. They do three re-tries and with the covers off you can hear the ignition spark (about ten regular clicks in three seconds), and then lock out. Irregular clicking sounds like thermal expansion to me. If you have a DVM and don't mind playing around inside you might find it interesting to hook up to the "modureg" valve which throttles the gas inlet. On the Halstead, the voltage modulates from 2.5 to 20 V according to the book, actually 4 to 18 on mine. But it should set to about 10 V for a few seconds during the ignition sequence. Drops back to ~ 4 once it is alight, and ramps up to full power over the next minute or so.
On mine I have just changed the board and it is better but still misbehaves. I've also found my thermistor is out of spec and when the new one comes I will measure it and post the data. If you *do* decide to change the board, give Geoff a ring (www.cetltd.co.uk) and see if he can do you a reconditioned one. About 55 instead of 200 in my case. Highly recommended.
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Newshound wrote:

Thanks for the suggestions. The "clicking" is definitely something like a relay. If the fan were running all the time, I would investigate the gas valve side, but it cuts in and out, although I can't really tell if there's a sequence of clicks and starts or stops or not.
Going in with a multimeter is the next step, but I will also try some systematic tests of the thermostat. If the boiler is hovering on the point of restarting, then presumably turning the stat up or down will make it more decisive.

I am aware of Geoff's reputation, but thanks for the reminder !
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