[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."