In an earlier contribution to this discussion,
JP wrote:

My 10 year old gas CH system has gone on the blink.

The boiler fires up fine on the Hot water mode, but does nothing when I
try run the CH.
Any clues as to what this utter novice should attempt next
TIA

An awful lot depends on the design of the system, and exactly what
components it has in it.

But, seeing as it's Christmas, I'll take a flyer and go for the most likely
cause. I'm going to guess that you've got a fully pumped Y-Plan system with
a 3-port mid-position motorised valve which directs the hot water from the
boiler either to the indirect coil in the HW cylinder or to the radiators or
to both at the same time. If I'm right, you have a problem with the valve -
or more likely with the actuator (the electrical bit) - which is failing to
move away from the HW position. Once the HW demand is satisfied, if the
valve doesn't move to either the mid or CH position, the boiler won't come
on because it's a microswitch in the actuator which actually switches it on.
So, my diagnosis - starting with the most likely - but based on very little
information(!) is:
* failed motor in valve actuator, or
* mechanical part of valve seized, or
* faulty room stat - failing to relay the CH demand to the valve

Clearly, you'll have to sort out the valve and/or stat, but to get some heat
in the meantime do the following:
1. The valve should have a lever on it which enables it to be moved manually
to the mid position. Operate the lever, and find a way of holding it in the
manual position (string, tape, etc. if it doesn't have a mechanical latch)
2. Turn the boiler stat down a bit to about 65 degC to prevent the DHW from
getting too hot
3. Turn the cylinder stat up to above 65 degC

The system will then run and will attempt to heat the HW to the cyl stat
setting - which it won't be able to do - BUT the radiators will get hot -
albeit not quite so hot as usual.
--
Cheers,
Roger
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