On 11/12/2017 13:32, newshound wrote:

It's a combi, isn't it? So there are in effect two heat exchangers, the
one built into the "firebox" which heats the radiator water, and the DHW
secondary heat exchanger. When you call for hot water, the diverter
valve puts the "radiator water" through this secondary HX to heat cold
water coming from the rising main.

It's not uncommon for this secondary HX to block, but that affects the
DHW rather than the heating.

Kettling and an overheat alarm does sound consistent with blockage of
the HX which sits above the burners.

I'd be inclined to think you've done well to get 18 years out of a
combi. These have quite a few other "bits" to go wrong.

Indeed. Diverter valve was replaced after a few years but didn't bother
when it packed up 2nd time round as only really use it for heating.

Last year was the first time I've had to do anything to it. Fault code
showed as burner not igniting when it was but was then going out.
That just required a quick clean of the flame sensor thermocouple, that
was it so all in all it's been fantastic given the amount of use it's
had over the years.

It's a bit strange that the pump is running and hot water is coming from
the burner side into the pump but nothing coming out of the pump.
It had been slowing down over the last few weeks with water flow to the
C/H must have been slowing down as boiler would burn and temp would rise
quickly then shut down so I think a flow blockage somewhere or pump
impeller has done something strange. end of pump shaft is spinning and
srewdriver on the ear certainly sounds like it's pumping.

I have 2 motorised port valves, 1 for upstairs on c/h timer and one for
downstairs on H/W timer. I don't think they could both have stopped
working.... Hmm... just off to see if they're both opening...
If timer isn't firing either of them then the water has no-where to go...