I would definitely be of the opinion that if the pump head/impeller was installed during this build up that the impeller eye/vanes could be some what occluded, I include a picture of my 18 year old Salmson pump head which was still running perfectly after 18 years and is still in spotless condition used in a vented system (with the odd drop of inhibitor thrown in) so when/if you remove the pump head you should be able to see the impeller and make sure and clean out any deposits between the vanes and also the impeller eye.
Don,t know what your boiler output is but assuming a maximum deltaT of 25C then 15kw needs a circ flow of 8.6 LPM, 20kw 11.5 LPM, 25 kw 14.3 LPM and 30 kw 17.2 LPM, all at full firing.
If you have a UPS2 6M (60) pump running on speed2 which makes it a 5M pump and if you look at the pump curves or confirm that it is a ups 2 then if CH only on from cold that it might be a bit of a struggle to circulate enough water to keep the temp at 25C with anything bigger than a 20kw boiler but if you can also (first thing in the mornings) have both CH & HW on together until the boiler cuts out normally at its setpoint temperature (say 30 minutes or so) then one would expect plenty of circulation.
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A view of the impeller eye (not a great photo)
That's clean for 18 years! I will give the impeller some TLC.
This morning the house seemed to heat up ok all aside from that one rad again. The boiler trips out most often with overshoot I think - if the current temp exceeds the target by more than 5 deg it shuts down for a few mins.
The boiler is an 18KW unit, I have rated it down using a diagnostic command to 11Kw as I only have 9 radiators and I thought if it is putting less energy into the water it will not trip up so often. the temp increase should be steadier.... The pump is a UPS2.