Boiler overheating?

[towelman]

Hi
I have a Keston c36 (yes I know....) and have a number of people baffled on a problem.
We had the problem originally of the CH only kicking in when the HW was demanded. The HW usually works, but the CH is sporadic. It does sometimes work on the timer in the mornings 30% of times, but rarely in the evening!!
So we have, tested the diaphragm (holding pressure fine), drained system twice and applied centinel sludge remover, replaced the prv.
The symptoms now are:
Boilers starts up, pipe gets hot, display reads that temp is getting to above required setting, boiler stops, heat in pipe reduces, first radiator gets warm at best,others remain cold. Flow pipe remains cold.
The temp reading always seems to think its up to temperature and so switches off. (i'm presuming here that this display is showing the system temp, ) So it seems to me its overheating next to the boiler and unable to get the heat away, but I'm far from understanding it. Turning the HW on seems to help this, but not always.

Any ideas? My repair man has drawn a blank!
Thanks!

 

[45yearsagasman]

Circulation problem would be my first thought from your symptoms. Pump performance or a restriction somewhere. I can't believe your man has not checked this basic first step.

 

[towelman]

Thanks for your advice. Strangely enough I've tried it with the cover to the boiler off and it seems to be better, ie not getting so hot. All Rads are now hot. Is there a chance an air inlet blockage could cause it to overheat?
I'll try for a day or two and see if it keeps going....

Cheers

 

[twgas]

that's a room sealed boiler - do not run it with the case off.
I would agree it is circulation, possibly a blockage somewhere,
the air intake is almost certainly not blocked.

 

[towelman]

Ok thanks, have replaced the cover!!
The air intake is clear, so I'll see if it overheats again now the covers back on. I presume if its circulation I need to get the system flushed professionally? Is there anything a keen diy er can do beyond adding the anti sludge chemicals?
cheers

 

[yodaveg]

Might be just air? Try running the heating with just a couple of the rads on, the pump may be able to shift any air in to them. Also check the isolation valves that are under the boiler on the flow and return pipes are fully open. Make sure your rad (trv's) valves havent stuck in the shut position.