Central heating overheat

[Bob Marr]

Can anyone help with normal vented heating system glowworm boiler 50FF
1. Overheating boiler tripping out but only when just water heating on.
2. feed and expansion pipe getting hot.
3. hot water pumping into FE tank
4.Pump has been checked and appears ok
5. motorised valve working ok.
My thoughts are there is a blockage in the T where the FE pipe joins the main heating. The pipes around that area are attracted by a magnet. Does this sound logical or is there something else I should look at.
Incidentally there is a shut off valve on the FE pipe near where it joins the main heating. It is the same as used on domestic basins toilets etc. Should this be there? surely it would easily block being of such a small diameter.
Any help would be much appreciated.

 

[45yearsagasman]

View media item 55283
Classic symptoms of blockages. Only proper way is to cut out and clean/replace affected pipework. If the system has been pumping over for a while then a flush would also be beneficial. That isolating valve is best replaced with a 15mm gate valve, or removed completely. Personally I would fit a full bore lever type valve.