Thorn Heating System Fault

[MickeyV]

Good afternoon people, my first time here and I hope to get some answers.
My system is 30years old and still going strong apart from one thing, whenever I switch the water or heating on, the water in the header tank in the loft starts to overflow to the outlet pipe outside from under the guttering.
I have checked in the loft and at the header tank....ball cock in good condition and working normally...BUT once the water/central heating is switched on the boiler in the kitchen fires up and is forcing water back to the header tank via its feed pipe.....the water being returned is hot!
Do I have an airlock in the boiler.....have bled the whole heating system and everything is running with clear water and no air......Stumped.

 

[doitall]

Any idea where the extra water is coming from, is the ball valve running for example when its overflowing

 

[petit_pablo]

It sounds like its possibly blocked somewhere. Close coupling where the cold feed and open vent tee into the system is a favourite.

 

[doitall]

It sounds like its possibly blocked somewhere. Close coupling where the cold feed and open vent tee into the system is a favourite.

If that was the case the system would be full of air.

 

[MickeyV]

All ball valves are open, pump in airing cupboard has a 3 port mid position valve above it which allows for flow in one direction for central heating and switches over for the hot water flow to the main tank when required.
Both have the same effect when activated..the boiler pushes water back to the header tank
The pump which is a WILO Gold 50 has 3 settings, this one is set at No 2.
Could that setting be too fast?
Thats as much information as I can give you gentlemen.