water running from overflow

[feeltheheat]

i have a potterton kingfisher 2 boiler. When the boiler has been running for some time and gets hot, it the starts to spill water out of the over flow. The boiler is not over heating, the boiler stat and cyl stat are ok. I had the system power flushed today because of this problem but it is still happening. When the water came fom the over flow it was very dirty which prompted me to get it cleaned out professinally. ANY IDEAS. the system is open vented and on an 's' plan.
Cheers

 

[gas4you]

Back to basics, have you tried setting the ball valve in the F&E tank to give a lower water level?

Is the HW cold storage cylinder higher than the F&E?

If so the coil may have gone in the HW cyl and it is back filling the F&E tank.

 

[bengasman]

lower the level in the f&e tank