Central heating sometimes pumps over

[giles_lisa]

Hi there. I had a problem withy my open vented heating system sometimes pumping over into the F&E tank. I could stop it by running the hot tap.
I have just had the boiler and cylinder replaced and motorised valves installed and still the system sometimes pumps over.
I think that as the open vent pipe just lips over the F&E tank expansion may sometimes fill the F&E tank then as the open vent is then under water this siphones from the system.
If I extend the open vent to 2 feet above the tank and leave sufficient gap so it cannot siphone do you think this would be an end to my problem?????
Cheer
Giles