heating vent pipe is venting hot water and the cold feed from tank is getting hot and pumping hot water into the boiler. the boiler is a potterton netaheat mk2 1016.

mick burns, February 2013

This can happen when the pump is running but valves like thermostatic radiator valves have shut. A closed motorised valve will do the same.
You need to find out why the pump is trying to push water where it cannot go-hence the reason why it is pushing the hot water up the expansion pipe.
Be warned, if your header tank is plastic and the water in the tank gets too hot, then the tank could fail with devastating results

GB in Wales, February 2013