My combi is upstairs and is an open vented installation. Recently the expansion tank has been overflowing (through the warning pipe) .
I think it is due to the fact that when the water expands it obviously fills the tank - when it contracts it allows more water in. Then the next time the heating goes on it expands and overflows.
The tank is very small and I think this is the problem. However, space is a bit limited due to all the cross timbers. Are there any other solutions - I am even thinking of converting it to a closed system (it is a Worcester Heatslave) but I am a bit reluctant in case the 12 year old systems starts to leak if the pressure is higher. I guess I would have to plumb in a pressure release vent pipe to the outside wall.