My central heating system has a cylinder and a small tank in the loft so I think it counts as 'open vented'. I don't go into the loft too often but I have noticed that whenever the heating is on, hot water gushes quite fast into the small tank (which I presume is the expansion tank). See photo. The water doesn't overflow, so presumably it is returning to the system by another pipe which runs from the base of the tank. It's not a trickle; the hot water is pouring in steadily at about a pint a minute. Is this right? I was under the impression the expansion tank was a sort of pressure relief system that only operates in some circumstances.