Glow worm Ultimate boiler, with what I can see to be an open vented system.
Not had the system apart. Other than the above fault, it would seem to be operating normally.
Layout very similar to this -
Up in the loft this morning replacing a float valve on the cold water tank I noticed that the feed and expansion tank was hot/steaming. When the main hot water tank was heating there was a continual hot water flow into the feed and expansion tank from what looks like a vent pipe. The steam in...