Answer: -I don't know if these will have enough detail for you, but you could try: http://www.diyfixit.com/nflash/CentralHeating/CHSystems/systems.htm -Hopefully this is the vent from the top of the secondary cylinder not the primary vent. Is the cold feed to the bottom of the hot cylinder obstructed? Is the cold header filling properly? Is there a gate valve between it and the cylinder - should be fully open if there is. Sludge in cold tank or blockage in pipe? -Imagine closing off the supply from the loft tank to the cylinder, then opening the bath tap. The water in the vent would run down, and once it was gone the flow would stop. If the rate of flow of water from the tank to the cylinder was too low you'd sort of see the same effect, except it would tend to recover every so often. This sounds like what you're getting. I'd check that this is flowing freely. Is the feed from tank to cylinder an unrestricted 22mm? Stuck valves? Is the pipe from the tank to the cylinder sloping down or at least horizontal all the way along? If not it may have an airlock at the top and be acting more like 15mm or less pipe. Scale plugs? BTW, if you're posting ASCII art it helps to use a fixed width font like courier.