Thank you very much to those who replied to my post about this subject. Much appreciated. I turned on the hot water and the vent pipe is pouring hot water into the expansion tank continuously. Scary. I have shut the system down pending a plunber arriving. My own feeling is that the problem is being caused by sludge in the system (despite inhibitor being put in two months ago when a new pump was fitted). A neighnour has suggested it may also be a fault with the hot water cylinder in the airing cupboard. Anyone know how much it would cost to a sealed system?