Hi. I've read a number of posts but none quite seem to match my particular scenario so I'm doing the recommended thing and starting a new post. I've had a couple of problems. The worst has been lots (10 to date) of radiators springing leaks. One went 15 months after being replaced the first time. Most of this has probably been caused by the system never being topped up with Fernox over the last 10 years and what with radiators being taken off for decorating and thus taking any protection that was in the system out over time it not so unexpected. However, in the course of this I've found something else that I now know to be bad and probably adding signifcantly to the radiator corrosion problem. The header tank vent pipe constantly runs hot water into the header tank, probably aerating it much like the little water feature does on my pond. While the fish like it, the radiators don't I'm sure. The water in my central heating system is pure and clear. Its been flushed out a number of times due to having to drain down the heating system because of the failing radiators. The header tank is spotless. The heating system has in it (to date) 8 litres of Fernox MB-1. The system is a Potterton Profile 80e, a Grundfos super selectric 15-60 (on setting 2 I think), and there are two Honeywell V4043H valves, one for the CH and one for the HW. Both are working. The expansion pipe from the header tank is not blocked. We get lovely hot radiators and lovely hot domestic hot water. If the HW valve is open and the CH valve is closed, the vent does not run and the header tank stays cold. If the CH valve is open, and the HW valve is open or closed, the vent runs and the header tank gets hot water in it. The header tank overflow is not running, ie no evidence of the hot water cyclinder leaking water back into the central heating system. A couple of radiators do need to be bled every now and again, but I'm assuming thats because of the aeration occuring in the header tank. There are air type and kettling type noises that come from the boiler and pipework around the pump, but they're not constant, almost sounds like a big air bubble rushing around part of the system on a regular basis, although it sometime comes and goes. So, I'm baffled by the running vent but I know I need to get it resolved or I'll have my new rads eaten away before I know it. If there was a blockage in the CH side that might explain it but all my rads get hot. And the vent doesn't run if just the HW is on. But it does run even when the HW is on when the CH is on.... which I can't get my head around. So sorry for the long post, but anyone got any ideas as what I need to look at? Thanks. Mark.