Hi all, hit a tricky one here. Having recently noticed pump/boiler getting very noisy and looking up potential causes I decided to run some Fernox cleaner through the central heating system via the header tank. Haven't been up to the loft in a long while and not anywhere near the header tank for months and months. So what I found was condensation and steam and a very full header tank with constant running water from the vent pipe. Water level is just below the overflow outlet and didn't appear to be running into the overflow. After a week of letting the Fernox do its job I have now been up, tied up (and today changed) the float valve and drained the CH system. Then refilled the system and as soon as I switched on the CH control at the boiler went straight into the loft and the vent pipe was already flowing out water, not even hot yet. I've scoured Internet for a difinitive cause but am struggling. Does anyone have a clue what this could be and if there is a fix I could do? Many thanks in advance.