Hi, My boiler is currently cutting out after a minute or so. It starts banging and sputtering and throwing water out of the vent pipe. System - Gloworm fuelsaver 35f, hot water cylinder, 3 port valve, grundfos pump, 4 rads. So I'm thinking a couple of things; 1 - Air lock, but no air is coming out of any of the rads. 2 - Sludge or other type of blockage. We have an air separator just after the flow pipe comes into the airing cupboard. When the banging starts, I notice the heat travelling up the vent pipe and not towards the pump. Once the banging stops, there is a lot of gurgling in the pipes and the heat eventually travels along to the pump. However by this time the boiler has overheated and cut off, and then the process just repeats itself like this! So does anyone have an idea if its no 1 or no 2, or maybe something completely different? If it is a sludge blockage, can I do anything about it myself. I have tried to search for a how to on cleaning system. Is it drain some water off with hose pipe, add cleaner into small tank, let it run for a few days, then drain and refill with inhibitor? Also I'm in a 1st floor flat, the drain valve is at the floor level at bottom of airing cupboard but this is not the lowest point as the boiler flow and return go under the floor. Any help much appreciated.