My initial problem was with a large downstairs radiator that does not get hot. It's the furthest from the boiler and the other 7 rads on the system are fine. I decided to drain and flush the system (I have 8mm piping) and took the offending radiator off to flush it out. It was mucky but not that bad. My fix didn't work.
If I switch my pump up from speed 2 to 3, the radiator gets hot but then the pump sometimes pushes water up into the header tank and air gets in. Within 24 hours I have 2 upstairs radiators almost full of air.
The pump tees into the feed pipe from the header tank via a valve. I assume this is some sort of bypass circuit. I've got the valve almost off in the hope that it would stop air getting in but it still does. Any ideas please?