Always getting air lock when refilling system

[markyhud]

I have drained and refilled my system a few times (with some cracking advice off here) and each time I still end with an air lock. After draining down I re-open the valve in the f&e tank in the loft, let it fill and then start bleeding downstairs rads first to encourage filling. Once all the downstairs is done I move upstairs and again bleed each rad in turn I then bleed the screw on the pump and finally the vent in the airing cupboard next to the coil. I then have the boiler set at 0 and turn on hot water for a few seconds, go upstairs and bleed the pipework again. I repeat this a few times and then turn on the hot water and turn the boiler up. I then switch off the hot water and then turn on the central heating by itself. I work round each rad and bleed any air.
After all this I still end up with air in the pipework and coil. Grrrrrr!!!
As the system is initially refilling should I leave the vent open next to the coil to encourage filling of the airing cupboard pipework?
I was thinking of partially draining it down (via lowest rad) and then using cold mains water and refill from the bottom up. I am hoping this would fill the pipework and coil and push any air up into the tank in the loft. Is this feasible?

 

[markyhud]

OK so I took some time to read up on f&e tanks, positioning of vent pipe etc as I am continuosly chasing air round this system. When I loooked at the f&e tank it was about 3/4 full when cold. Now I believe this should be about 1/3 full or approx 100mm. So i bailed some water out and bent the arm of the ball cock. After this I noticed the vent pipe (highest section above the tank before the elbow) was only 300mm above my new water level. From reading I know there is a calculation for the correct height of this and I think mine might be poorly configured. I then started to think that may be the pump was drawing in air through this vent pipe as the system was always very noisy on start up and it seemed like it was starting at the boiler. So I also turned the pump from speed 2 to 1. Initial impressions are that it's quieter but will have to wait and see. It's a 3 bed 2 storey house with 10 rads. They all heat up but some are hotter than others on speed 1. I could put up with this if it will solve the noise issue.
I've also noticed that the auto air vent next to the hot water cylinder actually does vent automatically but in order to do this it has to weep slightly. I have put a towel underneath and keep checking on this as it's obviously not ideal. Are the cannister type auto air vents any better than the cheap slot head threaded type.
I think i'm finally getting somewhere but will definitely need the help of a professional to come and have a look and possibly fit a filter

 

[diamondgas]

Aso you reduce the pump speed you then show up potential system balance issues! Lower speed will emphasise poor balance IMO

Regards the auto vent, a 'float' type are best in my opinion, much better than the 'expanding cloth' type you have fitted?

Thing regarding the open vent; if it's fitted in the correct place you'll not get air drawn in through it......... also to achieve air being drawn in, you have to have a really poorly restricted circulation that produces such a negative pressure on the vent ...? If the reduction in speed reduces the noise then let it run that way till it's quiet then try stepping it up to two! If the noise returns the turn it back down...