We have a nearly new central heating system (installed 2 years ago), (Worcester Bosch combi boiler Greenstar jr , standard double convector radiators, plastic pipe throughout the house) and I'm having problems with some of the radiators heating up at the top but not at the bottom so I was looking for some advice as to what might be causing it? I very much doubt this is sludge in the system as we had inhibitor put in when installed (just to be sure I have checked the water which is clear and smells of inhibitor, and have also done a test with a shiny nail in a jar of water which has stayed clean in the system water, so I'm pretty sure its there). I have also ruled out the TRV as I've tried swapping this with one from another room which has made no difference. I have been able to temporarily rectify the problem in most rads by shutting them down and running the system for an hour, then turning them back on. This seems to fix it for a while, but then if the heating has been turned off for a bit (like over the summer) it comes back again. Its nearly always in the lounge radiator, which is one of the three closest to the boiler. I'm guessing somehow its air trapped in the radiator (which seems strange as its cold at the bottom, not the top, and I get nothing but water when I try to bleed it)? If so is there something clever I can do to clear it without draining down the system? Or could it be something else? Thanks 
