Our old sealed CH system would always get a lot of air in after it was drained down, resulting the boiler overheating and cutting out. I learned in time that the boiler inlet and outlet pipes, above the boiler and a couple of inches below the ceiling, needed to be loosened to purge the air as part of bleeding the system after draining down, and one plumber installed bleed valves in them. After the latest draindown, the boiler isn't overheating but some air remains in the system, enough for the circulation pump to make whooshing sounds from time to time. Not loud but a bit irritating and none of the air ever gets to a bleed valve where I could deal with it. The photo of the top of the boiler shows that the back bleed valve rises a little above the pipe. The front one is teed off horizontally or a little below horizontal, so the air doesn't rise into it. OK for purging large amounts of air, useless I fear for that last little bit. So my question is, how to get this last air out? I'm running the hot water and switching to central heating for a few minutes each day, hoping to move the air into the rads where I can get at it. But each time I switch back to hot water, after a minute or two the air whooshes through the pump again. It may be a little better, but this has lasted several weeks and two plumber's visits. My first thought is that I cut a hole in the ceiling to the loft space above and get a plumber to turn the horzontal tee vertical to a pipe into the loft, with a bleed valve, auto or manual, on the end. Do people think that's likely to be the source of the problem? If so, can a plumber do what I ask, in terms of being practical and within regs? Or do you have any clever tricks for this sort of problem? Many thanks in advance.