This Really is The Last of The Radiators.

It seems the problem with the basement radiators was an air lock. I shutdown all the other radiators in the house and then opened each in the basement one by one, so it had the full force of the pump. Lots of gurgling ensued and after bleeding, all is well. Reasonably well.....

There is a balance problem. These three in the basement are furthest from the boiler so pose the greatest resistance to water flow. As a consequence they are not getting sufficient water to heat up fully, even with the lockshield valve fully open. I think I am going to have to close some of the other rads in the house down a bit to improve it. The official way to achieve this is with a differential pipe thermometer to measure the temperature difference across each radiator. It should be around 11 degrees. Starting at the rad closest to the boiler, with the thermostatic valve fully open (better still, head removed), the lockshield is "tuned" to give the required temperature difference. Hopefully, by the time you reach the furthest radiator there is still sufficient capacity in the valves to achieve the 11 degrees, even if the last one is fully open.

I guess if the 11 degrees can't be achieved on that last rad, it means the pump is undersized.

I don't have a differential wotsit, so am applying guesswork. It is better than it was, though I think the basement radiators still have a much greater temperature drop than the others. Fortunately, the basement is partially underground and well insulated. Even 3 under-performing radiators seem to be keeping it warm.

Strangely, one of the double panel rads in the lounge is only getting hot in the front panel. The back panel must be blocked with something. You can take the bleed screw out completely (these rads have a separate bleed screw for each panel) and nothing ensues - no water, no air, nothing. I think it is going to have to come off and get the full colonic irrigation treatment.