I recently drained my heating system to replace the radiator thermostatic valves and now cannot get the boiler to keep running. I suspect I have air locks either in the boiler or in the feed/return pipes to the boiler. The boiler is a Baxi back-boiler situated downstairs, with the pump and bleed valve in the airing cupboard upstairs. The pump has been bled (as have the rads). Is there a technique I can use to purge the air from the boiler and main pipes? I was thinking of attaching a hose to the boiler drain tap and opening the main drain valve to force water through under pressure to try and remove the airlock. Is this a reasonable idea or do you have an alternative suggestion?

If you have standing radiators, each one will have to be bled.

Thanks for the feedback. I've already bled the radiators starting with the lowest ones and working through them all. I've bled the pump, bled the valve at the top of the system. Then after a ful day I've re-bled the radiators (in fact I've done them all three times now). I am getting no air out the radiators now (just water) and nothing out of the pump (occassionally it will release a tiny amount of air but not enough to stop it running).

When the boiler trips out the pump continues running so I'm fairly confident the problem is located either in the boiler or feed & return pipes to the boiler, neither of which have belled-off points, although I have tried opening the boiler drain-off tap which did initially release lots of air.

I've tried this again since with less success (ie. Less air in the water but still small amounts) which is why I was wondering if pressurising the system with a hose pipe might push any air locks out. I haven't heard of anyone trying this before but see no reason why I shouldn't try it, but thought I'd ask the experts on here before giving it a go.