Hi . either a major blockage or air in the system that caused the pump to run dry and burn out . if the only change was a drain down and new rad fitted then it has to be an airlock. Look for manual air release valves at all high points. You might have to add a couple. Check the new and old pipework for natural air traps , ie a loop or drop going down or up.. If all else fails
Cap off vent and feed and fill via the mains/ or change to a pressurised system
thanks. This problem is taking forever to fix. also very worryingly my coal fire which should heat water / heaters is not doing anything now - what on earth... - and is it safe???
I think you shouldnt use the fire till its sorted as it may boiler dry and crack. I recently had to change most of the pipe work on a back boiler . completely scaled up in areas from the time when it was gravity and not pumped