I have been having problems draining my central heating system. I have two draincocks in the garage underneath my combination boiler. One is 1100mm from the floor, the other is 200mm up. I have no problems with the higher draincock and water flows out as soon as I open it. Nothing drains out of the bottom draincock however, which means that I can't empty the radiators downstairs. I am about to change the radiator in the hall which will involve some adjustments of the pipework. Obviously I won't be able to do this work if I can't empty the lower half of the system.
Has anybody got any ideas why the lower draincock won't work? Is there something obvious that I am not doing?