Are you confused on how to drain central heating system? There are many ways to do this task. Although you can find many more ways, I’ve found this way easiest.
Open the drain-valve. Using the radiator key, open the upstairs radiator bleed valves a few turns, to allow air into the system, to allow water to flow out. When the water level is below the top of the downstairs radiators, open those too (unless you are only changing upstairs radiators, or fitting new valves). While the system is draining, check the header tank to ensure that no water is being fed back into it from the float-valve.
Check that draining has finished, remove the hose, close the drain-valve. Close off all the DOWNSTAIRS radiator bleed valve screws. If changing radiators or valves, or adding THERMOSTATIC RAD VALVES (TRV’s), now is the time to do it. If adding inhibitor, cleanser, descaler, or sludge remover, now is the time to add it to the system, in the header tank. FOLLOW THE MANUFACTURERS INSTRUCTIONS, AND ANY SAFETY RELATED LEAFLETS.
Only when you are sure that all the radiators are bled of air, should you allow the boiler to fire-up (put it on a cool boiler thermostat setting at first). After a couple of hours circulating, go around all the radiators again, and them put your boiler thermostat up from cool, to its normal setting. Some cleansers and descalers require the system to run hot for a number of hours before draining down again. FOLLOW THE MANUFACTURERS INSTRUCTIONS CAREFULLY.