The first thing to do is make sure the boiler is completely cooled down and switched off. Next turn the off the water supply to the heating header tank (unpressurised systems only), the stop cock may be in airing cupboard or in the loft near the header tank, if the stop cock appears seized and you don’t want to force it you can tie the ball cock up with a piece of string (see picture).
Note! There will be a large tank for the cold water and a small tank for the central heating, also you could turn the water off at the mains stop cock, but you then have no water supply to make a cup of tea.
Pressurised System is only connected to the mains water supply when being pressurised, so there is need to isolate them when draining.
Locate the drain tap their maybe more than one or none, it is always best to use a hose clip when connecting the hose pipe and a towel to soak up any drips.
Be careful when draining not to let it drain onto any flower beds or grass as the chemicals could kill them.
After about 10mins open the bleed valves upstairs and 5mins later open the downstairs valves, you will never get all of the water out as some pipes will be lower than the drain tap (under floor etc.). Once drain closed all the bleed valves and drain tap.
Tip! You can use a self cutting drain off tap if one is not fitted.
Check that all the drain taps and bleed valves are closed, then follow one of the below.
It is best to add an inhibitor to the tank to prevent corrosion (Fernox etc.)
Then turn on the water, starting at the downstairs radiators first, go round one by one until the water comes out of the bleed valves you should be able to hear the air coming out, then go upstairs and bleed the radiators upstairs.
It is best to add an inhibitor to system (click here to see how).
The system is filled using a filling loop (may be silver braided hose or solid pipe) it will have two small taps, turn one tap (1 in picture) to the fully open position (they will only turn 90 degrees and maybe quite stiff) then turn the second tap (2 in picture) slowly until normal pressure is shown on gauge normally around 1bar (marked green see picture).
Starting at the downstairs radiators first, go round one by one until the water comes out of the bleed valves you should be able to hear the air coming out, then go upstairs and bleed the radiators upstairs. Keep an eye on the pressure topping up if necessary.
You may find you also have a bleed valve for the hot water near the tank (see picture).