How to drain the central heating water out of a Vaillant combi boiler.
(My boiler was losing pressure, so I checked a few things and worked out that the Schrader valve had stopped working properly in the expansion vessel. So I needed to empty the water out of the boiler to replace the valve.)

TURN OFF THE POWER TO THE BOILER:
Open the front cover.
Turn off the power.
Turn off the power at the wall switch too.
Close the front cover.

DRAIN THE WATER OUT OF THE BOILER:
Isolate the boiler from the rest of the heating system by turning off the drain valves with a 4mm Allen key, so that the slot is pointing across the pipe.
Attach an 8mm diameter hose to the drain valves and use a bucket to catch the water.
Open the drain valve using an 8mm ratchet spanner.
Wait forever...

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