draining central heating system

[bowie77]

hi all,
i need to install a new radiator to a room without an existing one so i need to drain the system. i have a vailant ecotec pro boiler fitted. i did drain the system a couple of years back to move a radiator but i then had to have the water sensor replaced, not sure if i did something wrong.
so how do i drain it correctly?
cheers

 

[denso13]

There should be a drain cock sticking out somewhere. One of these -

Maybe outside below floor level, sometimes below the floor internally.

 

[bowie77]

cheers.
what do i with regards to the boiler, ie switching off, leaving it, resetting it? and re filling it. dont want to balls up the sensor again