how do i switch an to central heating ?

How do I use the central heating control box connected to the main boiler, which can switch the box from water to the central heating? Where at the bottom of the box there is three letters showing (W)(M)(H) and at the moment the switch is on (H) but the central heating for the home working. So how does this control box work? And what does these letters mean.

Joy Gregge
September 2010