Some boilers have an internal timer and others have an external timer - they are not all the same - You do not mention the make / model of your boiler and you do not say if it's an internal or an external timer.
The general case is that you can set a boiler to be always on - always off - on once per day - on twice per day. Often there are settings to control H/W and C/H separately and some models have alternative weekday and weekend setting options.
On my External timer/programmer the instructions are printed on the inside of a pull-down flap.
Good luck...
Peccavi, February 2010
