Some timers are built into the boiler case - others are remote from the boiler. You don't say which make or model of boiler or timer.
The remote electronic timers usually have separate programmes for hot water and central heating. Each may be selected to Never On / Always On / On Twice each day / On once each day. Many have a separate Weekday / Weekend setting option. You open a flap and read the panel that explains how to programme it.
Good luck...
Peccavi, February 2010
