Siemens RWB29 Instructions Manual page 18

What is a programmer?
... an explanation for householders
Programmers allow you to set 'On' and 'Off' time periods. Some models switch the
central heating and domestic hot water on and off at the same time, while others
allow the domestic hot water and heating to come on and go off at different times.
Set the 'On' and 'Off' time periods to suit your own lifestyle. On some programmers
you must also set whether you want the heating and hot water to run
continuously, run under the chosen 'On' and 'Off' heating periods, or be
permanently off.
The time on the programmer must be correct. Some types have to be adjusted in
spring and autumn at the changes between Greenwich Mean Time and British
Summer Time.
You may be able to temporarily adjust the heating programme, for example,
'Override', 'Advance' or 'Boost'. These are explained in the manufacturer's
instructions.
The heating will not work if the room thermostat has switched the heating off.
And, if you have a hot-water cylinder, the water heating will not work if the
cylinder thermostat detects that the hot water has reached the correct
temperature.