Honeywell CM901 User Manual page 17

WHAT IS A PROGRAMMABLE ROOM THERMOSTAT?
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.
Programmable room thermostats need a free flow of air to sense the temperature, so
they must not be covered by curtains or blocked by furniture. Nearby electric fires,
televisions, wall or table lamps may prevent the thermostat from working properly.
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