Challenge DL06-1 TIMER Instruction Manual page 8

5. To override the timer action slide the switch to either (0) for heat off or (1) for heat
on. The clock timer will continue to run but will no longer control the heater.
OPERATION with TIMER in 'I' (ON) position
Make sure HEATER switches are also in an ON position for the appliance to
warm up and set the THERMOSTAT dial to the desired temperature level.
(NOTE at MINIMUM 'Frostguard' setting the unit will only function when the
ambient room temperature drops below around 7 degrees centigrade)
With heater switches in the OFF position the unit will not warm up, even when
the TIMER is in the 'I' (ON) position