Webasto Air Top EVO 40 Operating Instructions Manual page 7

Heater is on.
Turn the function selector switch to the required heating
function.
.4 To set heating temperature
The heating temperature can be set infinitely from 5 °C to 35 °C.
Heater is on.
Heating function selected.
To increase heating temperature
Turn rotary knob clockwise.
To decrease heating temperature
Turn rotary knob anticlockwise.
.5 To switch on ventilation
Heater is on.
Turn function selector switch to ventilation.
Turn rotary knob to set fan level.