When I set the ventilation system to windscreen with the fan, the air conditioning comes on, even if I switch it off when the light comes on. I assumed the temperature of the air coming in onto the windscreen should be the same as the temperature outside, unless the air conditioning button has been pressed. Am I correct? I have put a thermometer on the vent and when the temperature outside was 25degrees C, the air coming in on the windscreen was 12 degrees with no air conditioning switched on. 

I don't know if it's a fault, but I think mine does the same.

If you demand "demist", at least with a similar VW Polo, all air is diverted to the windscreen and the chiller is switched on regardless if you had previously had the chiller switched off, this will be to maximise the demisting to the windscreen.

Edited 22 July, 20205 yr by rum4mo

The aircon comes on as it dehumidifies the air to assist clearing the windscreen.