How to use that Thumbwheel under the dashboard clock
 

An oft asked question this one. So here are the instructions taken from the original Rover 75 handbook. Good luck! :)

The thumbwheel below the clock works as follows:

'air supply from the face level vents is further controlled by the heater bypass control in the centre of the fascia panel.

The purpose of this feature is to enable cooler air to be directed towards the face at those times when the heater is required to provide hot air in order to keep the interior of the car warm-particularly useful during winter. The bypass is controlled by turning the thumbwheel:

Towards the BLUE spot to open the bypass

towards the WHITE spot to close the bypass

If the air distribution control is set to foot, screen or window vents:

Heater bypass closed-all air flow is prohibited

Heater bypass open- unheated (fresh) air supply only

if the air distribution control is set to face or foot and face vents:

heater bypass closed-heated air supply (temperature as heater setting)

heater bypass open-a blend of heated and unheated air (which has bypassed the heater) providing a cooler output'

Easy isn't it. So it appears that to get fully heated air from the face vents your climate or air con needs to be set to :

heater bypass closed

AND

air distribution set to face, or foot and face vents.

All other settings will give you fresh or cooled air.