On the ground before TO power, the system is in open circuit configuration (or in closed configuration when the skin temperature is below the ground threshold).
On the ground after TO power, and in flight, the ventilation system is in closed-circuit configuration.
OVRD:
When either switch is on OVRD:
Closed-circuit configuration.
Air from the a/c system is added to ventilation air.
The blower fan stops if the BLOWER pushbutton switch is in the OVRD position.