Calculation of flow in an air system

A method for determining an air flow in a vehicle air conditioning system, the system comprising: at least one inlet; a plurality of outlets; at least one air duct for guiding air from the inlet to the outlets, each air duct being characterized by a predetermined flow resistance coefficient; at least one flap being characterized by a flow resistance based on a degree of opening of the flap; and a fan for creating an air flow from the inlet to an outlet; the method comprising the steps of; determining a total equivalent flow resistance between a predetermined location in the system and the interior of the vehicle; wherein flow resistances are treated as resistances in an electric circuit, and a total equivalent flow resistance is determined. Treating a pressure drop as equivalent with a voltage drop, the air flow can be determined, as the pressure is equal to the resistance times the flow squared.