* Mode Selector: This is the control that typically has settings for "Floor," "Defrost," "Vent," and often a combination of these. If the mode selector is set to a position that combines "Floor" and "Defrost" (or a similar setting depending on your specific van), then air will flow to both the floor vents and the windshield.
* Temperature Control: While not directly *causing* the dual airflow, the temperature setting can influence the *amount* of air going to each vent. If you have the temperature set relatively cool, more air may be directed to the defrost (windshield) to quickly cool down the interior.
* Blending Door Actuators: These are small motors within the HVAC system that control the direction of the air. A malfunctioning blending door actuator can sometimes cause unexpected air distribution, even if the mode selector seems correctly positioned. If one is stuck or failing, it might cause a combination of airflow you don't intend.
In short: It's almost certainly a combination of the mode selector being in a position that sends air to both the floor and defrost vents, and potentially some influence from the temperature setting. If this behavior is unexpected or unintended and you cannot control it using the mode selector, then a malfunctioning blending door actuator is a possibility requiring further investigation.
