A blend flap in a car is a component of the HVAC (heating, ventilation, and air conditioning) system that controls the flow and mixture of hot and cold air. It is usually located behind the dashboard and is responsible for directing the air from the heater core or the air conditioning evaporator, depending on the desired temperature.

The blend flap operates by either opening or closing to create a pathway for the air to pass through. When the blend flap is fully open, it allows hot air from the heater core to circulate into the cabin, providing warmth. Conversely, when it is closed, it redirects the air from the air conditioning evaporator, resulting in cooler air.