Why does hot air come out when the car's air conditioning and heater are not turned on?
2 Answers
The reason why hot air blows out of the vents when the car's air conditioning and heater are not turned on is that the car's air conditioning has switched to external circulation. This phenomenon mostly occurs in vehicles equipped with automatic air conditioning. To ensure air flow, the automatic air conditioning switches to external circulation. As the vehicle speed increases, outdoor hot air is drawn into the cabin, making it seem like hot air is being emitted from the vents. When the automatic air conditioning is turned off, it actively switches to external circulation or foot mode to ensure air circulation inside the cabin. While the vehicle is moving, the negative pressure generated inside the cabin causes air to enter the air conditioning system from the external circulation, creating a free convection. The faster the vehicle speed, the more noticeable the convection becomes, making it feel like hot air is still coming from the vents. This is a normal phenomenon. In addition to the air conditioning automatically switching to external circulation, another possible scenario is that the temperature control is not set to the coldest state. If the air mix damper is open, heat from the engine coolant can also enter the cabin through the damper, causing the cabin temperature to rise. This is also a normal phenomenon.
I've been driving for over a decade and often encounter the issue of hot air blowing without turning on the heater, especially during summer highway driving. This mainly happens because the car's heater core is always operational, with the engine's coolant constantly flowing through it and releasing heat. Outside air is forcibly drawn in to carry away this hot air. If the ventilation system is set to the default external circulation mode, hot air can easily enter through the air intake. This is part of the car's design, intended for quick warm-up or defogging preparation. My suggestion is to switch to internal circulation mode when uncomfortable to prevent outside air from entering, while also regularly checking the air filter and duct seals to prevent excessive heat infiltration. I've also found that turning off external circulation before parking effectively reduces hot air buildup—a handy trick during hot weather.
