Why is hot air coming out of the car's air conditioning vents?
3 Answers
Hot air coming out of the car's air conditioning vents is due to the external circulation being turned on, allowing hot air from the engine compartment to enter the cabin. The reasons for hot air coming out of the car's air conditioning vents are as follows: Heat absorption by the exhaust system: When the vehicle is in motion, the incoming wind absorbs heat from the engine radiator and exhaust system, which then enters the cabin through the air distribution box. This is why you may feel hot air coming out of the vents even when the air conditioning is turned off. Caused by heat conduction and radiation: Another reason is due to heat conduction and radiation. There is a small heater core in the middle of the dashboard, which is separated from the air ducts by only a thin panel. Heat from the heater core can radiate into the air distribution box, causing the air inside to become warm.
I was also very frustrated when I first encountered this issue - the AC was set to the lowest temperature but still blowing hot air. The most common cause is accidentally setting the AC mode to heating (check if the red temperature indicator light is on). Another possibility is a severely clogged cabin air filter preventing cool air from entering, leaving only hot air circulating. Abnormal heat transfer from the engine coolant system can also affect this, especially in older vehicles with coolant leaks or insufficient coolant levels. Have you noticed? After being parked in direct sunlight, the AC needs several minutes to start cooling properly, during which time the air does feel stuffy and warm. If all these checks show no issues, I recommend directly measuring the refrigerant pressure - if the low-pressure line isn't cold, it likely indicates either refrigerant leakage or compressor failure.
Last time on the highway, I suddenly had hot air blowing, and after an emergency stop to check, I found three hidden dangers: first, the air conditioning button was set to manual heating mode, and switching it to AUTO fixed the issue; secondly, leaves were stuck in the air conditioning filter, causing insufficient airflow; most critically, the cooling fan fuse had blown, preventing the radiator from cooling down. In such cases, it's recommended to first check if the engine coolant temperature exceeds the limit, then listen for the compressor's startup sound. Sometimes, letting the system recover by stopping for half an hour can also help. Regularly replacing the refrigerant is crucial—my neighbor's car hadn't had it changed in four years, leading to hot air issues, which were resolved after a refill.
