What is the reason for the warm air not being hot in external circulation but hot in internal circulation?
3 Answers
The reason for the warm air not being hot in external circulation but hot in internal circulation is: the external environment temperature is low, and when in external circulation, the cabin is connected to the outside, allowing cold air to enter. When internal circulation is turned on, it means external circulation is turned off, isolating the cabin from the outside, so no cold air enters, hence the external circulation is not hot while the internal circulation is. The methods for using air circulation in cars are: 1. Use internal circulation during traffic jams and external circulation during high-speed driving; 2. The use of internal and external circulation should be combined with air conditioning and heating; 3. Use external circulation during long-distance driving to allow fresh outside air to enter the car, avoiding low oxygen levels inside the car.
A veteran driver with over 20 years of experience tells you, this issue is most likely caused by air leakage in the recirculation flap. When using fresh air mode, cold outside air should be introduced, but if the flap doesn't seal properly, the cold air mixes in and dilutes the warm air. Don't rush to dismantle the dashboard - first check if your cabin air filter is clogged like a rag. A dirty filter reduces fresh air flow to a mosquito's hum, making the heat feel weak. Try switching from recirculation (when it's warm) to fresh air mode and listen carefully for any rattling sounds from the flap movement. If the filter is clean and the problem persists, it's probably aged seal strips on the flap causing air leaks - this requires dashboard removal to fix.
From a mechanic's perspective, there are three main leakage points causing cold air in external circulation mode: 1) The firewall intake damper doesn't close tightly, allowing cold air to directly enter the air duct; 2) The heater core gets clogged with scale like a sieve, preventing sufficient heat transfer when outside cold air passes through; 3) Broken temperature blend door shafts in older vehicles. Interestingly, such issues frequently occur in cold regions because rubber seals freeze and lose elasticity. Just last week, we repaired an old CR-V where the footwell air duct on the passenger side came loose, sucking in nothing but cold air from the chassis during external circulation.
