Why would the heater blow warmhot air while driving but cold when sitting still?

When driving at highway speeds, a car engine and its components (including the radiator) are exposed to high airflow from moving the vehicle. This high airflow helps cool the engine and allows it to maintain lower temperatures. When a car is sitting still, this high airflow is gone and the engine relies more on its own cooling system components (like fans and water pumps) to keep the temperature in check. This may result in the engine running hotter when sitting compared to when driving.

If the heater blows hot air while driving and cold air while sitting still, it could indicate a problem with the vehicle's cooling system or the heater itself. When driving, the hot coolant flows through the heater core and the high airflow from driving helps transfer the heat to the passenger compartment. When sitting still, the decreased airflow may be insufficient to effectively transfer heat from the heater core to the passenger compartment, resulting in cold air coming out of the vents.

Some potential causes for this issue include a coolant leak, a faulty thermostat, or a problem with the heater fan or its controls. It is important to diagnose and address the root cause of this issue to ensure proper heating performance, engine temperature regulation, and overall vehicle reliability. If the issue persists or if you notice other symptoms, it's recommended to have your car inspected by a qualified mechanic.