Changing antifreeze has little to do with whether warm air is hot or not. Antifreeze has the functions of antifreeze, anti-boiling, anti-rust and anti-foaming. If the warm air is not hot, you can consider whether the thermostat is stuck or damaged, whether the water pipe of the warm air box is blocked, whether the air mixing switch or air mixing motor is damaged, and so on.
If you know the original brand and model, add the same product. If you don't know the original brand and model, change to a new car for maintenance. Replacing antifreeze is one of the most important maintenance items.