Why does the 15 Passat models have warm air only when the heater is set to 25?

The reason why the 15 Passat models only have warm air when the heating air is adjusted to 25.

1. The heating water tank is clogged and needs to be cleaned or replaced.

2. The water pump is damaged due to loss of rotation, and the warm air flow valve and coolant circulation valve are damaged.

3. There is no exhaust after replacing the coolant. The cylinder gasket is damaged. When the indoor temperature sensor in the cab detects 25 degrees, it will not blow warm air for heating. When the indoor temperature is lower than 25 degrees, heating will start.