Why is the BMW's heater not blowing hot air?

[OAaron]

There are several reasons why the BMW's heater may not be blowing hot air: Thermostat stuck open or opening too early: This causes the cooling system to enter the major circulation phase prematurely. When the external temperature is very low, especially while driving, the cold air quickly cools the antifreeze, preventing the engine coolant temperature from rising, which in turn prevents the heater from blowing hot air. Damaged or slipping water pump impeller: This results in insufficient flow through the heater core, causing inadequate heat transfer. Air lock in the engine cooling system: An air lock can lead to poor circulation in the cooling system, resulting in high coolant temperature and the heater not blowing hot air. If there's always air in the cooling system, it's likely due to a damaged cylinder head gasket allowing combustion gases to enter the cooling system. If the inlet pipe of the heater core is hot while the outlet pipe remains cool, this indicates a clogged heater core that needs replacement.