Thermostat stuck open
If the thermostat is stuck open, the coolant will immediately circulate from the engine to the radiator. This can keep the heater from blowing hot air for a while. It may take a lot longer for the heater to start blowing hot air.
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Heater Blows Cold Air
If the heater is blowing cold air, the thermostat may be stuck open. It is possible for the heater to blow hot air, but it will take much longer as the coolant is circulated and cooled immediately.
Under normal operation, the thermostat stays closed to keep the coolant in the engine until it reaches a certain temperature. Once this temperature is reached, the thermostat opens up, and the coolant circulates to the radiator where it is cooled. The heated coolant is passed through the heater core which heats the air before it enters the passenger cabin.