On some cars, the temp adjust knob controls the flow of water through the heater core, thereby regulating the temperature of the heater core itself. On other vehicles, particularly newer models, the knob controls a blend door that determines the degree of airflow that passes through the always-hot heater core.

Do all other HVAC controls work properly? When you work the mode control can you shift airflow around the defroster/floor/dashboard ducts?