Air from the evaporator is forced through or around the heater core and is heated as required. The air-mix door controls the amount of heating. At the maximum heat position, all air flows from the evaporator through the hot heater core and into the passenger compartment. In the opposite position, the door forces all of the air to bypass the core so that no heating takes place and cold air enters the passenger compartment. In the middle positions, some of the air passes through the heater core and some bypasses the core, until air is mixed in the passenger compartment to the requested temperature.