There are two different types of car heaters: basic heaters and climate control heaters. Some cars with climate control have dual climate control. Dual climate control works in one of two ways. It can allow the passenger to control his own heat settings (if the vehicle is so equipped). Dual climate control may also be used in vans with two heating and air conditioning systems, one for the front and one for the rear.
A car heater has five main parts to help bring heat--which is generated by hot water running through the engine--into the cab of the vehicle: the heater, core, control head, blower motor and blower motor resistor. There is also a fuse and the wiring to and from the control head.
The water, when circulating though the engine and radiator, also circulates through the heater core. This keeps the heater core hot, even when the heat is not being used. To keep the heat in the core or to allow it into the cab when needed, the heater core employs doors. The doors open and close based on the user's input via the control head.
The doors may be operated via vacuum, also picked up from the engine, or via electric pulses. Depending on the lever moved on the control head, various doors open and close, diverting air to the feet, the front of the dash or the top of the dash (for defrosting). Each door has its own motor, which is fed by an individual vacuum tube. The doors change position when the vacuum is applied (moving the levers on the control head). In electrical systems, the doors work the same, but they are controlled with an electrical pulse sent through the system when the user moves the levers on the control panel.
To change how much heat is let into the cabin, another lever on the control head dictates fan speed. The electrical impulse is sent through the blower motor resistor (this is the part that turns the fan on low, medium or high, when the user sets the fan switch), then to the blower, spinning it faster or slower according to user input.