The heater core in a vehicle works much like the radiator does; hot coolant passes through tubes that help cool the liquid down. They are much smaller than the radiator, however, and are not used as a primary cooling system. Heater cores also use cooling fins, and are normally made out of aluminum for maximum heat transfer.
The heater core is usually located under the dash of the automobile, and in most cases is very difficult to get to in case repair is needed. The two heater hoses that jut off of the water pump carry the water into and out of the cab of the vehicle, where it passes through the heater core. Once it cycles through the heater core, it is directed back out of the cab via the return heater hose. It is then dumped back into the main system where it flows through the block and radiator again.
While the coolant is in the heater core, a fan blows the hot air that is coming off of the heater core through the ducting of the car, ultimately resulting in what we call the "heater." Once the heater controls are turned on a valve opens. This allows the heat from the core to pass through the vent system and enter the cabin. Depending on the fan speed, the hot air will come out slower or faster. Older systems required someone in the passenger compartment to manually turn the valve on or off via a lever. Today, it is done using engine vacuum and solenoids.
Like radiators, over time, heater cores will wear out. Corrosion is a major enemy to heater cores, and can be a source of blockage for the coolant because it effectively narrows the tubes. This can cause unusual amounts of pressure, which in turn can lead to leaks. If the coolant is not flushed and changed regularly, the small tubes in the core can get clogged up with residue, reducing or completely eliminating coolant flow.
Only water-cooled engines use a heater core. Air cooled models such as an older Volkswagen use only air from the motor's cooling fan, which is channeled into the passenger compartment and used as heat. The reason that turning on the heater sometimes works to cool down a hot engine is that it uses essentially the same process the radiator does. By turning the heater on full blast, the fan blows cooler air over the heater core, and effectively cools the liquid inside a bit. While this is not sufficient to use as the vehicle's only cooling system, it can help in a pinch.
