A heater core is a small radiator, located under the dashboard of the vehicle and consists of conductive aluminium tubing with cooling fins to increase surface area. Hot coolant passing through the heater core gives off heat before returning to the engine cooling circuit.
The fan for the vehicle's ventilation system forces air through the heater core to transfer heat from the coolant to the cabin air, which is directed into the vehicle through vents at various points.
Once the engine has warmed up, the coolant is kept at a more or less constant temperature by the thermostat. The temperature of the air entering the vehicle's interior can be controlled by using a valve limiting the amount of coolant that goes through the heater core. Another method is blocking off the heater core with a plastic door, directing part (or all) of the incoming air around the heater core completely, so it does not get heated. Some cars use a combination of this system.
Simpler systems allow the driver to control the valve or door directly (usually by means of a rotary knob, or a lever). More complicated systems use electronics to control the valve or doors.
Cars with dual climate function (allowing driver and passenger to each set a different temperature) may use a heater core split in two, where different amounts of coolant flow through the heater core on either side to obtain the desired heating.
In a car equipped with air conditioning, outside air is first forced through the air conditioner's evaporator coil. This can be thought of as a heater core filled with very cold gas, which cools rather than heats the incoming air. In order to obtain the desired temperature incoming air may first be cooled by the air conditioning and then heated again by the heater core.
The heater core is made up of small piping that has numerous bends. Clogging of the piping may occur if the coolant system is not flushed or if the coolant is not changed regularly. If clogging occurs the heater core will not work properly. If coolant flow is restricted, heating capacity will be reduced or even lost altogether if the heater core becomes blocked. Control valves may also clog or get stuck.
Another problem that may occur is if one of the connections to the heater core begins to leak. This may first be noticeable by smell (ethylene glycol is widely used as coolant and has a sweet smell), later it may cause (somewhat greasy) fogging of the windshield above the windshield heater vents.
Electrolysis can cause excessive corrosion leading to the heater core rupturing. Coolant will spray directly into the passenger compartment followed with white colored smoke, this can create a large driving hazard.
Because the heater core is usually located under the dashboard inside of the vehicle and is enclosed in the ventilation system's ducting, servicing it often requires disassembling a large part of the dashboard, which can be time-consuming and therefore expensive.
Engines that do not have a water cooling system can not use a heater core; one alternative is to guide air around the (very hot) engine exhaust manifold and then in to the vehicle's interior. Temperature control is achieved by mixing with unheated outside air. Depending on the design, this can cause a safety issue where a leak in the exhaust system will begin to fill the passenger cabin with deadly fumes.