A heater core is a radiator-like device that heats the cabin of a vehicle. Hot coolant from the vehicle's engine passes through a winding tube of the core, which transfers heat from the coolant to the cabin air. Fins on the core tubes increase the surface area for transfer of heat to the air, which a fan forces across them and into the passenger compartment.

The internal combustion engine in most cars and trucks is cooled by a mixture of water and antifreeze that is circulated through the engine and radiator by a water pump to enable the radiator to give off engine heat to the atmosphere. Some of that coolant can be diverted through the heater core to give some engine heat to the cabin.

A heater core is a small radiator under the dashboard of the vehicle, and it consists of a conductive aluminium or brass tube 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 squirrel cage fan of 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 registers 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 a valve that limits the amount of coolant that goes through the heater core; another method is to block off the heater core with a door, directing part (or all) of the incoming air to bypass the heater core, so that it does not get heated (or re-heated if the air conditioning compressor is active). Some cars use a combination of these systems.

Simpler systems allow the driver to control the valve or door directly (usually by a rotary knob or a lever). More complicated systems use a combination of electromechanical actuators and thermistors to control the valve or doors to deliver air at a precise temperature.

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.

In a car equipped with air conditioning, outside air, or cabin air if the recirculation flap has been set to close the external air passages, is first forced, often after being filtered by a cabin air filter, through the air conditioner's evaporator coil. This can be thought of as a heater core filled with very cold liquid that is undergoing a phase change to gas (the evaporation), a process 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. In a vehicle fitted with manual controls for the heater and air conditioning compressor, using both systems together will dehumidify the air in the cabin, as the evaporator coil removes moisture from the air due to condensation. This can result in increased air comfort levels inside the vehicle. Automatic temperature control systems can take the best course of action in regulating the compressor operation, amount of reheating and blower speed depending upon the external air temperature, the internal one and the cabin air temperature value or a rapid defrost effect requested by the user.