Heater core

Some of that coolant can be diverted through the heater core to give some engine heat to the cabin, or adjust the temperature of the conditioned air.

A heater core is a small radiator located under the dashboard of the vehicle, and it consists of conductive aluminium or brass 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 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.

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 a combination of electromechanical actuators and thermistors to control the valve or doors to deliver air at a precise temperature value selected by the user.

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.

If coolant flow is restricted, heating capacity will be reduced or even lost altogether if the heater core becomes blocked.

However, 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.