A heater core is very much just a smaller version of a radiator, but instead of being used to cool the coolant in the motor, it uses the heat from that coolant to warm the car’s interior. It works, in essence, by diverting coolant from the rest of the car’s cooling system.
As the car’s engine warms up while running, the antifreeze/coolant absorbs the heat, and is pumped round the cooling system by the water pump. It passes through the main radiator to cool it below the boiling point, and the whole system is kept at a constant temperature by the thermostat.
When you turn on the heat inside the car, a valve is opened so hot coolant can flow to the heater core (on some cars), or a door is opened to direct air through the already hot heater core. With the coolant typically at close to 200 degrees, it can quickly deliver a significant amount of hot air to your feet or the foggy windshield.
The simplest systems moderate the temperature by use of a blend door, mixing outside air with air from the heater core. This allows you to tailor the warmth of the air you get inside the cabin.
Dual zone climate control systems have a heater core that’s spilt in two, allowing the driver and front-seat passenger to choose their own cabin temperature, and often with a fan triggered by a temperature sensing fan or blend door. Some high-end luxury models and large SUVs even an extra heater core, to allow rear-seat passengers to regulate their own temperatures as well.
If your engine has fully warmed up and you’re on the move, the heater core can quickly indicate your car’s cooling system has developed a fault. If the heater suddenly starts to blow out cold air, the car has most likely developed a leak and lost all its antifreeze.
Without coolant the warning light or temperature gauge may not even indicate a problem, because it is not able to read the temperature of the now empty water passages. If, however, the heater suddenly blows cold, and the temperature starts to climb, the belt that drive the water pump (or the pump itself) has most likely failed. In either case, you should stop as soon as is safely possible and switch off the engine to avoid causing permanent damage from overheating.
The heater core is a mostly maintenance free part of the car, but you should be sure to check the hoses that carry the coolant to it periodically; about every 6,000 miles or six months. When flushing the coolant (every 30,000 miles or two years for most cars), be sure to run the engine with the heater on to flush new antifreeze into the heater core. Changing the core itself, on the rare occasion that it begins to leak, typically requires removing the dashboard and is very labor intensive, but your Haynes manual has step by step details.