Car Tech

Illustrated Guide to Your Car’s Heater

When it’s cold and wintry, being warm is the best. But what actually happens in your vehicle when you turn on the heat? Why does it take a moment for the heat to arrive on a cold morning? What’s going on at the other end of the climate control console to keep you and your passengers warm?

Below, we’ll take a simplified look at how your ride’s heater system works, the jobs of various components inside of it, and what might cause it to work poorly if something goes wrong. Numerous processes kick into action to provide a warm cabin in the coldest months of the year – so let’s dive in.

Your Engine

Your ride’s engine is the heart and soul of its heater – since it’s the heat from your vehicle’s engine that’s captured by the heater system and used to warm the cabin. This is why, on a cold morning, there’s no heat for the first few moments that the engine is running. When it’s 20 below, it takes a moment or two before the engine generates enough heat to share with the vehicle’s cabin. But, how does that work?


Engine coolant, sometimes called water (though this is technically incorrect), is a liquid. It’s also the vehicle that transports heat from your car’s engine to your car’s cabin. A device called a coolant pump (or water pump) drives several litres of engine coolant through passageways built into the engine at all times while it’s running. The flowing coolant absorbs heat, and therefore cools the engine, from the inside out. Engines need constant cooling to run properly, even if it’s very cold outside.

As the coolant flows through the engine’s passageways, it becomes very hot – so it flows into the radiator to cool off. In the radiator, cool outside air is used to reduce the temperature of the coolant flowing through it. The cooled coolant is then pumped back into the engine, and the process continues.

The process of moving hot coolant from the engine to the radiator is constant, and runs in a never-ending loop. It’s not unlike the human circulatory system: as it is with your heart, blood and veins, the engine cooling system never stops pumping, and the fluid is always moving.

The Heater Core

The heater core is one of the most vital parts of your vehicle’s heating system. It’s like a radiator, but smaller, and it’s typically located somewhere inside the dashboard. The heater core, as well as a few associated components, are responsible for controlling the temperature of the air being pushed into your vehicle’s cabin by the blower fan.

In some vehicles, the heater core is sealed into a passageway. All air entering the vehicle via the climate control system must pass through this passageway, and therefore, the heater core. In this setup, a valve controls the flow of hot engine coolant into the heater core. Crank the heat, and that valve allows the maximum possible flow of hot coolant through the heater core, which powerfully warms the air being pumped into the cabin. When you turn the heat down, the valve closes partially, slowing the flow of hot coolant into the heater core. Turn the heat off, and the flow of coolant is blocked, and the heater core, and incoming air, quickly become cool.

In other vehicles, the heater core has no control valve, and hot coolant passes through it at all times. Controlling the cabin temperature in this setup is achieved using something called a blend door, which is a flap that opens and closes to control how much of the air entering the cabin passes through the heater core, and how much does not.

Put simply, some vehicles control cabin heat by controlling the temperature of the heater core, by manipulating coolant flow. Other vehicles control cabin heat by altering the proportion of incoming air that must pass through the heater core.

Heat Control

All vehicles are different, and some control cabin temperature based on the above principles in different ways. Vehicles with dual-zone climate control may have two heater cores, with one responsible for each climate control zone. Simpler climate control systems use a single heater core.

In some applications, the flow of coolant through the heater core (and the flow of air past it), are controlled via electric motors. Other applications use cables, and others still manipulate climate control hardware with vacuum actuators.

If you drive a newer vehicle with an advanced climate control system, chances are that changes you request to cabin temperature via the climate control console are handled by the actions of one or more electric motors that adjust blend doors, deep inside your dashboard.

If you drive an older vehicle, chances are that the heater control dial simply manipulates a cable that controls the position of the heater core’s valve, and therefore, the amount of coolant flowing through it.

Supplemental Heat

These days, bigger and bigger vehicles are being fitted with smaller and smaller engines, which makes it more challenging to warm the vehicle cabin. As a result, many automakers are turning to supplemental electric heaters, which help to keep the engine coolant (and therefore, the cabin) hot, even in extremely frigid temperatures.

Problems with Your Heater

If your heater isn’t heating well, problems may exist with one or more of the components listed above.

Over time, engine coolant can become dirty, and full of contaminants and gunk that may settle and build up in the heater core. This reduces the flow of hot coolant through the heater core, and makes it harder to warm the cabin. The best defence against a clogged heater core is to change and flush your engine coolant not a moment later than the interval set out in your service manual. A technician may also be able to professionally clean and flush a clogged heater core, restoring its effectiveness.

Another common cause of poor heater performance is a clogged cabin air filter. This is not a part of the heating system per se, but this filter pre-cleans dirt and dust from the air being drawn into the cabin. Eventually, this filter clogs up and needs replacing, though many owners fail to replace their cabin air filter regularly, if at all, which slashes airflow into the vehicle’s cabin, making it harder to heat. If your ride’s heater seems to suck, a clogged cabin air filter is a prime suspect.

Other causes of poor heater performance can include problems with cooling system plumbing and parts, or low coolant levels, possibly caused by a leak. A technician can quickly determine and address the cause of lousy heater performance.

Finally, though fairly rare, the motors or actuators that control the blend doors in your climate control system can wear out or fail, making it difficult to control the cabin air temperature.