How Does A Heater Core Work; Signs Your Heater Core Is Failing

Otherwise known as your heating, ventilation and air conditioning system (HVAC), the heating and cooling systems of your vehicle are very important in keeping you comfortable when you are driving down the road. So let’s jump in and find out, how does a heater core work.

While having no air conditioning system sucks, having a heater core that does not work is unsafe in cold climates. Having windows that fog up and snow and ice build up on the windshield creates a driving hazard that can be dangerous.

The heater core can be a mysterious component as it is tucked in behind and underneath the dashboard. Let’s take a look and learn more about how a heater core works.

How Does A Heater Core Work?

Your car’s heater core is a component that is known as a heat exchanger. The type of heat exchanger used for a heater core is known as a liquid to air type exchanger. 

Air that is flowing across the heater core absorbs the heat from the hot liquid (engine coolant) inside. This causes the air to warm up and in turn keep you and your passengers toasty warm. 

While it may seem like a simple device, when you look at it in a little more detail it starts to look like a very complicated system. There are many different components that allow a heater core to function correctly. 

Interior photo of a vehicles center vents and heater controls

Engine Coolant

The heater cores in your vehicle rely on the vehicle’s cooling system to provide it with a source of hot engine coolant flow.

This hot engine coolant is a water and antifreeze mixture that travels through the heater hoses and is used to warm the cold air coming into the core and converting it to warm air.

Water Pump

image of a water pump and the water pump impeller

The water pump is an important component of the cooling system. Without it the cooling system would not function.

The water pump provides a flow of coolant through the system allowing the hot coolant to be circulated through the cooling system.

Heater Core Tubes

The core tubes are made of aluminum and are what allow the coolant to travel inside of the heater core. They are bent and routed to allow the longest possible route for coolant to travel through this maze of a cooling system.


Attached to the core tubes are a series of aluminum fins. Engine coolant passing through the core tubes heat the aluminium fins.

As air passes over the fins inside of the heating system heat is transferred into the surrounding air that is passing by to heat it up. 

The core tubes and fins have to work together to function properly. Engineers try to allow as much tubing and fins as possible to make the heating system for efficient.

The more tubing and fins there are present in a heater core the more surface area for the heat to transfer to the air passing by. 

HVAC Housing

This component does a great job at hiding behind and underneath the dashboard of your vehicle. Just because it is hiding does not mean the job it does is not important. 

The HVAC housing contains a series of passageways, doors, actuators, and also provides a housing for the heater core and evaporator.

It allows air to flow through the right vents at the right time at the right temperature making sure you are comfortable when you are driving your vehicle. 

Image of heater vents and radio controls on a vehicle with a dark interior

Blower Motor

This is the last piece of the puzzle. Like we discussed earlier the heater core is a heat exchanger that requires air entering the car’s interior to flow across it heating it up. 

Yup, you guessed it. The air for this process comes from the blower motor assembly. It is an important part of how does a blower motor work.

How Does The Heater System Work Together

The heating system in your car is a highly complex system. It can really be thought of as an orchestra, all the pieces working together to complete a job well. 

The system starts with the blower motor which provides various amounts of airflow based on the speed that you select on the rotary controls. 

This air flows through your cabin filter and into the ductwork of the HVAC housing. As the air is traveling through the system it reaches a door that provides a route to either the heater core or evaporator. This door is called the temperature door or temp door. 

In this case, the air is going to be directed to the heater core. As it travels through the heater core assembly the air is heated up by the hot coolant that is flowing through the heater core. 

This hot air continues to travel through the HVAC housing ductwork and comes to another door. This door is known as the blend door. 

The blend door directs the airflow to the selected outlet, either to your feet, vent, defrost or a combination. 

As the heat exits the vents it is directed to and warms your body ultimately making winter or cold weather driving much more comfortable. 

Replacing A Heater Core Is A Large Job

Because of the location of the heater core on your vehicle, inside of the HVAC housing and underneath the dash. Heater core replacement is a very expensive job to have completed on your vehicle. 

Depending on your level of do-it-yourself repair, heater core replacement is generally above the skill level of most at home mechanics. 

So if you think your car’s heater core needs to be replaced, you first need to be sure. What are the signs that your heater core needs replaced and heater core replacement is needed?

6 Symptoms Of A Failing Heater core

There are a handful of very common symptoms that you will notice if your heater core is failing and needs to be replaced.

Sweet Smell Inside Your Vehicle

When you enter the vehicle do you notice a sweet smell? Is the smell that you notice new. If so, the most probable cause of the issue is a coolant leak from your heater core. 

The sweet smell comes from the coolant that is circulating through it. As it leaks into the HVAC housing the blower motor blows the fumes and tiny droplets through the system and out the ductwork. Resulting in the sweet smell that you notice.

Wet Floor (Usually passenger side)

A wet floor when it is not raining outside or if you have ever had wet feet inside of the vehicle is caused by a heater core that is leaking externally. 

Depending on where it is leaking it can leak out of the HVAC housing. The heater core is generally located on the p/s of the HVAC housing so this issue most commonly shows up on the passenger side floor of the vehicle. 

If you notice a wet floor on the drivers side, please do not rule out a bad heater core as an issue as they can be located on the drivers side as well. 

A good way to tell if it is coolant is by rubbing the carpet with your fingers and getting them wet. Rub your fingers together, is the fluid slippery? If so this is a sign that coolant is leaking inside of the vehicle. 

Heater Is Blowing Cold

This can be caused by a heater core that is partially clogged. Depending on how bad the clog inside of the heater core is it can be partially or fully clogged. 

If the heater core is fully clogged, the coolant that normally travels through the heater core tubes is not able to pass through easily. Because there is no coolant to heat the cold air passing over the heater core, there is no warm air present. 

A partially clogged heater core can cause other symptoms. Depending on which sections of the heater core are clogged. It can cause only one side of the heater core to provide heat and the other section only blow cold. 

cutaway of a heater core showing the coolant channels

This generally results in noticing that one side of your vehicle blows warm and the other side of your bad heater core blowing cold. 

If your vehicle has these symptoms your heater core may be possible to clean by flushing the heater core

Engine Running Warmer Than Usual

This heater core symptom is generally the result of it leaking coolant. Over time if the leak is not noticed or repaired the coolant level will be low. 

When the coolant is low in your engine, it will result in the engine possibly overheating and may also result in your heater system blowing cold air. 

Inside Of Windshield Is Fogging 

If the windshield is fogging up in the passenger compartment of your vehicle this is a sign that your heater core is leaking. 

This will be most noticeable when the heater is placed in the defrost mode and the air is directed towards the windshield. 

Coolant leaking into the HVAC housing causes the air to be damp. As this air travels through the system and is directed onto the windshield it causes the inside to fog up. 

When you wipe the fog off, you may also notice that it does not come off well and leaves a streaky finish. This is caused by the coolant contaminating your windshield. 

Heater Core Leaking? What Next

If your heater core is leaking you need to get it repaired as soon as possible. If you allow the heater core to leak it can cause your engine to overheat.

Also be aware of the issues that coolant leaking inside of your vehicle can cause. The coolant overtime will soak the underlayment of the floor and the carpet. 

The underlayment is a foam layer that sits underneath the carpet and provides a cushion to your feet. Once this underlayment is soaked in coolant, it can be very difficult to fully dry out. 

The only real chance you have of removing the coolant and the smell that comes with it, is to fully remove and replace the carpet and underlayment. Resulting in hundreds if not thousands of dollars in damage.

Is It Safe To Drive If My Heater Core Is Leaking?

Yes, it is safe to drive if your heater core is leaking but you need to be cautious. If the coolant is not checked or topped up the engine coolant level will be low. 

If the coolant level gets low it will result in the coolant system not being able to cool the vehicle. This will result in your engine overheating which can cause internal engine issues. Such as, blown head gaskets or worse case, an engine failure.

As you can see the heater system in your vehicle has very complicated systems that we sometimes think they are as simple as a rotary knob on the dash of our vehicle.

The system does more than just deliver air to heat inside of your vehicle. Hopefully you have learned more about how a heater core works and understand its function on your vehicle.

Staff Writer
Staff Writer
Our team of staff writers goes above and beyond in creating material that our readers love. They bring unique skills to the team that always provides a fresh perspective on the automotive industry.

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