The Heater Core Explained

On a Q & A website I write for the questions about the heater core and how it works dominate the queue! Here I will explain the heater core in terms that everyone can understand and learn how this device operates.

A typical heater core. This is out of a 1993 Ford Explorer.

The heater core works similar to a radiator. Hot engine radiator fluid flows through coils in it and then passes back out to be cooled in the engines radiator.

When the hot radiator fluid flows through the heater core the heat can be utilized to warm the interior of your vehicle. When you switch the heater on low-med-high, on your dashboard, a fan in front of the heater core turns on and blows across the hot coils. The hot air flows into the cab of the vehicle warming it up.

I bought a Ford Explorer some years ago when I moved to a region with snow. Upon 4 wheeling one cold, snowy, day we all hopped into the Explorer and I fired up the heater to high. Air was blown into the cab of the Explorer, but it was straight up COLD air! We were freezing! I popped the hood to see what the problem may be and to my disappointment the prior owner had rerouted the hoses from the heater core! This is what I saw under the hood!

Heater Core hoses spliced together.

This is a common practice when they notice their heater core has gone out and they don’t know how to fix it. Or, they don’t have time to fix it. The two adapters coming out from the firewall that lead into the heater core inside were plugged with blue caps.

Outlets at the firewall plugged.

Thus, the source of heat to warm the interior of the Explorer had been completed removed. A crude picture of the situation is below.

There was nothing I could do out in the field to fix this. Since it was bypassed in such a way it meant that the heater core was not working anyways. We wrapped ourselves up in several blankets and drove back to town freezing our asses off.

The heater core uses the heat absorbed from the engine to warm the interior of your car. When you start your engine for the first time the engine is cold. If you’ve ever turned on the heater immediately after starting the cold engine you’ll notice that only cold air blows out. See the illustration below.

The heater core is receiving cold air from the engine. With the heater fan turned on you will only receive cold air. That’s all that is available to blow out.

After the engine warms up the picture below shows the cool radiator fluid rising out of the lower radiator hose cold, or cool, from the radiator cooling it. It flows through the engine absorbing the heat in the engine. When it exits the engine through a hose, near the heater core, the radiator fluid is hot. It flows through the heater core. If you turn on the heater the fan will blow the warmth of the heater core into the cab of your vehicle. The radiator fluid flows back out of the heater core and heads for the radiator at the front of the engine to be cooled down and sent back through the system. That’s how simple the heater core system works.

When the heater core goes bad you will have several indicators:

  • The passenger side floor will be wet.
  • The windshield will fog up especially where the vents come out of the dashboard.
  • You will smell radiator fluid inside the cab.
  • Drips will be coming from under the dashboard.
  • You will have very little, or no, heat when the heater is activated.

The heater core has a reputation for being very difficult to access. I’m sure that is why the previous owner had rerouted the hoses, to avoid having to replace the heater core that is difficult to access. It IS NOT difficult. There are maybe two models that make the heater core difficult to access IE: The Mustang and Corvette are two that I know of. They are difficult merely because there are more panels to remove. I have heard many people say the entire dash board must be removed. This is not true. If you are removing the entire dashboard you are attacking the heater core from the wrong direction or you have some foreign model that is built from the inside out!

If you notice any of the symptoms above of a bad heater core purchase a new one at your local auto parts store or online (this leads to the heater core products on Amazon).

These are the steps I had to take for my Explorer:

If not already removed, remove the heater core hoses from the pipes protruding through the firewall under the hood (radiator fluid may come out).

Locate the cover for the heater core under the glove box or behind it.

Remove the screws holding the cover in place.

When I unscrewed the cover screws, since the heater core had already been detached under the hood, it practically fell into my hand!

Slide it out carefully.

Now that you have the old one out, slide the new one in. Make sure the pipes go through the firewall when your putting it in. Attach any screws you’ve removed. Put the cover back on with it’s screws.

Under the hood, attach the hoses to their appropriate pipe coming through the firewall. One is outlet and the other is inlet. Secure the hoses to the pipes with hose clamps.

Top off the radiator because it may have lost fluid during this process of removing hoses.

Fire up the engine and check for any leaking on your hose clamps under the hood and below the heater core inside the car. If nothing is leaking, turn the heater on.

Congratulations! You’re done!

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