How To Flush A Heater Core; A Step-by-Step Guide

Is your heater system in your vehicle blowing cold even with the heat turned on in the high position? This is a symptom of a clog. Replacing a heater core can get expensive, flushing the heater core is a simple job that could save you thousands. We have done this job many times and put together a thorough step-by-step guide on how to flush a heater core below. 

Before you jump into this project head first it is important to understand what the heater core does in your vehicle. 

It Is Important To Know What A Heater Core Does

Most heater cores are simple devices that function as what is known as a heat exchangers. Engine coolant travels through the heater core in a series of tubes. These tubes are attached to the heater core fins. As the coolant travels through the tubes the fins absorb the heat from the hot coolant and heat up. 

When air is forced through the heater core, the cold air absorbs the heat from the fins and heats up the air that is passing through. This is the warm air that we feel coming from the car’s heater vents when we drive.

When a heater core clogs it is due to mineral build up, dirt, and other forms of material that coat the inside tubes of the heater core. This eventually builds up and clogs the tubes and does not allow coolant to pass. 

The process of flushing the heater core is meant to “break” apart these materials and flush them out of the heater core. This allows coolant to pass through easily again, allowing the heater core to function properly.

Let’s jump in and learn more about how to flush a heater core.

Tools Needed To Flush A Heater Core

Flushing a heater core will require many different types of tools. Here is a detailed list of the tools that you will need to flush a heater core:


Used to remove the hose clamps that secure the heater hoses

Radiator Hose Pick Set

This is a specialized tool that is designed to help remove radiator hoses. They help remove those pesky stuck on hoses that sometimes are very hard to remove. 

Socket Set

Used to remove any trim pieces or parts that need to be removed to access the heater hoses attached to the core

Drain Pan

Used to capture any coolant and debris when flushing the cooling system. Coolant is a hazardous material and should never be dumped on the ground. Contact your local landfill to learn more about how to properly dispose of this liquid in your area.

Jack and Jackstands

As always, work safe. If you need to raise your vehicle, always use jack stands on a safe working surface

Garden Hose

Used to flush the heater core system.

Garden hose and hose side water outlet

Air Compressor

This tool is nice to have but is not a necessity. Air pressure can be used to remove the water that is left in the heater core. It can also break up the clogged debris, helping to remove it. 

Second Hose

This hose can be used to attach to the heater hose and direct the water to the drain pan. Using a second hose will decrease the mess that you make when flushing the clogged heater core. 

How To Flush A Heater Core; Detailed Step-by-Step Guide

Flushing a heater core is a very simple do-it-yourself job, however there are a few tips to help make the job easier. We have detailed these steps below in this detailed guide on how to flush a heater core. 

Gain Access To The Heater Hoses

Image of a firewall with heater hoses clearly labelled.

The first step in flushing the heater core is locating the heater hoses that are attached to the heater core. They will be located on the car’s firewall, underneath the hood. Generally you will find them located on the passenger side area of the vehicle. You will need to remove both the inlet and outlet hoses to flush the core correctly.

You may need to remove some components to gain access to the inlet and outlet hoses. If you cannot find where the heater hoses are located, check an online service manual or possibly a quick internet search.

Before you start any coolant system repair make sure you relieve any coolant system pressure. Let the vehicle cool down fully and remove the radiator cap.

Remove heater hoses 

Using a screwdriver loosen the clamps that attach the hoses to the heater core. Once they are loosened move them down the hose. 

Depending on how old your vehicle is, the hoses may be very difficult to remove from the heater core inlet and outlet hose. If you have a radiator hose pick set this is where they can be used.

Be careful when removing the hoses. If you use a sharp tool, the hoses can be quickly damaged. If this happens you will have no choice but to replace the hoses. 

Flush the heater core

Once the coolant hoses are removed, attach the second hose to the outlet hose of the heater core. Direct this hose into the drain pan. Using the garden hose apply water pressure to the inlet hose, dirty water and coolant should be seen coming out of the second hose.

Apply water with the garden hose until it is completely clear on the second hose. 

Once you have clear water in one direction you need to reverse the hoses. This will make sure that you backflush the heater core. So fasten the second hose to the inlet hose and flush with the garden hose on the outlet hose.

Flushing in both directions breaks up the mineral deposits that are present in the system. This means that the system gets flushed better than only flushing in one direction. 

Blow Out With Air Compressor

If you have an air compressor, remove any leftover water or remaining coolant from the heater core flush by applying small amounts of compressed air.

It is VERY IMPORTANT that you DO NOT over pressurize the heater core. Using an air compressor can apply over 100 psi to the heater core. This can result in immediate heater core failure. Be careful. 

Re-Install Hoses

Once the heater core is flushed you will need to re-install the hoses that were removed. If the hoses are difficult to re-install, try a rubber safe lubricant to make them easier to install.

A small spray of a silicone lubricant can help to ease the installation. Just as the hoses are easy to damage when they are removed, they can also be damaged when installing just as easy. Be careful.

Tighten the hose clamps and make sure you do not overtighten.

If you had to remove any trim panels or any other parts to gain access to the heater core, re-install them now.

Fill the coolant system with proper coolant

Image of a coolant reservoir lid

Once you have completed the heater core flush, top up the coolant with a pre mixed coolant that meets the standards for your vehicle. 

If you do not use pre-mixed coolant, make sure you use distilled water. If you use plain water directly from the hose that you used to flush the system. You can introduce minerals into the cooling system. 

These minerals in the water increase the chance of mineral deposits in the cooling system. This leads to a build up of mineral deposits and conditions such as clogged heater cores, reduced cooling performance, etc. 

Kind of defeats the purpose of flushing the heater core. Always make sure to use premixed coolant or distilled water to prevent this.

Bleed The Heating System

Once the coolant is topped up you need to bleed the air out of the cooling system. Air pockets in the cooling system can get trapped.

Trapped air in the system will keep coolant from flowing through the system. This can cause many different issues with your vehicle.

Engine overheating is the most common issue when the air pockets in the cooling system have not been properly removed. If there is an air pocket in the heater core, it will not allow coolant to flow through the heater core itself. This will result in the heater not blowing warm.

Dispose Of Old Coolant

When you are finished make sure that you dispose of the old coolant correctly. Engine coolant is a hazardous material and cannot be dumped on the ground or poured down the sink.

After you have topped up the coolant with new coolant keep the empty jugs. You can than fill the empty jugs with the coolant that you just flushed out of the cooling system.

Contact your local landfill, they will most likely have a hazardous material drop off area or have a special day when you can drop off these types of materials.

Sometimes, these materials can even be dropped off free of charge.

Flush Your Complete Cooling System

When you are flushing your heater core it is the time to flush your entire cooling system at the same time. 

Flushing the entire cooling system will make sure that any old coolant is removed and fresh coolant is traveling through the entire cooling system.

How Much Does A Heater Core Flush Cost?

If you do not have access to the required tools to complete a coolant flush or you are not mechanically inclined. Or, like many others these days, lack the time to complete the job yourself. There are always automotive professionals that are able to complete a heater core replacement or heater core flush for you.

However, how much does a heater core flush cost? A heater core core flush will cost between $120 to $300 depending on the vehicle. There are many things that will change the cost of what you are charged to complete a heater core flush. 

Things such as access to the heater core hoses, how many parts need to be removed, and even the type of coolant that the engine requires can all cause the price to increase quickly. 

Just like hiring someone to do any job. Make sure you get a price quote before getting the job done. This will give you a chance to compare prices of various providers and choose which repair shop will give you the best treatment.

Flushing a heater core can be an excellent project to take on if you are trying to learn more automotive skills. It requires a few simple tools along with some household items to complete the job.

We really hope this step-by-step guide on how to flush a heater core has been valuable. If you have any questions or any other tips and tricks that you would like to share with other readers. Please feel free to leave them in the comments below. We always like to hear from our readers.

John Morris
John Morris
John Morris is the technical editor for His years of experience in automotive repair as well as an automotive professor have prepared him to ensure that even the most technical information is accurate and concise at all times.

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