Cracked Heat Exchanger in Furnace – How To Troubleshoot?

A cracked heat exchanger is one of the worst things that can happen to your furnace.

Heat exchangers can crack due to overheating, lack of maintenance, malfunctioning burners, and corrosion. The most dangerous thing about a crack in this component is that toxic combustion gases might get released into your house. Such an issue can’t be fixed, you would either have to get a new heat exchanger or a new furnace.

How do these heat exchangers even work and does every furnace have one? Let’s dive deeper into the topic.

What Is a Heat Exchanger?

A heat exchanger is an element that is designed to transfer heat between two mediums. Those can be two fluids, a gas and a fluid, or even two gases. The heat transfer usually occurs through a solid separator – its main job is to prevent the mixing of the two mediums.

Heat exchangers can be made from a wide range of materials depending on the intended application and the type of heat exchanger. The most common materials used to manufacture such elements include stainless steel, copper, and titanium, and the actual heat exchanger is usually comprised of shells, coils, tubes, fins, plates, and adiabatic wheels.

In HVAC systems, heat exchangers can be used for both heating and cooling. Cooling coils usually feature refrigerant or chilled water, while the most common substances for heating coils are steam, water, and a special water-glycol mixture.

How Does a Heat Exchanger Work?

In a gas furnace, a heat exchanger is a component made out of metal coils that connect to the burner assembly on one end and the flue pipe on the other. To create the heat that is going to warm up the air, the furnace combusts fuel. The hot combustion gases then move through the heat exchanger and warm up the air that passes over the coils.

Within the heat exchanger, the air and the gas are separated. That is an important safety measure as it prevents exposure to dangerous combustion products.

An electric furnace can have a component called a ‘heat exchanger’, but it’s not the exact element that we have been talking about. There isn’t a second medium used in an electric furnace that heats the air – it is warmed up directly by the heating elements.

Read: How To Troubleshoot And Reset Furnace?

The Two Types of Heat Transfer Mechanisms

Some gas furnaces have a single heat exchanger, while others feature two.

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Primary heat exchanger

In furnaces with one heat exchanger, the medium does not undergo any phase change throughout the process of heat transfer. The combustion gases simply transfer the heat through the heat exchanger’s coils and then leave the system through a flue pipe.

Secondary heat exchanger

If your furnace has two heat exchangers, it means that you have a high-efficiency unit. In such systems, the medium does experience a phase change. In gas furnaces, the combustion gases get turned into water (that’s why a high-efficiency furnace has a drainage system as well).

The secondary heat exchanger usually has a set of fins that allows the combustion gases to transfer even more heat. The gases then cool down, turn into liquid, and the condensate gets removed through the condensate drain.

Read: How To Reset Furnace Control Board?

What Can Make a Heat Exchanger Crack?

  • A poorly sized furnace – if the heating unit is too big for your house, the furnace will be constantly turning on and off. This will make the heat exchanger expand and contract way too often
  • Lack of proper maintenance – ideally, you would want to schedule a professional furnace check-up twice a year. The heat exchanger can get dirty over time. The lack of maintenance will lead to both reduced heat transfer and erosion issues.
  • Age – if your furnace is over 15 years old, then some of its components might begin to fail due to general wear and tear. The heat exchanger, for example, can corrode and develop a crack.
  • Overheating – this is one of the most common reasons for cracks in a heat exchanger. When the furnace does not receive enough airflow, the element overheats and starts suffering from excess stress because of constant contraction and expansion. In the absolute majority of cases, overheating would be caused by a clogged air filter.
  • Malfunctioning burners – if there is an issue with your furnace’s burners, the unit might end up becoming hotter, than it should. Even though heat exchangers are created to be able to withstand high temperatures, the abnormal increase in heat can make the element crack.

How to Check for Cracked Heat Exchanger in Furnace

You should look out for the following symptoms:

Soot build-up

A heat exchanger that has developed a crack will lead to incomplete combustion which generates black carbon buildup. If you have a cracked heat exchanger, you will see a thick layer of soot buildup as soon as you open up the unit and get to the gas chamber (this is usually performed by an HVAC technician).

A cracked furnace

Check the outside of the unit. In some cases, if the heat exchanger has a crack, you will be able to detect a crack on the outside of the furnace as well.

Read: Should You Repair Or Replace A Furnace?

Corrosion on the exterior of the furnace

High-efficiency furnaces can develop a crack, in case there is an issue with the unit’s drainage system. If the moisture doesn’t get removed from the furnace, it can lead to corrosion not only on the heat exchanger but also on the exterior of the unit.

On top of that, combustion gases include chloride gases that can get released through the heat exchanger’s crack and cause corrosion of the whole unit.

Smell of formaldehyde

The fumes coming from a heat exchanger can smell like formaldehyde.

A yellow furnace flame

Under normal circumstances, the flame in a furnace should be blue. A yellow flame is a sign of either a dirty burner or a cracked heat exchanger.

The furnace’s heating capacity has decreased

A damaged heat exchanger won’t be able to transfer heat to the air as effectively.

The carbon monoxide detector is going off

Carbon monoxide might get leaked right into your house through a crack in the heat exchanger.

Read: What Are Common Furnace Repair Problems?

How to Fix a Cracked Heat Exchanger?

Cracked heat exchangers cannot be fixed or welded, according to industry standards and safety guidelines. The best way to deal with such a problem is to remove the heat exchanger and replace it with a new one or even get a new furnace.

How Much Does It Cost to Replace a Heat Exchanger?

The actual heat exchanger can cost anywhere between $350 and $850, but you should be prepared to pay a small fortune for the installation of this component.

The heat exchanger is located deep inside the heating system. The HVAC technician would have to disassemble the whole system, remove the heat exchanger, and then reassemble the furnace. It might take the professional quite a few hours to do that, so the overall cost of heat exchanger replacement can be as high as $3,000.

If your unit is 15+ years old, then it might be more practical to invest this sum in a new furnace.

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Is a Cracked Heat Exchanger Dangerous?

If you have a cracked heat exchanger, dangerous gases will get leaked into your house. Those include carbon monoxide – the ‘silent killer’.

If any of your family members are experiencing the following symptoms, it means that they are, most likely, suffering from carbon monoxide poisoning, and you should leave the house and invite an HVAC professional as soon as possible.

  • Headache
  • Weakness
  • Dizziness
  • Vomiting
  • Chest pain
  • Upset stomach

Read: How Can I Avoid Furnace Repairs?


If you have a gas furnace, you should look out for the symptoms of a cracked heat exchanger – soot build-up, corrosion, a strong smell, and a decreased heating capacity.

The issue should be addressed right away as a crack in the heat exchanger can lead to carbon monoxide poisoning.