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How to Identify a Cracked Furnace Heat Exchanger

You notice a strange smell coming from your gas furnace – could it be a cracked heat exchanger? Upon further inspection, you also see that there is rust along the exterior of the system. You wonder: Is this something I need to call an HVAC company about?

The answer: Yes, immediately! Both of those signs could point to a cracked heat exchanger, which, if not handled soon, poses a serious health and safety risk to your family.

Heat exchangers are the largest component of your home’s heating system, and over the course of its lifespan, it can begin to develop cracks. When you have a cracked furnace heat exchanger, the system may leak combustion gasses. This can lead to dangerous levels of carbon monoxide entering your home.

What Do Heat Exchangers Do?

A furnace heat exchanger is a crucial component in home heating systems, responsible for a vital role in maintaining indoor comfort. Its primary purpose is to transfer heat generated during fuel combustion (like natural gas or oil) into the air circulated throughout your home. This process ensures warm and comfortable indoor temperatures during cold seasons.

What Causes a Cracked Furnace Heat Exchanger?

A heat exchanger’s lifespan can vary depending on factors such as usage, maintenance, and the quality of the unit. On average, well-maintained furnace heat exchangers typically last 15 to 20 years. However, some may reach 20 to 30 years or more with proper care.

If your furnace’s heat exchanger is between 10 and 20 years old, any cracks that have developed are likely from normal use. If your heat exchanger is less than a decade old, however, other factors may be to blame, including:

Poor Maintenance

Neglecting regular furnace maintenance can contribute to a damaged heat exchanger. Accumulated dirt, debris, and corrosion can weaken the metal over time, making it more susceptible to cracking. An overly clogged or dirty furnace filter or obstructed air ducts can also cause heat exchanger cracks.

Excessive Heat

High temperatures within the furnace, often caused by issues like restricted airflow from blocked registers or a malfunctioning thermostat, can accelerate wear and tear on the heat exchanger, potentially causing cracks. 

Improper Sizing

Improperly sized furnaces may turn on and off frequently. This is known as “short cycling.” Short cycling causes your heat exchanger to expand and contract more than necessary. This overuse of your system may eventually lead to cracks before you’d normally see them if your heating system was the correct size for your home.


Exposure to moisture and corrosive substances can lead to rust and corrosion, weakening the metal and making it more prone to cracking.

Manufacturer Defects

Though rare, manufacturing defects or the use of poor-quality materials can contribute to premature heat exchanger failures.

Why Are Cracked Heat Exchangers a Problem?

If you suspect your gas furnace has a cracked heat exchanger, it’s important to call a heating and cooling technician to diagnose the issue immediately. A cracked heat exchanger poses serious health risks for a couple of very important reasons:

Carbon Monoxide Leaks

The furnace heat exchanger contains and burns off carbon monoxide, sulfur dioxide, and nitrous oxide. A leak means they could escape into your home’s air ducts. Carbon monoxide poisoning is known to cause illness, including flu-like symptoms, or, in extreme cases, death. 


When a mixture of gas and hot air combust in the heat exchanger, uncombusted gas builds up until it reaches a high enough volume to be explosive. When it becomes explosive, it combusts and forces flames and air pressure out of any accessible hole, such as a cracked heat exchanger. According to the National Fire Protection Association (NFPA), heating equipment is to blame for one in six home structure fires across the U.S.

Signs of a Cracked Furnace Heat Exchanger

Detecting a failed heat exchanger is crucial for your safety and the proper functioning of your heating system. Signs of a cracked heat exchanger may include:

  • Unusual Odors: Cracks can allow combustion gasses to escape into your home, producing unusual smells.
  • Visible Rust: The presence of rust on the exterior of your furnace or on the heat exchanger itself can indicate potential cracks. 
  • Carbon Monoxide Detector Alerts: If your carbon monoxide detector goes off or indicates elevated carbon monoxide levels, it’s a sign of a potential issue with your heat exchanger.
  • Soot Accumulation: Cracks can disrupt the combustion process, leading to soot or black residue on or near the furnace.
  • Visible Cracks: In some cases, you may physically see cracks on the heat exchanger if you inspect it closely enough.
  • Inconsistent Heating: If certain rooms in your home are consistently colder than others, it could be due to a cracked furnace heat exchanger as it may struggle to distribute heat evenly.

What Do You Do If Your Heat Exchanger Is Cracked?

You may be unaware of a cracked furnace heat exchanger unless your carbon monoxide detector beeps, but you may also notice some of the other signs mentioned above. Unfortunately, you can’t really know for sure that your heat exchanger is cracked without an HVAC technician’s professional inspection.

A cracked furnace heat exchanger typically cannot be repaired and usually requires a complete furnace replacement.

Schedule a Furnace Check-Up with Douglas Cooling & Heating Today

If you suspect a cracked furnace heat exchanger, contact your local HVAC technicians for assistance right away. In the Birmingham, Alabama area, Douglas Cooling & Heating has earned a reputation as the most trusted heating company, boasting years of experience in the field.

Our regular furnace maintenance and professional inspections are instrumental in preventing and detecting potential heat exchanger issues before they become safety concerns. Better yet, enrolling in our 24/7 Club Membership provides year-round care for your home’s heating and cooling systems, ensuring their continuous performance.

Don’t hesitate to give us a call or reach out online to schedule a heating tune-up today. You can also schedule service for your other heating, air conditioning, or indoor air quality equipment to ensure your entire HVAC system operates efficiently and safely.

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