Published on Dec 28, 2020|Updated on Oct 16, 2025|By: Matthew Showers

Cracked Heat Exchangers in Furnaces: What Should We Do?

Cracked Heat Exchangers in Furnaces
Key Takeaways
  • Safety First: While cracked heat exchangers may not directly produce CO in all cases, they represent a serious system malfunction requiring immediate shutdown and professional replacement
  • Detection Methods: Use combustion analysis comparing blower-on vs. blower-off readings, visual inspection, and watch for flame rollout when the blower starts
  • Gun-Type Burners Exception: Positive pressure systems like Trane Voyager RTUs can push flue gases directly into the air stream through cracks, creating immediate CO hazards
  • Liability Protection: Always turn off gas and power when discovering cracks, document findings, and clearly communicate safety risks to customers

Cracked Heat Exchangers in Furnaces: What Should We Do?

As heating season gets underway for many of us, we will inevitably find heat exchanger cracks in furnaces, whether in someone’s home or on a commercial RTU at an office building.

Many, if not all, of us have been told that a cracked heat exchanger is a hazard to people from a carbon monoxide poisoning standpoint.

But is this true?

What are the actual safety hazards of allowing a furnace to operate with a compromised heat exchanger?

Check out three cracked Carrier heat exchangers on the same roof.

The Real-World Discovery

This train of thought began on a recent service call that left me disgusted with our industry’s standards.

I was asked by my boss to investigate an issue after another company had condemned a heat exchanger. My task was simple: report back the model and serial numbers so we could provide the building owners with another quote for replacement.

I was also told that the other company had “repaired” the cracked heat exchanger, so I needed to find the defect myself.

What I found was shocking: the other company had simply applied some RTV silicone to the opening and put it back into operation. This is not only ineffective but potentially dangerous.

Before I left, I turned off both the natural gas and power supply. I wasn’t going to be held liable if someone suffered because I was the last technician on site.

Understanding the Physics of Heat Exchanger Cracks

Unfortunately, there aren’t many comprehensive resources when it comes to safety and cracked heat exchangers. However, understanding the basic physics helps us grasp the real dangers.

From a general physics standpoint, if a crack is large enough, it will disrupt the burner flame once the blower starts up. This happens because the positive pressure of the blower fan pressurizes the heat exchanger, and the added pressure forces the flame to roll out of the burner section.

Additionally, we can find potential cracks via combustion analysis based on how readings change with the blower on compared to when the blower is off. For technicians serious about proper heating system maintenance, understanding these diagnostic techniques is crucial.

I encourage anyone who works with combustible appliances to download and read Jim Bergmann’s free guide, found at TruTech Tools’ site (https://www.trutechtools.com/Downloads), for more information on this topic.

You can also listen to Jim discuss combustion analysis on this episode of the HVAC Know It All Podcast.

The Cracked Heat Exchanger Myth: Separating Fact from Fiction

I found one article written in October 2004 from Contracting Business talking about CO myths (https://www.contractingbusiness.com/archive/article/20864508/carbon-monoxide-lets-stop-the-madness).

The main myth the author wants to dispel is that heat exchanger cracks directly cause CO poisoning. From a scientific standpoint, this isn’t entirely accurate because the excess warm air from the blower adds combustion air which dilutes the flue gases.

From the experience of numerous technicians, most CO issues come from:

  • Improper venting (blocked vents)
  • Recirculation of flue gases
  • Spillage due to multiple appliances sharing a common vent
  • Poor combustion due to lack of maintenance

However, it’s crucial to understand that while cracks might not directly produce CO, they still represent a serious safety hazard that requires immediate attention.

Check out Matt’s post from Instagram finding a cracked heat exchanger.

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Components, Heating Systems
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Matthew Showers

Matthew Showers is an HVAC service technician, performing work in both the residential and commercial fields.  His specialty is in inverter and communicating systems.  Follow Matt on Instagram

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