As an inspection professional, one of your most critical responsibilities is educating homeowners about the risks associated with heating systems, particularly the often-overlooked hazard of thermal decomposition (aka pyrolysis). Many homeowners believe that because they have used their fireplace, heating appliance, or chimney for years without an issue, their system is safe. Your role is to help them understand why this assumption can be dangerously misleading.
Key Concepts to Explain to the Homeowner
1. What Is Thermal Decomposition?
Thermal decomposition is a slow process where combustible materials—like wood framing, insulation, or drywall—break down over time due to continuous or intermittent exposure to heat. Even if the temperature never reaches the textbook ignition point, repeated exposure as low as 170°F can weaken the material. When conditions are right temperatures as low as 170°F have been reliably documented to lead to ignition. That is why listed systems are tested. If they exceed 170°F in concealed areas they fail their listing tests.
Homeowners should understand that this process is gradual and may take years to become a problem, but when the right conditions align, a fire can start inside walls, ceilings, or hidden areas without any warning.
2. Why It’s a Concern
The most dangerous fires are the ones you never see coming. If materials around your heating appliance or chimney have been slowly degrading for years, it only takes a little extra heat—maybe from a larger fire, prolonged burning, or a new homeowner’s different usage habits—for ignition to occur.
At this point, show the homeowner any signs of heat damage, charring, or clearance violations that could contribute to thermal decomposition.
3. What Causes It?
Give homeowners relatable examples:
- Improper Installation – “If insulation or framing was installed too close to the fireplace, stove, or chimney, it can break down over time and may transfer heat to adjacent combustibles.”
- Liner or Chimney Damage – “A cracked or deteriorated chimney liner can let heat escape into areas not designed to for inspection or to contain combustion byproducts.”
- Oversized or Undersized Inserts – “If an insert was added that doesn’t match the original fireplace’s design or isn’t tested for that specific application, it might create excessive heat transfer to nearby combustibles.”
- Changes in Use – “Maybe the system was used infrequently before, but increased use, burning hotter fires, or using different fuel types can accelerate the problem.”
How to Present Your Findings
1. Keep It Factual
Homeowners may feel defensive or overwhelmed, especially if they have used their system for years without issue. Avoid scare tactics and instead focus on the facts:
“This isn’t about whether there’s been a fire before—this is about how heat exposure over time can lower the ignition temperature of materials around your system.”
2. Show, Don’t Just Tell
Use visual evidence whenever possible:
- Photos of damage – Show cracks, charring, or clearance violations.
- Infrared imaging (if available) – Demonstrate heat transfer issues.
- Code references or manufacturer specifications – Point out the required clearances and how the system compares.
3. Offer Clear Next Steps
Homeowners need to know what action to take:
- Further Inspection: “A Level III chimney inspection can determine if there’s hidden damage or clearance issues behind the walls.”
- Repairs or Modifications: “The liner needs replacement, damaged materials need to be removed or replaced, or additional protection may be required to bring the system up to meet or exceed minimum standards.”
- System Replacement: “Where significant thermal decomposition or charring has occurred, continued use is not recommended, and replacing or redesigning the system is the best course of action.”
Handling Homeowner Pushback
Homeowners often respond with: “But we’ve never had a problem before.”
Your response should emphasize prevention: “That’s exactly why thermal decomposition is so dangerous—it takes years to develop, but once the materials are weakened, it only takes a little extra heat to cause ignition. This is why professional inspections are so important. We’re catching issues before they lead to a fire.”
Final Thoughts: Your Role as an Educator
Your job isn’t just to inspect—it’s to inform and empower homeowners to make safe choices. By explaining thermal decomposition clearly, showing them evidence, and providing actionable next steps, you can help them understand and address this hidden fire risk before it becomes a disaster.
Homeowners trust your expertise. The more clearly and confidently you communicate the risks, the more likely they are to take the necessary steps to ensure their home is safe.
Example Text to Include In Your Reports
If used combustibles too near one or more appliances, fireboxes, chimneys, or systems may overheat and potentially ignite, leading to a structure fire.
THE HIDDEN DANGER OF THERMAL DEGRADATION
You’ve used your fireplace, heating appliance, chimney, or vent for years without a problem—so it must be safe, right? That’s the trap. Thermal degradation (pyrolysis) is a slow, silent threat. Combustible materials and insulation too close to a heat source—whether a fireplace, stove, furnace, chimney, or vent—can break down over time with exposure to temperatures as low as 170°F. As this happens, they require less and less heat to ignite. If minimum standards are not met and thermal degradation has occurred a routine fire or normal operation could be enough to start a hidden blaze inside your walls.
What’s Makes A System Hazardous?
1) Bad Installation: Combustibles or insulation placed too close to the appliance, chimney, or vent.
2) Hidden Damage: Cracks or damage in the liner, firebox, or venting allow heat to escape, accelerating thermal degradation.
When the conditions are right, a fire can start where you’ll never see it—until it’s too late.
“But We’ve Never Had a Problem Before…”
That’s exactly why fires from thermal degradation are so dangerous—they take years to develop. Maybe you’ve only used your appliance occasionally, but what happens when usage increases? What if a new owner burns hotter, longer, or more often? That unseen damage doesn’t go away. It just waits.
Heating systems don’t get safer with time—they get riskier as the system itself, or the materials around it, degrade. The only way to control that risk is to take action before a fire happens.