What You’ll Learn
What is an combustion analysis report actually telling me about my furnace?
A combustion analysis measures how safely and efficiently your furnace burns fuel. Understanding the numbers helps homeowners see whether their system is operating properly, where efficiency may be lost, and how professional tuning improves performance.
In this blog:
The technician finishes the furnace tune-up and hands over a report.
It’s full of numbers.
Oxygen. Carbon monoxide. Efficiency percentages. Stack temperature.
The homeowner scans the page, nods politely, and sets it aside.
For many Pennsylvania homeowners, a combustion analysis report feels more like a medical chart than a service document.
The good news?
Those numbers are not random. When interpreted correctly, they tell a detailed story about how safely and efficiently your furnace is operating.
What Is a Combustion Analysis and When Do You Get One?
A combustion analysis is a diagnostic test technicians perform on propane or gas furnaces during a tune-up to evaluate combustion performance, efficiency, and safety.
Using specialized testing equipment, technicians measure how the furnace burns fuel and how efficiently it transfers heat into your home.
Homeowners typically receive a combustion analysis during:
- Annual furnace tune-ups
- No-heat or performance service calls
- Furnace troubleshooting
- Efficiency evaluations
- Safety inspections
Good technicians don’t perform combustion analysis simply to check a box.
They use it to understand how the system is operating and whether adjustments can improve performance.
This is especially important with propane systems, where proper air-to-fuel balance directly affects efficiency and safe operation.
Why Good Technicians Document “As Found” and “As Left”
One of the most valuable parts of a combustion report is something many homeowners overlook:
Two sets of numbers.
“As Found”
These readings show how the furnace was performing when the technician arrived.
This is the baseline.
It reflects the real operating condition of the system before any maintenance, cleaning, or adjustments take place.
“As Left”
These readings document how the furnace performs after service is completed.
This may include:
- Burner cleaning
- Air adjustments
- Sensor cleaning
- Draft or combustion tuning
- General furnace maintenance
The difference between these numbers matters.
Good technicians document both because they provide:
- Transparency
- Proof of work performed
- Measurable improvement
Instead of simply saying, “The furnace is better now,” the report shows exactly how performance changed.
At Kauffman Gas, technicians believe homeowners should understand the condition of their system—not just trust that work was completed.
Breaking Down
the Numbers on
Your Report
At first glance, combustion reports can look overwhelming.
But most focus on five key measurements.
Oxygen (O₂)
Oxygen readings help technicians understand the air-to-fuel mixture inside the furnace.
Combustion requires oxygen.
But too much oxygen can signal that excess air is entering the burn process.
This can mean:
- Reduced efficiency
- Improper burner adjustment
- Draft issues
- Heat leaving through exhaust instead of heating the home
Too little oxygen can also create combustion concerns.
The goal is balanced combustion—not simply more air.
Carbon Monoxide (CO)
This is often the number homeowners notice first.
Carbon monoxide is produced when fuel does not burn completely.
Small measured amounts inside exhaust gases can be normal, but elevated readings demand attention.
Higher CO levels may indicate:
- Incomplete combustion
- Dirty burners
- Airflow problems
- Venting issues
- Improper burner operation
Because CO affects safety, technicians monitor these readings carefully during service.
This is one reason combustion analysis matters so much.
Problems are often identified before homeowners notice symptoms.
Carbon Dioxide (CO₂)
Carbon dioxide readings provide another window into combustion quality.
Generally speaking:
- Efficient combustion produces more CO₂
- Poor combustion produces less CO₂
Technicians evaluate CO₂ alongside oxygen readings to understand how effectively fuel is burning.
This number is not viewed in isolation.
It helps complete the overall picture of combustion.
Stack Temperature
Stack temperature measures how hot exhaust gases are as they leave the furnace.
This answers an important question:
How much heat is staying inside the system, and how much is escaping?
If temperatures are too high, valuable heat may be leaving through the vent instead of warming your home.
If temperatures are unusually low, it may point toward combustion or venting concerns.
Proper stack temperature supports:
- Better efficiency
- Stable combustion
- Safer furnace operation
Combustion Efficiency
Efficiency readings estimate how effectively the furnace converts fuel into usable heat.
Higher numbers generally indicate better fuel utilization.
But this number requires context.
Efficiency alone does not tell the whole story.
Technicians evaluate it alongside:
- Oxygen levels
- CO readings
- Stack temperature
- Burner performance
A furnace can show strong efficiency while still having combustion or safety concerns.
That is why professional interpretation matters.
The Three Operating States Technicians Watch
Combustion is not static.
A furnace behaves differently at various points in the heating cycle.
That is why skilled technicians sometimes record readings during three separate operating states.
“Light Off”
Despite the “off” in the name, the “Light off” state refers to the system startup.
This is the moment ignition begins, and the burners first establish flame.
Technicians watch for:
- Delayed ignition
- Rough startup
- Flame instability
- Sudden combustion changes
Problems during startup can create wear, inefficiency, or unsafe combustion conditions.
“Steady State”
Steady state is where combustion stabilizes.
This is the normal operating mode.
Most combustion readings are evaluated here because the furnace has reached a consistent performance level.
Technicians confirm:
- Stable flame
- Balanced combustion
- Safe exhaust readings
- Proper efficiency
This is the condition homeowners rely on most during heating season.
“Shutdown”
The heating cycle doesn’t stop mattering once the burners shut off.
Shutdown readings help technicians observe how the system behaves as combustion ends.
Large changes during shutdown may reveal:
- Draft problems
- Venting concerns
- Pressure changes
- Combustion instability
This is why good diagnostics sometimes extend beyond steady-state readings alone.
A furnace may appear normal while running but behave differently during transitions.
What a Good Combustion Analysis Report Looks Like
Not all combustion reports are equally detailed.
A strong report should include:
- Clear combustion readings
- “As found” and “as left” measurements
- Technician observations
- Notes explaining adjustments or recommendations
- Documentation of system performance
Good service is not just about collecting numbers.
It is about understanding what those numbers mean—and helping homeowners understand them too.
When technicians take time to document, explain, and optimize performance, the report becomes more than paperwork.
It becomes proof of how your furnace is operating.
Know What Your Furnace Is Really Telling You
Your furnace may look fine from the outside.
But combustion analysis tells the deeper story.
It reveals:
- How efficiently fuel is burning
- Whether heat is being wasted
- If safety concerns exist
- How professional tuning improved performance
Instead of guessing, you get real data about your system.
Contact Kauffman Gas for a professional combustion analysis and see how your furnace is truly performing.
FAQs
Is combustion analysis only for older furnaces?
No. Combustion analysis benefits both older and newer propane or gas furnaces. Even modern systems can develop airflow, combustion, or venting issues that affect efficiency and safety.
Should I be worried if carbon monoxide appears on my report?
Not necessarily. Small CO readings inside exhaust gases may be normal. Technicians evaluate the amount and operating conditions to determine whether the system is combusting safely.
Why are there two sets of numbers on my report?
Good technicians often provide “as found” and “as left” readings. This shows how the furnace performed before service and documents improvements after cleaning or tuning.
What if my efficiency reading seems lower than expected?
Efficiency numbers should always be interpreted alongside other measurements. Oxygen levels, combustion quality, and stack temperature all help explain overall furnace performance.
How often should combustion analysis be performed?
Combustion analysis is commonly performed during annual furnace maintenance, especially on propane and gas systems. Regular testing helps maintain efficiency and catch developing problems early.