Combustion Analysis Results
What You'll Learn
- What measureQuick displays on the combustion results screen: CO, CO2, O2, stack temperature, draft pressure, and efficiency
- How to interpret ambient CO readings and when to escalate or evacuate
- How to interpret flue CO readings using air-free CO thresholds
- What O2 and CO2 percentages tell you about combustion quality and excess air
- How stack temperature indicates heat transfer efficiency
- How draft pressure confirms safe venting and what positive or zero draft means
- How measureQuick calculates combustion efficiency from these inputs
- How these results feed into the venting pass/fail subsystem
- When combustion analysis is critical and when it can be deferred
What You'll Need
- Device: iPhone (iOS 15+) or Android phone/tablet (Android 10+) with measureQuick installed
- Account: measureQuick account with active subscription
- Combustion analyzer: Sauermann Si-CA series or AccuTools BluFlame, paired with measureQuick over Bluetooth
- Ambient CO monitor: UEI COA2, Sensorcon Inspector, or equivalent standalone ambient CO meter
- Prerequisite knowledge: Combustion analysis fundamentals (E16) covering what each measurement is and how it is captured
- Time: 10 minutes to read; assumes combustion data has already been captured per E16
What measureQuick Displays
After completing a combustion analysis in the gas furnace or boiler workflow, measureQuick presents the following measurements on the results screen:
| Measurement | Unit | Source |
|---|---|---|
| CO (raw) | ppm | Combustion analyzer flue probe |
| CO (air-free / COaf) | ppm | Calculated from raw CO and O2 |
| CO2 | % | Combustion analyzer flue probe |
| O2 | % | Combustion analyzer flue probe |
| Stack temperature | F | Combustion analyzer thermocouple |
| Draft pressure | inWC | Combustion analyzer or paired manometer |
| Combustion efficiency | % | Calculated from stack temp, O2/CO2, and fuel type |
Each measurement has a role in evaluating whether the appliance is operating safely and efficiently. The sections below cover how to read each one.
measureQuick combustion results screen showing all seven measurements with pass/fail indicators
Interpreting CO Readings
Carbon monoxide is the primary safety indicator. measureQuick displays both raw CO and air-free CO (COaf). Always use COaf for threshold evaluation, because raw CO is diluted by excess air and understates the true concentration.
Ambient CO
Before the appliance fires, measure ambient CO in the equipment room using a standalone CO meter. This establishes the baseline.
| Ambient CO (ppm) | Interpretation | Action |
|---|---|---|
| 0-9 | Normal background levels | Document and proceed |
| 9-35 | Elevated; a CO source exists in the space | Identify the source before continuing. May be the appliance under test, another appliance, an attached garage, or an external source |
| 35+ | Dangerous; occupant health risk | Ventilate the space immediately. Do not proceed with testing until the source is identified and ambient CO drops to safe levels. Follow your company's evacuation protocol |
After the appliance fires, recheck ambient CO in the supply and return air. If supply air CO is higher than return air CO, the appliance is contributing CO to the airstream, which indicates a cracked heat exchanger or flue gas spillage.
Flue CO (Air-Free)
Flue CO is measured inside the flue pipe where concentrations are much higher than ambient.
| Flue COaf (ppm) | Interpretation | Action |
|---|---|---|
| 0-25 | Normal combustion | Document and continue |
| 26-100 | Elevated; incomplete combustion | Investigate: dirty burners, improper gas pressure, cracked heat exchanger, insufficient combustion air supply |
| 100-400 | High; appliance needs service | Do not leave the appliance running unattended. Schedule immediate repair. Determine whether the appliance can operate safely until service is completed |
| 400+ | Dangerous | Shut down the appliance. Do not restart until the cause is identified and corrected |
CO tends to spike during startup (first 30-60 seconds), then settle. Evaluate after at least 5 minutes of steady-state operation.
Interpreting O2 and CO2
O2 and CO2 are inversely related. As O2 in the flue gas goes down, CO2 goes up, indicating more complete combustion with less excess air.
O2 Percentage
| O2 (%) | Interpretation |
|---|---|
| Below 4% | Too little excess air. Rich combustion. CO risk increases. Investigate combustion air supply and burner adjustment |
| 4-9% | Normal range for natural gas appliances. Adequate excess air for complete combustion without excessive dilution |
| Above 9% | Too much excess air. Diluting the flue gases with cold air, reducing efficiency. Check for draft issues, oversized flue, or air leaks at the draft hood |
CO2 Percentage
| CO2 (%) | Interpretation |
|---|---|
| 8-10% | Normal range for natural gas. Indicates reasonably complete combustion |
| Below 8% | Too much excess air or incomplete combustion. Corresponds to high O2 readings |
| Above 10% | Very little excess air. Monitor CO closely; the margin for incomplete combustion is smaller |
O2 and CO2 together tell you the combustion quality story. High O2 with low CO2 means excess air is diluting the flue gases, carrying usable heat up the flue. Low O2 with high CO2 means tighter combustion, which is more efficient but leaves less margin for error.
Interpreting Stack Temperature
Stack temperature is the temperature of the flue gases measured at the probe insertion point. The key metric is net stack temperature: stack temperature minus combustion air temperature.
| Equipment Type | Typical Stack Temp (F) | Notes |
|---|---|---|
| 80% AFUE standard furnace | 300-500 | Higher end indicates more heat loss up the flue |
| 90%+ AFUE condensing furnace | 100-150 | Low because the secondary heat exchanger recovers most of the heat |
| Atmospheric water heater | 300-450 | Varies with firing rate and flue configuration |
| Boiler | 300-550 | Depends on boiler type and load |
What high stack temperature means: The heat exchanger is not transferring enough heat to the air (or water). Possible causes: dirty heat exchanger, excessive firing rate, low airflow across the heat exchanger, or a degraded heat exchanger surface.
What low stack temperature means on non-condensing equipment: If a standard-efficiency furnace shows stack temperatures below 300F, check for restricted flue, recirculation of flue gases, or incorrect fuel type. On condensing equipment, low stack temperature is expected and desirable.
Interpreting Draft Pressure
Draft pressure confirms that flue gases are moving up and out of the building. measureQuick displays draft in inches of water column (inWC).
| Draft (inWC) | Interpretation |
|---|---|
| -0.02 to -0.05 | Normal negative draft for natural-draft equipment at the draft hood. Gases are exhausting properly |
| Weaker than -0.02 | Marginal draft. May be acceptable on calm days but could reverse under adverse conditions (wind, exhaust fans, dryer operation) |
| 0.00 or positive | No draft or backdraft. Flue gases are not exhausting or are spilling into the space. This is a safety failure. Shut down the appliance and investigate |
Induced-draft and direct-vent equipment create their own draft mechanically, so draft readings at the combustion blower may show different values than natural-draft equipment. Refer to the manufacturer's specifications for expected draft ranges on induced-draft units.
Common causes of poor draft:
- Undersized or deteriorated flue pipe
- Blocked or partially obstructed chimney
- Competing exhaust appliances (bath fans, range hoods, dryers) depressurizing the combustion air zone
- Inadequate combustion air supply
Combustion Efficiency
measureQuick derives combustion efficiency from stack temperature, O2 (or CO2), combustion air temperature, and fuel type. This represents the percentage of the fuel's heat energy that is transferred to the building rather than lost up the flue.
| Measured Efficiency | Interpretation |
|---|---|
| Within 2-3% of AFUE rating | Normal. Real-world conditions produce some variation from rated |
| 5%+ below AFUE rating | Degraded performance. Investigate stack temperature, excess air, heat exchanger condition, and airflow |
| Well above AFUE rating | Unusual. Verify measurement accuracy. On condensing equipment, efficiency can vary with return water temperature or load |
Efficiency is a useful summary metric, but it does not replace the individual measurements. A system can show acceptable efficiency while producing dangerous CO levels. Always evaluate CO and draft independently of the efficiency number.
How Results Affect the Venting Subsystem
The venting subsystem (pf_venting) is one of measureQuick's 19 pass/fail evaluations. It considers CO levels, draft adequacy, and temperature rise to determine whether the appliance is venting safely.
From measureQuick's V12 diagnostic database, covering over 200,000 total diagnostic tests: 29.6% of gas-fired systems fail the venting subsystem. Nearly one in three gas appliances tested has a measurable combustion or venting problem.
A venting failure does not always mean dangerous CO. It can be triggered by:
- Elevated (but not dangerous) CO levels
- Marginal draft pressure
- Temperature rise outside the manufacturer's specified range (often an airflow problem, not a combustion problem)
- A combination of individually borderline readings that together indicate a system operating outside safe parameters
When pf_venting fails, review each individual measurement to determine the root cause. The fix for a temperature rise failure (adjust airflow) is very different from the fix for high CO (clean burners, check heat exchanger, verify gas pressure).
When Combustion Analysis is Critical
Combustion analysis is required on every gas-fired appliance service call. It is especially critical in these situations:
- Annual maintenance on gas furnaces and boilers. The only way to verify safe operation is to measure it.
- After any burner, gas valve, or heat exchanger service. Any work that affects combustion must be verified with measurements afterward.
- Comfort complaints involving gas equipment. Temperature rise and efficiency issues often stem from combustion or venting problems.
- New installation commissioning. Baseline combustion readings document proper setup and provide a reference for future comparisons.
- When ambient CO is detected. If any CO monitor in the building shows elevated readings, combustion analysis on every gas appliance is mandatory.
The 29.6% failure rate means that skipping combustion analysis on a gas appliance is, statistically, skipping a problem nearly a third of the time.
[Visual Reference] A venting fail result displays a red "Fail" banner on the pf_venting subsystem. Tapping the banner expands a detail panel listing each combustion measurement that contributed to the failure. Failed measurements are highlighted in red with their actual values shown alongside the acceptable thresholds - for example, "CO: 156 ppm (threshold: 100 ppm)" or "Draft: +0.02 inWC (must be negative)." Passing measurements within the same subsystem appear in green. This expanded view makes it clear which specific conditions triggered the fail, guiding the technician to the root cause.
Tips & Common Issues
Stack temperature is high but CO is normal
This usually indicates an airflow problem, not a combustion problem. The burners are burning cleanly (low CO), but heat is not being transferred efficiently to the air because airflow across the heat exchanger is low. Check static pressure, filter condition, and blower speed.
CO is elevated but efficiency looks fine
Efficiency calculations weight stack temperature and excess air heavily. A modest CO elevation may not move the efficiency number. Never use efficiency alone as a safety indicator. CO thresholds exist independently of efficiency.
Draft readings fluctuate
Fluctuating draft is common when competing exhaust appliances cycle on and off. Test draft with all other exhaust appliances running (worst-case scenario). If draft becomes positive under worst-case conditions, the venting system is inadequate for the building's exhaust load.
Condensing furnace shows very low stack temperature
This is normal. Condensing furnaces extract so much heat that flue gases cool below the dew point, which is why they produce condensate. Stack temperatures of 100-130F are typical and indicate the secondary heat exchanger is functioning correctly.
Temperature rise fails but combustion readings are clean
Temperature rise is primarily an airflow measurement, not a combustion measurement. If CO, O2, and draft are all normal but temperature rise is outside the manufacturer's range, the problem is almost certainly restricted airflow or incorrect blower speed. This is a duct system or blower issue, not a combustion issue. The venting subsystem will still flag it because temperature rise is part of the evaluation.
Reference Material
Download: Combustion Quick Start Guide (PDF)
Related Articles
Prerequisites (complete these first):
- Combustion Analysis Fundamentals - What each measurement is and how to capture it
- UEI COA2 CO Monitor - Pairing and using the UEI ambient CO monitor
- Sensorcon CO Meter - Pairing and using the Sensorcon ambient CO meter
Follow-up articles (next steps after this one):
- Venting Diagnostics - Deep diagnostic approach for venting failures
- Fault Types: Minor vs. Major - How to classify combustion findings by severity
- Gas Furnace Workflow - Full workflow that produces the combustion data interpreted here
Related in the same domain:
- Combustion Probe Placement - Proper probe placement that affects measurement accuracy
- Component Pressure Drops - Airflow restrictions that cause temperature rise failures
Need Help?
If you have questions about interpreting combustion analysis results in measureQuick:
- Check the Related Articles section above
- Contact measureQuick support: support@measurequick.com
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