What You'll Learn

• What a combustion analyzer measures: CO, CO2, O2, stack temperature, draft pressure, flue gas velocity, and manifold pressure
• How to interpret CO readings and the action thresholds: 0-25 ppm normal, 26-100 ppm investigate, 100+ ppm shut down
• What combustion efficiency means and how it is calculated from stack temperature and excess air
• How measureQuick captures combustion data from Sauermann analyzers over Bluetooth
• How the venting subsystem pass/fail (pf_venting) works and what it evaluates
• Why 29.6% of systems fail the venting subsystem in measureQuick's diagnostic database

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 (direct Bluetooth integration with measureQuick), or AccuTools BluFlame
• Gas-fired equipment: Furnace, boiler, or water heater in operation
• Ambient CO monitor: A standalone ambient CO meter for monitoring the space during testing
• Prerequisite knowledge: Combustion probe placement (F7) - where to insert the flue probe
• Time: 15 minutes to read; 20-30 minutes for a complete combustion analysis in the field

What Combustion Analysis Measures

Combustion analysis evaluates how completely a gas-fired appliance burns its fuel and how safely it vents the combustion byproducts. A combustion analyzer samples the flue gases leaving the appliance and measures their composition.

Here are the seven primary measurements and what each one tells you.

1. Carbon Monoxide (CO)

CO is a colorless, odorless gas produced by incomplete combustion. It is the primary safety measurement in combustion analysis.

What the analyzer reads: CO concentration in the flue gas stream, measured in parts per million (ppm).

Action thresholds:

These thresholds apply to air-free CO (COaf), which is the CO concentration adjusted to remove the dilution effect of excess air. Raw CO and air-free CO are different numbers. measureQuick displays both values. Use air-free CO for threshold evaluation.

Jim Bergmann explains the ambient CO check in the gas furnace workflow: the technician measures "the ambient CO, we're looking at the return, we're looking at the supply, and we're making sure it's falling below nine parts per million and there's no difference between supply and return air that would indicate the furnace is contributing CO into the space."

2. Oxygen (O2)

What it tells you: How much excess air is being drawn into the combustion process beyond what is needed to burn the fuel.

Normal range: 4-9% O2 in the flue gas for natural gas appliances. Below 4% indicates insufficient combustion air (rich burn, CO risk). Above 9% indicates too much excess air (poor efficiency, draft issues).

The O2 reading is the primary input for calculating excess air and combustion efficiency.

3. Carbon Dioxide (CO2)

What it tells you: How completely the fuel is being burned. CO2 is the desired product of complete combustion.

Normal range: 8-10% CO2 for natural gas. Higher CO2 generally means more complete combustion and less excess air. CO2 and O2 are inversely related - as one goes up, the other goes down.

4. Stack Temperature

What it tells you: The temperature of the flue gases leaving the appliance. Combined with the combustion air temperature, this determines how much heat is being lost up the flue.

Normal ranges:

• 80% AFUE furnace: 350-500F stack temperature
• 90%+ AFUE condensing furnace: 100-130F (much lower because the secondary heat exchanger recovers additional heat)

The difference between stack temperature and combustion air temperature is the net stack temperature, which is the primary driver of efficiency loss.

5. Draft Pressure

What it tells you: Whether the flue gases are being properly exhausted from the appliance.

Measurement: Negative pressure in the flue measured in inches of water column (inWC). A properly drafting natural-draft appliance shows -0.02 to -0.05 inWC at the draft hood. Induced-draft furnaces create their own draft via the combustion blower motor.

As Jim Bergmann notes, the induced draft setup involves "coming off the combustion blower motor" with "a T that's tied in" to measure draft pressure alongside the pressure switch.

Why it matters: Inadequate draft can cause flue gas spillage into the living space. Positive pressure in the flue (backdrafting) is a safety hazard.

6. Flue Gas Velocity

What it tells you: How fast the combustion gases are moving through the flue pipe. Higher velocity means more aggressive venting.

This measurement is less commonly captured than the others but is available on analyzers that include a pitot tube or differential pressure-based flow measurement.

7. Manifold Gas Pressure

What it tells you: The gas pressure at the furnace's gas valve. This confirms the gas valve is delivering the correct pressure to the burners.

Normal values: 3.5 inWC for natural gas (standard), though manufacturer specifications take precedence. As described in the gas furnace profile: "the maximum gas supply pressure is seven, the minimum is five, and 3.5" for manifold pressure.

Manifold pressure is measured with a manometer connected to the gas valve pressure tap, not with the combustion analyzer. measureQuick captures it as part of the gas furnace workflow using a paired Bluetooth manometer.

[Visual Reference] The combustion analysis screen displays live readings in a vertical list of labeled channels: CO (ppm), CO2 (%), O2 (%), stack temperature (F), and draft pressure (inWC). Each channel shows the current value streaming from the connected Sauermann analyzer, with stability indicators next to each reading. The CO channel is at the top since it is the primary safety measurement. Draft pressure appears as a negative value when the flue is drawing correctly. At the bottom of the channel list, combustion efficiency is calculated automatically from the captured values.

Combustion Efficiency

Combustion efficiency represents how much of the fuel's heat energy is being transferred to the building versus lost up the flue. It is calculated from stack temperature, combustion air temperature, O2 percentage, and the fuel type.

Efficiency Categories

The AFUE rating on the data plate is the manufacturer's design-condition efficiency. The measured combustion efficiency in the field will vary based on actual conditions: burner cleanliness, heat exchanger condition, air supply, and venting configuration.

A measured efficiency significantly below the rated AFUE indicates a problem: dirty burners, a degraded heat exchanger, improper gas pressure, or restricted airflow.

What Affects Efficiency

• High stack temperature: More heat going up the flue, lower efficiency
• High excess air (high O2): Diluting the flue gases with cold air that absorbs heat and carries it up the flue
• Low CO2: Same issue viewed from the other direction - less complete combustion means more waste

How measureQuick Captures Combustion Data

Bluetooth Integration with Sauermann Analyzers

measureQuick integrates directly with Sauermann combustion analyzers (Si-CA series) over Bluetooth. When paired, the analyzer streams live combustion data to the app:

• CO (raw and air-free)
• CO2
• O2
• Stack temperature
• Combustion air temperature
• Draft pressure
• Calculated efficiency (gross and net)

The data flows into the gas furnace or boiler workflow automatically. You do not need to manually enter combustion values when using a paired analyzer.

The Sauermann Si-CA specs include:

• O2 sensor: field-replaceable, 0-25% range, +/-0.2% accuracy, 5-year life
• CO sensor: factory-replaceable, 0-8,000 ppm range, +/-8 ppm or +/-5% accuracy, 4-year life
• Draft pressure: -80 to +80 inWC
• Flue temperature: -4 to +2,282F
• Auto-zeroing, data logging, and PDF/CSV export

The Gas Furnace Workflow

In the gas furnace workflow, combustion measurements are captured alongside temperature rise, static pressure, and manifold pressure. As shown in the workflow walkthrough: "I've got a combustion analyzer, I'm going to be measuring my stack CO and my ambient CO as well as my combustion efficiency calculations. I've got a Tech DG-8 that I'm going to be using for measuring my gas pressure."

The workflow captures all measurements in a single test record, providing a complete picture of furnace performance in one visit.

Gas furnace workflow screen showing combustion data alongside temperature rise and static pressure measurements

AccuTools BluFlame Integration

The AccuTools BluFlame combustion analyzer also integrates with measureQuick. The BluFlame "has built-in" the ability to "get the T2 temperature, the supply air temp, return air temp, and very easily get the differential temperature" on the appliance, in addition to standard combustion measurements.

The Venting Subsystem: pf_venting

measureQuick evaluates combustion and venting data through the venting subsystem pass/fail indicator (pf_venting). This is one of the 19 subsystems evaluated during a gas appliance test.

What pf_venting Evaluates

The venting subsystem considers:

• CO levels in the flue (within acceptable thresholds)
• Draft pressure (adequate negative draft or proper induced draft operation)
• Temperature rise (within the manufacturer's specified range)
• Overall venting safety

A Pass result means the system's combustion and venting meet the evaluation criteria. A Fail result means one or more measurements are outside acceptable ranges and the system requires attention.

Industry Data: 29.6% Venting Failure Rate

From measureQuick's V12 diagnostic database (over 200,000 total diagnostic tests), 29.6% of gas-fired systems fail the venting subsystem. This means nearly one in three gas appliances tested shows a combustion or venting problem.

Common causes of venting failure include:

• Elevated CO from dirty burners or cracked heat exchangers
• Inadequate draft from undersized or deteriorated flue pipes
• Improper combustion air supply
• Temperature rise outside the manufacturer's range (often caused by restricted airflow, which is an air distribution problem that shows up as a venting symptom)

The 29.6% rate underscores why combustion analysis is not optional on gas appliance service calls. Nearly a third of systems have a measurable problem.

Performing a Combustion Analysis: Step-by-Step

Step 1: Zero the Analyzer Outdoors

Before entering the building, turn on your combustion analyzer outside in fresh air and let it complete its full zeroing cycle. The O2 sensor calibrates to 20.9% (atmospheric oxygen). The CO sensor zeros to 0 ppm. As noted in the gas furnace workflow: "I zero this analyzer outside."

Do not start the zeroing process inside the building or near exhaust sources. Indoor air may contain low levels of CO or other gases that give the analyzer a false baseline. If the analyzer zeros against contaminated air, every subsequent reading will be offset by that contamination. This was emphasized across multiple training events as one of the most common combustion analysis mistakes, particularly with Sauermann analyzers that auto-zero on startup.

Step 2: Check Ambient CO

With the zeroed analyzer, measure ambient CO in the equipment room before the appliance fires. Record the return air CO and supply air CO. Both should be below 9 ppm, and there should be no significant difference between the two.

If ambient CO is elevated before the appliance fires, the source is something other than the appliance you are testing. Investigate before proceeding.

Step 3: Fire the Appliance and Allow Stabilization

Start the appliance and allow it to run for at least 5 minutes (10 minutes for condensing equipment) before inserting the combustion probe. The appliance needs to reach steady-state operation before the measurements are meaningful.

Step 4: Insert the Probe and Read

Insert the combustion probe into the flue according to placement guidelines (see Combustion Probe Placement). If using a paired Sauermann analyzer, the readings stream directly to measureQuick.

Monitor CO first. If CO exceeds 100 ppm air-free at any point, shut down the appliance.

Allow readings to stabilize for 2-3 minutes before recording final values.

Step 5: Measure Draft

Check draft pressure at the draft hood (natural-draft equipment) or at the combustion blower tap (induced-draft equipment). Draft should be negative (exhaust gases moving up and out).

Step 6: Check Manifold Pressure

Connect a manometer to the gas valve pressure tap and verify manifold pressure matches the manufacturer's specification. Record the value in measureQuick.

Step 7: Evaluate and Save

Review all combustion values. measureQuick evaluates the venting subsystem and assigns pass/fail. Save the test record.

[Visual Reference] The completed combustion analysis screen shows all measurement channels populated with final values: CO, CO2, O2, stack temperature, draft pressure, and calculated combustion efficiency. Below the measurements, the venting subsystem pass/fail result (pf_venting) is displayed as a colored banner - green with "Pass" if all values are within acceptable ranges, or red with "Fail" if any measurement exceeds its threshold. Tapping the result expands a detail view showing which specific measurements contributed to the pass or fail determination.

Video Walkthrough

• What should you be looking at when doing a combustion analysis? (The basics!) (13 min, 57K views): - Comprehensive overview of combustion analysis fundamentals, covering what each measurement means and how to interpret the results

• measureQuick Gas Furnace or Boiler Workflow 2022 (31 min): - Complete gas furnace workflow walkthrough including combustion analysis, manifold pressure, temperature rise, and static pressure

• Jim Bergmann reviews combustion analysis, the BluFlame analyzer and measureQuick - Jim Bergmann covers combustion analysis principles, CO thresholds, draft testing, and how measureQuick integrates with the BluFlame analyzer

• The Critical Importance of Combustion Analysis ft. Sauermann Analyzers (9:30, 2.6K views): - Why combustion analysis matters for safety and efficiency, featuring Sauermann analyzer integration

• Sauermann Combustion Analyzer now in measureQuick (13 min): - How to pair and use the Sauermann Si-CA combustion analyzer with measureQuick

• 2025 HVACR Symposium: Sauermann Combustion Analyzers w/ Tyler Nelson (10 min, 1K views): - Tyler Nelson demonstrates Sauermann combustion analyzer features and measureQuick integration

• Gas Furnace Combustion Analysis Training with Tyler Nelson! (1:35): - In-depth combustion analysis training covering all measurement types and interpretation

• Tuning a Gas Furnace with Testo Smart Probes and the BluFlame Analyzer (19 min, 10K views): - Gas furnace tuning workflow using Testo Smart Probes for temperature/pressure and BluFlame for combustion

• AccuTools BluFlame Combustion Analyzer (35 min, 14K views): - Detailed BluFlame combustion analyzer review and measureQuick integration walkthrough

• Using Your Combustion Analyzer To Detect Exhaust Gas Recirculation (2:55, 2K views): - How to use combustion readings to detect flue gas recirculation issues

• Combustion Testing Home Appliances (12:37): - Applying combustion analysis to various home gas appliances beyond furnaces

Tips & Common Issues

CO readings spike during startup, then drop

This is normal. During ignition and the first 30-60 seconds of operation, burners may produce elevated CO as the flame stabilizes. Wait for steady-state operation before evaluating CO levels. If CO remains above 25 ppm air-free after 5 minutes of operation, investigate.

What is CO air-free, and why does it matter?

CO air-free (COaf) is the calculated concentration of carbon monoxide with the dilution air removed. When your analyzer samples flue gas, the raw CO reading includes excess air that dilutes the sample. Air-free gives you the true combustion chamber concentration by mathematically removing that dilution.

This is the number that matters for safety evaluations. Building codes and safety thresholds reference air-free values. A raw CO reading of 50 ppm might translate to 120 ppm air-free once the dilution is accounted for, potentially crossing an action threshold that the raw number would not.

measureQuick displays both raw CO and CO air-free. Always use the air-free value when evaluating against the action thresholds in the table above.

CO levels and legal liability

CO levels above 1600 ppm air-free have been cited as evidence in legal proceedings involving HVAC contractor liability. In one case referenced during training (Event 14), a 1600 ppm CO exposure resulted in brain injury, and the contractor's documentation (or lack of it) became central to the case.

Documenting your combustion readings with measureQuick creates defensible records. The measurements, timestamps, and diagnostic results are stored in the project record. If a question arises later about the condition of the appliance when you serviced it, you have instrument-verified data rather than just notes.

My analyzer shows different CO and COaf values

CO is the raw measurement from the sensor. COaf (CO air-free) removes the dilution effect of excess air to give a standardized value. COaf is always higher than raw CO because it represents what the CO would be without dilution. Use COaf for threshold evaluation.

Temperature rise is outside the manufacturer's range

The manufacturer's specified temperature rise (printed on the data plate or in the installation manual) assumes correct airflow. If temperature rise is too high, airflow is likely too low - check the filter, blower speed, and duct system. If temperature rise is too low, airflow may be excessive, or the gas valve may be delivering low manifold pressure.

The analyzer will not connect to measureQuick

Verify the analyzer is in Bluetooth pairing mode. For Sauermann analyzers, check that the analyzer's Bluetooth is enabled in its settings menu. In measureQuick, go to the Toolbox and scan for the analyzer. If it does not appear, restart both the analyzer and the app.

Do not use co_ambient column from the database

For anyone working with measureQuick's diagnostic database directly: the co_ambient column contains garbage data (epoch timestamps, not CO values). Do not reference it for ambient CO analysis. Ambient CO is captured as part of the workflow but is not reliably stored in that particular column.

Always use a standalone ambient CO monitor

The combustion analyzer measures flue CO. A separate ambient CO meter monitors the room air while you work. This is a safety requirement, not a recommendation. If the appliance is backdrafting or the heat exchanger is cracked, ambient CO can rise while you are focused on flue readings.

Reference Material

Download: Combustion Quick Start Guide (PDF)

Related Articles

Prerequisites (complete these first):

• Combustion Probe Placement - Where to insert the flue probe for accurate combustion readings

Follow-up articles (next steps after this one):

• Gas Furnace Workflow - Complete guided workflow for gas furnace testing including combustion analysis
• Venting Diagnostics - Interpreting venting failures and identifying root causes

Related in the same domain:

• Total External Static Pressure - TESP measurement, which affects temperature rise and combustion performance
• TESP Budget & 140% Rule - How restricted airflow compounds combustion problems
• Combustion Analyzer Pairing - Pairing Sauermann and BluFlame analyzers with measureQuick

Need Help?

If you get stuck or this article does not answer your question:

• Check the Related Articles section above
• Contact measureQuick support: support@measurequick.com