Heating stove carbon monoxide (CO) emission concentration testing is a critical safety evaluation that measures the CO content of flue gas and room air to verify that gas-fired, wood-burning, pellet, coal and kerosene heating appliances comply with national and international limits. Conducted by accredited third-party laboratories in accordance with EN, UL, ASTM, GB, JIS and ISO/IEC-referenced standards, this testing protects users from the odorless and potentially lethal hazards of incomplete combustion while supporting product certification, market access and design optimization.
Technical Overview
Carbon monoxide is produced whenever carbon-based fuels such as natural gas, LPG, wood, biomass pellets, coal or kerosene are burned with insufficient oxygen, poor air-fuel mixing, flame impingement on cold surfaces, or an obstructed chimney. Because CO is colorless, odorless and tasteless, occupants cannot detect dangerous accumulations without instrumentation, which makes objective laboratory measurement the only reliable basis for assessing emission performance.
Physiologically, CO binds to hemoglobin with an affinity roughly 200 to 250 times greater than oxygen, forming carboxyhemoglobin and progressively starving the body of oxygen. Exposure to a few hundred ppm can cause headache and dizziness, while concentrations above roughly 1000 ppm can rapidly become incapacitating or fatal. Consequently, even a well-designed heating stove must be proven, through standardized testing under both normal and abnormal operating conditions, to keep CO emissions below the limits defined by the applicable safety and emissions standards.
The regulatory framework for CO emission testing is built on regional and international standards families: the EN system for gas and solid fuel local space heaters, ANSI/UL and ASTM standards for the North American market, GB standards for the Chinese market, JIS standards for Japan, and supporting ISO/IEC references for laboratory competence. A typical compliance program combines type testing of a representative appliance with production surveillance to ensure that certified CO performance is maintained throughout manufacturing.
Testing Samples
Vented Gas-Fired Room Heaters and Wall-Hung Combination Boilers
These include independent gas-fired convection heaters covered by EN 613 and gas-fired combination heating and hot water boilers evaluated under EN 483, EN 15502 and GB 25034, with energy efficiency requirements addressed by GB 20665. Samples are tested with reference test gases at nominal, minimum and maximum supply pressures to capture the full range of field operating conditions.
Flueless (Unvented) Gas and Catalytic LPG Heaters
Because these appliances discharge combustion products directly into the occupied space, they are subject to the strictest CO controls. Testing follows standards such as EN 449 for dedicated LPG flueless space heaters and ANSI Z21.11.2 for unvented gas room heaters, which combine air-free flue gas CO limits with sealed-room CO concentration monitoring over extended operation.
Wood-Burning and Multi-Fuel Room Heaters
Freestanding wood stoves and multi-fuel heaters are tested under EN 13240, the EN 16510 series for local space heaters, and UL 1482 for solid-fuel-type room heaters. Testing uses specified fuel loads, wood species and moisture content, with CO measured in the flue during ignition, refueling and steady burning phases.
Pellet-Fired Stoves and Boilers
Automatically fed pellet appliances are evaluated under EN 14785, the EN 16510 series and ASTM E1509. Testing addresses CO emissions across the controller heat output range, including start-up and shutdown transients where automated ignition can temporarily elevate CO production.
Solid Fuel Cookers and Coal-Fired Water-Heating Stoves
Cookers with boilers and residential coal-fired heating stoves are covered by EN 12815 for residential solid fuel cookers and by applicable national and sectoral GB requirements for civil coal-fired water-heating stoves. These appliances are tested with the manufacturer-declared fuel types and maximum approved fuel loadings.
Kerosene and Oil-Fired Portable Heaters
Unvented kerosene-fired room heaters, evaluated under UL 647 and related regional standards, receive particular scrutiny because their combustion products enter the room directly. Testing includes normal operation, tilted-burner conditions and fuel-related fault scenarios.
Testing Items
Depending on the appliance category and the applicable standard, a CO emission test program typically covers the following items:
- CO concentration in dry flue gas at nominal heat input, corrected to the reference oxygen level defined by the standard, commonly 13% O2 for solid fuel local space heaters.
- Air-free CO concentration (values extrapolated to 0% excess oxygen) for gas-fired appliances, reflecting intrinsic burner quality independently of dilution air.
- As-measured CO alongside measured O2 and CO2, enabling verification of dilution, air ratio and combustion completeness.
- CO/CO2 ratio as an indicator of combustion quality during steady state and transient phases.
- CO at reduced heat output settings, at maximum and minimum gas supply pressures, and at the extremes of the adjustable air settings.
- CO under abnormal and fault conditions, such as partially or fully blocked flues, downdraft simulation, combustion fan failure, restricted air inlets, flame deflection and tilted appliance positions.
- Room air CO concentration for flueless appliances, monitored at defined sampling heights in a sealed test chamber over the operating period specified by the standard.
- Ignition and shutdown transient CO peaks, which for intermittent appliances can dominate total emissions in real use.
- CO emission factors expressed in mg/Nm3, g/h or g/kg of fuel where required by energy efficiency and ecodesign regulations, such as EU Regulation 2015/1185 for solid fuel local space heaters.
Testing Methods
Instrumentation and Calibration
Flue gas CO is measured with nondispersive infrared (NDIR) analyzers or calibrated electrochemical sensors, paired with paramagnetic or zirconia oxygen analyzers and NDIR CO2 measurement. Heated sample lines, condensate traps or gas dryers are used so that results are reported on a dry, water-vapor-free basis. All analyzers are calibrated immediately before testing with certified zero and span gases traceable to national metrology institutes, with linearity checks across the measurement range and documented response times to capture transients accurately.
Sampling and Test Setup
The appliance is installed in a test room following the manufacturer's instructions, connected to the specified flue or chimney configuration and loaded with the standard test fuel or reference gas. Flue gas is sampled at the position defined by the standard, typically at least two flue diameters downstream of the appliance outlet and well upstream of any dilution entry. For flueless appliances, multiple room sampling points are positioned at representative breathing-zone heights within a chamber of defined volume and air exchange rate.
Test Conditions and Sequences
Testing normally begins with a heat input verification at nominal conditions, followed by steady-state CO measurement averaged over the period specified by the standard. Additional sequences cover minimum and maximum settings, repeated ignition and refueling cycles for solid fuel appliances, and the prescribed abnormal-condition clauses that deliberately stress combustion stability. Gas appliances are tested with the reference test gas category declared by the manufacturer, at nominal pressure and at the declared extreme supply pressures.
Data Reduction and Corrections
Measured CO values are converted between ppm (v/v) and mg/m3 using the relation of approximately 1.25 mg/m3 per ppm at 0 degrees C and 101.3 kPa. Results are corrected to the reference oxygen concentration using the formula CO(ref) = CO(measured) multiplied by (20.9 minus O2(ref)) divided by (20.9 minus O2(measured)), and air-free values are obtained by extrapolating to 0% O2. Typical acceptance levels are on the order of 0.10% (1000 ppm) dry flue gas at 13% O2 for many EN-classified solid fuel local space heaters, with tighter limits, often in the range of 0.02% to 0.10% air-free, applying to gas-fired appliances and considerably stricter constraints applying to flueless types. All tests are performed under an ISO/IEC 17025 quality management framework, with documented traceability, environmental records and measurement uncertainty budgets.
Applications
CO emission concentration testing serves multiple purposes across the product life cycle, from initial design through market surveillance:
- Type certification and regulatory market access, including CE marking under the Gas Appliances Regulation (EU) 2016/426, ecodesign conformity, GB market acceptance and North American product listing.
- Research and development support for combustion optimization, including burner geometry, air-fuel ratio tuning, secondary and tertiary air design, baffle configuration and catalytic retrofit evaluation.
- Production quality assurance, routine factory checks and follow-up surveillance of certified designs.
- Comparative benchmarking and verification of declared emission values for energy labeling and public procurement tenders.
- Forensic and incident investigation following suspected CO poisoning events, reproducing the operating conditions that led to the accumulation.
- Supporting documentation for insurance assessments, installation approvals and recalls.
FAQ
What CO concentration is considered acceptable for a heating stove?
Acceptance levels depend on the appliance type and standard. Many EN-classified solid fuel local space heaters are limited to approximately 0.10% (1000 ppm) CO in dry flue gas at 13% O2, while gas-fired appliances are usually limited to air-free values between roughly 0.02% and 0.10%. Flueless appliances face the strictest requirements because their products discharge directly into the living space.
What is the difference between as-measured and air-free CO?
As-measured CO is the raw concentration in the diluted flue gas stream, whereas air-free CO is mathematically corrected to remove the diluting effect of excess air by extrapolating to 0% oxygen. Air-free values describe the intrinsic quality of the combustion process independently of how much secondary air the appliance draws in.
Why are CO results corrected to a reference oxygen level such as 13% O2?
Different appliances and operating settings introduce different amounts of dilution air, which would otherwise make CO readings non-comparable. Correcting all results to a common reference oxygen concentration normalizes the data so that emissions from different products can be judged against a single, fair benchmark.
How is room CO tested for flueless heaters?
The appliance is operated in a sealed test chamber of defined volume and ventilation rate, with gas analyzers sampling the room air at specified heights and locations over the full operating period required by the standard. The appliance passes only if the room CO concentration remains below the ceiling defined by the standard for the entire test.
Does a clean blue flame guarantee safe CO levels?
Not by itself. A blue flame under ideal conditions indicates good combustion, but faults such as blocked flues, restricted air supply, fan failure or flame impingement can cause CO to spike even in a well-designed gas appliance. This is why standards mandate abnormal-condition testing in addition to normal-operation measurements.
How often should heating stoves be retested for CO emissions?
Type tests are performed at design certification, but retesting is required after significant design changes, fuel changes or control modifications, and during periodic factory surveillance. Market surveillance authorities may also sample and test certified products already in circulation.
Can manufacturers perform CO testing in their own facilities?
In-house testing is valuable for development screening, but regulatory certification generally requires testing by accredited third-party laboratories operating under ISO/IEC 17025 with traceable calibration gases, calibrated instrumentation and witnessed test conditions. Accredited laboratory reports are the recognized evidence for market access and compliance declarations.