Combustion Safety Testing
Comprehensive combustion safety testing procedures including CO measurement in flue gas, ambient CO monitoring, draft testing, spillage assessment, and worst-case depressurization protocols.
- Measure and interpret CO levels in flue gas using a combustion analyzer
- Perform draft measurements and determine if natural draft is adequate
- Conduct spillage testing under worst-case depressurization conditions
- Apply BPI action levels for CO in ambient air and flue gas
Lección 1
CO in Flue Gas & Combustion Analysis
The Purpose of Combustion Safety Testing
Combustion safety testing is the single most important safety procedure performed by BPI Heating Professionals. Approximately 430 people die from unintentional carbon monoxide poisoning in the United States each year, and thousands more are hospitalized. Many of these incidents involve malfunctioning heating equipment. BPI requires combustion safety testing on every heating assessment, and testing must be performed before and after any work that could affect combustion appliance operation.
Combustion Analyzer Measurements
A combustion analyzer measures four key parameters when its probe is inserted into the flue gas stream:
Flue gas temperature - The temperature of combustion gases in the vent connector. Higher temperatures mean more heat is being lost up the chimney (lower efficiency). Normal range: 300-500 F for standard efficiency, 100-150 F for condensing.
Oxygen (O2) - The percentage of oxygen remaining in the flue gas after combustion. A small amount of excess air is normal and necessary for safe combustion. Normal range: 4-9% O2.
Carbon dioxide (CO2) - The percentage of CO2 in the flue gas, indicating combustion completeness. Higher CO2 (with normal O2) means more complete combustion. Normal range: 6-10% CO2 for natural gas.
Carbon monoxide (CO) - The concentration of CO in the flue gas, measured in parts per million (ppm). CO is a product of incomplete combustion and indicates a problem when elevated.
CO Air-Free Measurement
CO readings from a combustion analyzer are typically reported in two ways:
CO as-measured - The actual CO concentration in the flue gas sample, including dilution air. This reading can be misleadingly low if excess air dilutes the sample.
CO air-free - The CO concentration corrected to remove the dilution effect of excess air. This is the standard BPI measurement because it reflects the true combustion quality regardless of draft conditions.
CO air-free formula:
COAFppm = (20.9 / (20.9 - O2%)) x COppm
Where O2% is the percentage of oxygen in the flue gas sample. This formula normalizes CO to a theoretical zero-excess-air condition. For example, if the analyzer reads 50 ppm CO as-measured with 7% O2: COAF = (20.9 / (20.9 - 7)) x 50 = (20.9 / 13.9) x 50 = 75 ppm air-free.
An older formula using CO2 was common but the O2-based formula above is the current BPI standard because O2 is measured more accurately by electrochemical sensors than CO2.
BPI action levels for CO air-free in flue gas (ANSI/BPI-1200-S, at 5 min main burner):
| CO Air-Free (ppm) | Action Required |
|---|---|
| 0-25 | Normal combustion |
| 26-200 | Investigate cause; may exceed limits for some appliances |
| Above 200 | Exceeds limit for space heaters and water heaters - shut down |
| Above 400 | Exceeds limit for furnaces and boilers - shut down |
See module 04 (Carbon Monoxide Testing) for full appliance-specific limits including cooking appliances.
Common Causes of High CO
Incomplete combustion from:
- Insufficient primary air to the burner (blocked air openings, dirty burner)
- Flame impingement on a cold surface (cracked heat exchanger, misaligned burner)
- Overfiring (gas pressure too high, wrong orifice size)
- Sooting (oil burners - dirty nozzle, wrong spray angle)
CO Above Limit = Shut Down
If CO air-free exceeds the ANSI/BPI-1200-S limit for the specific appliance type (200 ppm for space heaters and water heaters; 400 ppm for furnaces and boilers), the appliance must be shut down immediately. Inform the homeowner in writing that the appliance is producing unsafe levels of CO and must not be operated until repaired by a qualified technician. Never leave a high-CO appliance running.
Combustion analysis measures flue gas temperature, O2, CO2, and CO to evaluate combustion safety and efficiency. CO must be reported as "air-free" using the formula COAFppm = (20.9 / (20.9 - O2%)) x COppm. Per ANSI/BPI-1200-S, CO air-free limits at 5 minutes vary by appliance: 200 ppm for space heaters and water heaters, 400 ppm for furnaces and boilers. Above the applicable limit, the appliance must be shut down immediately. Common causes of high CO include insufficient combustion air, burner misalignment, and overfiring.