Combustion Theory Reference

Why this matters

Combustion is fuel + oxygen producing heat, water vapor, carbon dioxide - and, when something goes wrong, carbon monoxide. Understanding the stoichiometry, the products, and what each reading on a combustion analyzer means is what separates a tune-up that improves safety from one that creates a CO incident. Every gas furnace, boiler, water heater, range, and pool heater service involves combustion.

The basic equation

Stoichiometric (perfect) combustion of natural gas (methane):

CH₄ + 2 O₂ → CO₂ + 2 H₂O + heat

In real life, fuel never combusts perfectly. Real combustion always has:

• Some excess oxygen (we add more air than theoretical to ensure complete combustion)
• Some incomplete combustion (CO formation, even in good conditions)
• Water vapor in the products
• Nitrogen from the combustion air (passes through unreacted, 78% of air is N₂)

Air-fuel ratio

For natural gas, stoichiometric air requirement is approximately:

• 10 cubic feet of air per 1 cubic foot of natural gas
• By mass: 17.2 lb of air per 1 lb of methane

Excess air: real combustion uses 20-50% excess air over stoichiometric to ensure complete combustion.

• "20% excess" = 1.20 × stoichiometric airflow
• Typical residential gas appliances: 30-50% excess
• Higher = cooler exhaust (less efficient)
• Lower = risk of incomplete combustion (CO formation)

Air-fuel ratio measurement: an O₂ reading in the flue tells you excess air.

• 0% O₂ = stoichiometric (theoretical perfect, never achieved)
• 4-8% O₂ = typical residential gas appliance, good combustion
• 8-12% O₂ = high excess air, lower efficiency
• 2-4% O₂ = approaching stoichiometric, CO risk grows

The flue gas analyzer - what the readings mean

A combustion analyzer (Testo 320, Fieldpiece SOX3, Bacharach Insight Plus) measures multiple gases simultaneously in the flue. The key readings:

Oxygen (O₂) %:

• Range: 0-21% (21% = ambient air, no combustion)
• Typical natural-gas appliance: 4-9% O₂
• Higher → more excess air, cooler exhaust
• Lower → less excess air, hotter exhaust, CO risk

Carbon Monoxide (CO) ppm:

• The critical safety reading
• Healthy combustion: <100 ppm "air-free" CO (corrected to 0% O₂)
• Marginal: 100-400 ppm air-free
• Unsafe: >400 ppm air-free
• Air-free correction adjusts measured CO upward to account for dilution by excess air; it's the meaningful number for safety

Carbon Dioxide (CO₂) %:

• Natural gas stoichiometric maximum: ~11.7% CO₂
• Typical: 6-10% CO₂
• Inversely related to O₂

Stack temperature:

• The exhaust temperature
• Typical 350-500 °F for 80% AFUE furnaces
• Typical 100-180 °F for 90%+ condensing
• Higher = more heat lost up the flue (lower efficiency)
• Used to compute combustion efficiency

Combustion efficiency:

• Calculated from O₂, stack temperature, and CO₂
• Tells you what % of fuel's energy is going to useful heat vs up the flue
• Tune-up goal: high efficiency without excessive CO

What different readings tell you

O₂ high (12%+), CO normal:

• Too much excess air
• Causes: incorrect manifold pressure, leaky combustion chamber, oversized burner
• Action: adjust primary air shutter (older), check gas valve outlet pressure

O₂ normal (4-9%), CO high (>400 ppm air-free):

• Incomplete combustion despite adequate air
• Causes: dirty burner, blocked secondary air, misaligned flame, cracked heat exchanger (CO from chamber bypassing)
• Action: clean burner, verify air paths, inspect heat exchanger

O₂ low (<3%), CO high:

• Inadequate combustion air
• Causes: combustion air supply blocked, draft inadequate
• Action: verify combustion air requirements (NFPA 54 / IFGC), check vent
• DANGEROUS: continue carefully or shut down

O₂ low (<3%), CO normal:

• Approaching stoichiometric without CO yet
• Combustion is hot but on edge - could spike CO with any disturbance
• Action: increase combustion air, recheck

Stack temperature high (>600 °F for 80% AFUE):

• Heat exchanger dirty (heat not transferring well to room air)
• Excess air too high (dilution cooling vs lost heat)
• Burner over-fired (more gas than rated)
• Action: clean heat exchanger, adjust air, verify gas pressure

Stack temperature low (<150 °F for 80% AFUE):

• Under-fired (less gas than rated)
• Air-fuel ratio off
• Possibly extracting too much heat (low gas pressure)
• Action: verify gas pressure

Manifold pressure

The gas pressure at the burner manifold determines firing rate.

Natural gas:

• Standard: 3.5" w.c. manifold pressure (varies slightly by appliance)
• High-fire on two-stage: 3.5" w.c.
• Low-fire on two-stage: 1.5-1.7" w.c.

Propane (LP):

• Standard: 10-11" w.c. manifold pressure

Measurement: digital manometer connected at the manifold pressure tap (usually a port on the gas valve outlet side).

Adjustment: gas valve regulator screw under the cap. Turn clockwise = increase pressure; counterclockwise = decrease. Small adjustments only (1/8 turn at a time).

Products of combustion

Water vapor (H₂O):

• Natural gas combustion produces ~2 lb H₂O per 1 lb fuel
• Visible as steam from flue in cold weather
• Condensing furnaces (90+ AFUE) intentionally condense this vapor to extract latent heat - produces acidic condensate (drained)
• Must be drained properly; otherwise erosion of heat exchanger and venting

Carbon dioxide (CO₂):

• Normal product of complete combustion
• Greenhouse gas but not toxic at flue concentrations
• Higher CO₂ = more complete combustion (good in residential terms)

Carbon monoxide (CO):

• Product of INCOMPLETE combustion
• ODORLESS, COLORLESS, lethal at low concentrations (<400 ppm in living space → death over hours)
• Always measure CO; never just assume the flame "looks good"
• Mechanical / chemical risk in any combustion appliance

Nitrogen oxides (NOx):

• Formed when N₂ in air reacts with O₂ at high temperatures
• Smog precursor
• Modern low-NOx burners minimize this

Soot:

• Solid carbon from extreme incomplete combustion
• Visible as black deposits
• Indicates serious problem; usually a fuel-rich condition

Combustion air requirements

NFPA 54 / IFGC require adequate combustion air for any indoor fuel-burning appliance. Standards depend on the appliance size and the room volume.

General rule: 50 ft³ of free space per 1000 BTU/hr of input rating.

A 100,000 BTU/hr furnace requires 5,000 ft³ of free space - about a 25 ft × 25 ft × 8 ft room. If the mechanical room is smaller, combustion air must be supplied via ducts to outdoors or to a larger interior space.

Direct-vent appliances: sealed combustion; air comes directly from outside through one of the concentric vent pipes. Combustion air room requirements don't apply. Becoming the modern standard.

References

• NFPA 54 / National Fuel Gas Code
• ANSI Z21.47 (gas-fired central furnaces)
• ANSI Z223.1 (general gas appliance safety)
• ASHRAE Handbook - HVAC Systems and Equipment
• AGA / GAMA combustion testing standards
• Manufacturer service manuals (Carrier, Trane, Lennox, Goodman, Rheem, Bradford White, AO Smith)
• Bacharach / Testo / Fieldpiece combustion analyzer documentation