Combustion Fundamentals Reference

Why this matters

Combustion appliances (furnaces, boilers, water heaters) burn fuel + need proper air + ignition + venting. Bad combustion = wasted fuel + carbon monoxide + equipment damage. The tech who measures + tunes combustion delivers safe + efficient operation. This is the field card.

Combustion basics

Fuel + oxygen + ignition + heat = combustion. Remove any one = no combustion.

Complete combustion: fuel + O2 → CO2 + H2O + heat. Clean.

Incomplete combustion: insufficient O2 OR temp → CO + soot + unburned fuel. Dangerous.

Stoichiometric air: theoretical exact air for complete combustion (14.7:1 air-to-fuel for natural gas by mass).

Excess air: extra air beyond stoichiometric; ensures complete combustion but reduces efficiency.

Fuel types + heat values

Natural gas (CH4): 1,000 BTU/cu ft; 21,500 BTU/lb; stoichiometric 9.5 ft³ air per ft³ fuel. Propane (C3H8): 2,500 BTU/cu ft; 21,500 BTU/lb; stoichiometric 24 ft³ air per ft³ fuel. Heating oil #2: 138,500 BTU/gal; 19,700 BTU/lb. Wood: hardwood (oak, maple) 22 - 25M BTU per cord; softwood (pine) 14 - 18M BTU per cord. Coal: anthracite 13,000 BTU/lb (legacy); bituminous 11,000 BTU/lb. Pellets: 8,000 BTU/lb.

Combustion air requirements (NFPA 54)

Natural-draft appliance (atmospheric): 1 sq inch of air per 1,000 BTU input.

Sealed combustion / Category I: combustion air drawn from outside.

Combustion air location: 1 high + 1 low (typical); allows convection.

Confined space (< 50 cu ft per 1,000 BTU): outside air required.

Unconfined space: indoor combustion air typically OK.

Burner types + ignition systems

Burner types: atmospheric (natural draft; gas+air mix at orifice; rises through HX; older units); power burner (forced fan combustion air; higher firing; boilers + commercial); inshot tube-style (most modern furnaces); sealed combustion (air + exhaust sealed from house via PVC); modulating (varies firing to load).

Ignition systems: standing pilot (continuously burning; inefficient); intermittent pilot (IPI; pilot lit on call; safer + more efficient); direct spark ignition (DSI; spark directly ignites burner); hot surface ignition (HSI; silicon nitride/carbide igniter glows; common modern furnaces); electronic ignition module controls sparking + timing + safety.

Flame sensor

Rod in flame stream: completes electrical circuit when flame present.

Rectification principle: AC voltage applied; flame rectifies to small DC current.

Common cause of "won't stay lit": dirty flame rod; clean with steel wool OR fine sandpaper.

Microamp meter reading: typical 2 - 8 μA when sensing good flame.

Combustion air ratio

Stoichiometric air-to-fuel ratio (AFR):

• Natural gas: 15:1 by volume
• Propane: 25:1 by volume
• Oil: 14:1 by volume

Lean combustion: excess air; cooler flame + complete combustion + lower efficiency.

Rich combustion: insufficient air; incomplete + high CO + soot.

Industry target: 20 - 50% excess air for safe + efficient operation.

Excess air determination

Measure O2 + CO2 in flue gas: combustion analyzer.

O2 reading:

• Natural gas: 3 - 9% O2 = 15 - 65% excess air
• Propane: 4 - 9% O2 = 20 - 60% excess air
• Oil: 4 - 8% O2 = 20 - 50% excess air

CO2 reading (calculated from O2 OR direct):

• Natural gas: max ~ 12% CO2 (stoichiometric); target 8 - 10%
• Propane: max ~ 14% CO2; target 10 - 12%
• Oil: max ~ 15% CO2; target 10 - 12%

Higher CO2 = lower excess air = more efficient.

Combustion analysis (commissioning + tune-up)

Use combustion analyzer in flue: target steady-state operation (5 min after startup).

Measure: O2, CO2, CO, stack temp, draft, efficiency.

Adjust for:

• O2: 6 - 10% atmospheric; 4 - 8% condensing
• CO: < 100 ppm air-free atmospheric; < 50 ppm condensing
• Stack temp: 350 - 500°F atmospheric; 100 - 150°F condensing
• Draft: -0.02" WC atmospheric; positive condensing

Adjustments: gas valve manifold pressure (3.5" WC natural gas; 11" WC propane); orifice size; combustion air supply.

Heat exchanger types

Atmospheric (clamshell): open burner under heat exchanger; products vent up B-vent.

Cast iron heat exchanger: long-life; commercial boiler.

Aluminized steel heat exchanger: standard residential furnace.

Stainless steel heat exchanger: condensing furnace; higher temp tolerance.

Modular heat exchanger: serpentine path; common condensing.

Condensing combustion

Extracts latent heat from water vapor in combustion products.

Stack temp very low (100 - 150°F).

Water condensate: drained.

Plastic vent (PVC OR CPVC): low temp allows.

Efficiency: 90%+ AFUE.

Sealed combustion: typical.

Drains: clear annually (mold + algae buildup).

Common combustion problems

Yellow flame (was blue): incomplete combustion; insufficient air. Check: blocked combustion air, dirty burner, gas pressure too high, low primary air.

Soot buildup: incomplete combustion ongoing; clean heat exchanger + diagnose root cause.

High CO: gas valve too high; burner misalignment; cracked heat exchanger.

Won't ignite: gas supply, ignition module, igniter, flame sensor, control board.

Ignites then shuts off: flame sensor dirty OR rectification failing.

Flame rollout: combustion products spilling out front of furnace; flue blocked OR draft insufficient.

Pilot OR ignition diagnostics

Standing pilot won't light: gas supply, thermocouple, manual valve.

Standing pilot lights but won't stay: thermocouple (replace).

Intermittent pilot won't fire: gas valve, igniter, flame sensor, control board.

Hot-surface igniter (HSI) tests: 40 - 75 ohms cold; glows at full power.

Spark igniter: visible spark across electrodes; replace if cracked OR weak.

Combustion safety devices

High-limit switch: shuts off burner if temp too high (overheat).

Roll-out switch: shuts off if flame rolls out (flue blocked).

Pressure switch: ensures inducer fan running before burner ignites (sealed combustion).

Spill switch: water heater; senses backdraft.

T+P (water heater): over-temp OR over-pressure relief.

Flame sensor: confirms flame present; safety shutoff if not.

CO concerns

Normal operation: < 50 - 100 ppm air-free CO; cleanly venting.

Trouble: > 200 ppm in flue; CO in living space; backdrafting.

Causes of CO in home: blocked flue, cracked heat exchanger, depressurized house pulling flue gas, atmospheric vent inadequate.

CO detector required (code): outside sleeping area + near fuel appliances.

References

• NFPA 54 (National Fuel Gas Code)
• IFGC (International Fuel Gas Code)
• ASHRAE Fundamentals Handbook
• Combustion analyzer training (Testo, Bacharach)
• Manuall internal: Common Codes Overview, Common Gauges + Meters