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: the theoretical exact air for complete combustion. For natural gas that is about 17:1 by mass, or about 9.5 volumes of air per volume of gas. (The 14.7:1 figure people reach for is gasoline's, out of engine work. It does not belong on a gas appliance.)

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 (direct vent): combustion air is ducted from outdoors to a sealed burner compartment, so the appliance takes no air from the room and no room-air opening is required for it.

Do not equate that with Category I. The vent categories describe the FLUE, not where the air came from. Category I is negative vent pressure and non-condensing, which is the ordinary atmospheric or induced-draft appliance on a B-vent. Category II is negative pressure and condensing, Category III positive pressure and non-condensing, Category IV positive pressure and condensing, which is where most sealed-combustion condensing furnaces and boilers on plastic vent actually sit. The category drives what vent material and joint you are allowed to use, so read it off the rating plate rather than inferring it from the burner.

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). State the basis with the number, because the volume figure and the mass figure for the same fuel are nowhere near each other:

• Natural gas: about 9.5 volumes of air per volume of gas; about 17:1 by mass
• Propane: about 24 volumes of air per volume of gas; about 15.7:1 by mass
• #2 oil: about 14:1 by mass. Oil is a liquid, so there is no useful volumetric air-to-fuel ratio for it

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