HVAC Systems Encyclopedia

A comprehensive encyclopedia of heating, ventilation, and air conditioning systems

Combustion Fundamentals

Combustion fundamentals govern the design, operation, and efficiency of all fuel-fired HVAC heating equipment. Understanding these principles is essential for optimizing system performance, minimizing emissions, and ensuring safe operation.

Stoichiometric Combustion

Stoichiometric combustion represents the ideal chemical reaction where fuel and oxygen combine in exact proportions to achieve complete combustion with zero excess oxygen. For natural gas (approximated as methane, CH₄):

CH₄ + 2O₂ → CO₂ + 2H₂O

This reaction requires 2 moles of oxygen (or approximately 9.52 moles of air, considering air is 21% oxygen by volume) per mole of methane. The stoichiometric air-fuel ratio for methane is approximately 17.2:1 by mass.

For fuel oil (approximated as C₁₂H₂₆):

2C₁₂H₂₆ + 37O₂ → 24CO₂ + 26H₂O

The stoichiometric air-fuel ratio for No. 2 fuel oil is approximately 14.7:1 by mass.

Actual combustion systems never operate at stoichiometric conditions due to mixing limitations, flame stability requirements, and the need to ensure complete combustion.

Excess Air Requirements

Excess air is the quantity of air supplied beyond the stoichiometric requirement. It ensures complete combustion by compensating for imperfect fuel-air mixing and provides operational stability. Excess air is expressed as a percentage above the theoretical air requirement:

Percent Excess Air = [(Actual Air - Theoretical Air) / Theoretical Air] × 100

Typical excess air percentages for HVAC equipment:

  • Natural gas burners: 10-20% (residential), 5-15% (commercial/industrial)
  • Oil burners: 15-25% (residential), 10-20% (commercial)
  • Coal burners: 20-40%

Excessive air reduces efficiency by carrying sensible heat up the flue. Insufficient air causes incomplete combustion, producing carbon monoxide (CO) and soot while reducing efficiency.

Combustion Products

Complete combustion of hydrocarbon fuels produces primarily carbon dioxide (CO₂) and water vapor (H₂O). Actual combustion also generates nitrogen oxides (NOx), and incomplete combustion produces carbon monoxide (CO) and particulates.

Carbon Dioxide (CO₂): Principal combustion product. CO₂ concentration in flue gas indicates combustion completeness. Natural gas combustion at 10% excess air produces approximately 9-10% CO₂ in dry flue gas.

Water Vapor (H₂O): Forms from hydrogen in fuel and moisture in combustion air. Natural gas produces approximately 2 pounds of water per pound of fuel burned. This water vapor contains latent heat recoverable in condensing equipment.

Nitrogen Oxides (NOx): Pollutants formed through three mechanisms:

  1. Thermal NOx: Forms at flame temperatures above 2800°F (1540°C) through oxidation of atmospheric nitrogen. Increases exponentially with temperature.
  2. Prompt NOx: Forms in fuel-rich flame zones where hydrocarbon radicals react with nitrogen. Significant in gas burners.
  3. Fuel NOx: Forms from nitrogen compounds in fuel. Primary mechanism in coal and heavy oil combustion.

Carbon Monoxide (CO): Indicates incomplete combustion. Results from insufficient oxygen, poor mixing, or flame quenching. Safe levels below 400 ppm air-free. Concentrations above 1000 ppm indicate serious combustion problems requiring immediate correction.

Flame Temperature

Adiabatic flame temperature is the theoretical maximum temperature achieved during combustion with no heat loss. For natural gas:

  • Stoichiometric combustion: approximately 3500°F (1930°C)
  • With 15% excess air: approximately 3350°F (1843°C)

Actual flame temperatures are lower due to heat transfer, dissociation, and incomplete combustion. Flame temperature affects NOx formation, heat transfer characteristics, and equipment material selection.

Combustion Efficiency

Combustion efficiency represents the percentage of fuel energy converted to useful heat, accounting for stack losses and incomplete combustion. ASHRAE defines combustion efficiency as:

ηc = 100 - (Stack Loss + Incomplete Combustion Loss)

Stack Loss Calculation:

Stack loss is the sensible heat carried away by flue gases:

Stack Loss (%) = K × (Tstack - Tambient) / CO₂%

Where K is a fuel-dependent constant (approximately 0.39 for natural gas, 0.38 for No. 2 oil).

For example, with natural gas at 8% CO₂, 350°F stack temperature, 70°F ambient: Stack Loss = 0.39 × (350 - 70) / 8 = 13.6%

Incomplete Combustion Loss:

Calculated from CO concentration:

Incomplete Combustion Loss (%) ≈ 0.53 × CO (ppm) / CO₂%

Total combustion efficiency = 100 - 13.6 - incomplete combustion loss.

Flue Gas Analysis

Flue gas analysis measures combustion products to assess efficiency and safety. Key measurements:

Oxygen (O₂) Percentage: Indicates excess air level. Typical values: 3-6% for gas, 4-8% for oil.

Carbon Dioxide (CO₂) Percentage: Inversely related to excess air. Maximum CO₂ indicates near-stoichiometric combustion.

Relationship: For natural gas, %O₂ + %CO₂ ≈ 12% (dry basis, approximate)

Carbon Monoxide (CO): Safety and efficiency indicator. Should be below 100 ppm air-free for safe operation.

Flue Gas Temperature: Direct efficiency indicator. Each 40°F reduction in stack temperature above dew point improves efficiency approximately 1%.

Modern combustion analyzers measure O₂, CO, CO₂, and temperature simultaneously, calculating efficiency directly.

Air-Fuel Ratio

The air-fuel ratio (AFR) is the mass ratio of air to fuel. It directly controls combustion completeness, efficiency, and emissions.

AFR = Mass of Air / Mass of Fuel

Equivalence Ratio (φ): Ratio of actual AFR to stoichiometric AFR:

φ = AFRstoichiometric / AFRactual

  • φ = 1.0: Stoichiometric combustion
  • φ < 1.0: Fuel-lean (excess air)
  • φ > 1.0: Fuel-rich (insufficient air)

Optimum AFR balances complete combustion, efficiency, and low emissions. Typically maintained at φ = 0.85-0.95 (5-15% excess air) for gas-fired equipment.

Heating Values

Heating value is the energy released per unit mass or volume of fuel during complete combustion.

Higher Heating Value (HHV) / Gross Heating Value:

Includes latent heat of condensation of water vapor formed during combustion. Represents total energy available if all water vapor condenses to liquid at 77°F. Standard rating basis for US equipment.

Typical HHV values:

  • Natural gas: 1000-1050 Btu/ft³
  • No. 2 fuel oil: 140,000 Btu/gallon
  • Propane: 2500 Btu/ft³

Lower Heating Value (LHV) / Net Heating Value:

Excludes latent heat of water vapor, assuming it remains as vapor in flue gas. Used in European efficiency calculations and for non-condensing equipment where water vapor is not recovered.

Relationship: HHV - LHV = Latent heat of water vapor formed

For natural gas: HHV ≈ 1.11 × LHV (approximately 10% difference) For fuel oil: HHV ≈ 1.06 × LHV (approximately 6% difference)

Condensing equipment can approach HHV-based efficiency by recovering latent heat through flue gas condensation.

ASHRAE References

ASHRAE Handbook - Fundamentals provides comprehensive combustion data:

  • Chapter 19 (2021): Energy Resources - heating values, combustion constants
  • Chapter 29 (2021): Combustion and Fuels - detailed combustion calculations
  • Chapter 28 (2021): Air-Heating and Air-Cooling Equipment - efficiency definitions

ASHRAE Standard 103 defines combustion efficiency measurement methods for residential equipment.

Practical Implications

Understanding combustion fundamentals enables:

  1. Proper burner adjustment for maximum efficiency and minimum emissions
  2. Accurate efficiency calculations from flue gas measurements
  3. Diagnosis of combustion problems through flue gas analysis
  4. Selection of appropriate excess air levels for different fuels and equipment types
  5. Evaluation of condensing versus non-condensing equipment based on heating value recovery potential
  6. Compliance with emission regulations through NOx and CO control
  7. Safety assurance through CO monitoring and proper combustion air supply

Combustion optimization requires balancing competing objectives: maximum efficiency demands minimum excess air, while complete combustion and flame stability require adequate excess air. Low NOx operation requires reduced flame temperature, which may increase CO formation. Field adjustment must account for these tradeoffs while maintaining safe, efficient operation within manufacturer specifications and applicable codes.