Gas Heating & Combustion Formulas

This section provides essential formulas for calculating gas heating performance, efficiency, combustion characteristics, and pressure conversions. Ideal for HVAC technicians, engineers, and combustion system professionals.

Heat Transfer & Airflow Calculations

• Gas Heat CFM Calculation:
  CFM = (Heater Input BTU × Heat Exchanger Efficiency) ÷ (1.08 × Temperature Rise °F)

  Note: If the blower motor is in the airstream, add its BTU output to the heater output.

• Heat Exchanger Output:
  Output BTU = Input BTU × Heat Exchanger Efficiency

Combustion Timing & Efficiency

• Time Rating (sec/ft³):
  Time = Correction Factor × 3600 × (BTU/ft³ of Corrected Gas) ÷ Heat Exchanger BTU Input

• Heat Exchanger Efficiency:
  Efficiency (%) = 100% - Flue Loss (%)

• Flue Loss:

  Determined by comparing the CO₂ reading and the flue temperature (refer to ANSI Standard Z21.47)

Gas Content & Correction Factors

• Corrected Gas Content:
  Corrected = Actual Gas Content × Gas Content Correction Factor

• Gas Content Correction Factors:

  • From wet to dry: Wet Gas BTU/ft³ × 1.0177

  • From dry to wet: Dry Gas BTU/ft³ × 0.9826

Correction Factor (Temp & Pressure)

• Correction Factor Formula:
  CF = (Corrected Barometric Pressure × 520) ÷ [(Gas Temp + 460) × 30]

• Corrected Barometric Pressure:
  = (Barometer – Temp Correction) + Gas Pressure in Inches of Mercury (Hg")

Pressure Conversion

• Gas Pressure to Inches of Mercury (Hg"):
  Inches of Water × 0.07355

  Examples:
  7" W.C. × 0.07355 = 0.52" Hg (Natural Gas)
11" W.C. × 0.07355 = 0.81" Hg (LPG Normal Fire)
13" W.C. × 0.07355 = 0.96" Hg (LPG High Fire)

Overfire Rates

• Overfire Rate Increase:
  = Heat Exchanger Input × 1.12 (NG)
= Heat Exchanger Input × 1.09 (LPG)

  Corresponds to a 1.25% gas pressure increase.

CO & CO₂ Combustion Analysis

• Air-Free CO Calculation:
  Air-Free CO = (Ultimate CO₂ ÷ Actual CO₂) × Actual CO (PPM)

• Convert PPM to % CO:
  PPM ÷ 10,000

  Or move decimal 5 places to the left.

Ultimate CO₂ Calculation

• For Natural Gas:
  Ultimate CO₂ = [ (Actual Gas Content – 970) × 0.003846 ] + 11.7

• For LPG:
  Ultimate CO₂ = [ (Actual Gas Content – 2466) × 0.001184 ] + 13.73

Summary Table (Key Gas Formula Constants)