Boiler and Burner Formula Cheat Sheet
The conversions and rules of thumb that come up most in boiler rooms, with a worked example for each. Fuel heat contents vary by supplier, so use your own bill's numbers when you have them.
Boiler output
1 boiler horsepower (BHP) = 33,475 Btu/hr
1 BHP = 34.5 lb/hr of steam from and at 212°F
1 BHP ≈ 9.81 kW
MBH = 1,000 Btu/hr. Example: 300 BHP × 33,475 = 10,042,500 Btu/hr, about 10,043 MBH. For sizing a boiler from the load, see How to Size a Steam Boiler.
Fuel input
Input = output ÷ efficiency. Example: 10,042,500 Btu/hr ÷ 0.82 ≈ 12.25 million Btu/hr.
Natural gas: about 1,036 Btu per cubic foot. Gas flow (cfh) = input (Btu/hr) ÷ Btu per cubic foot. Example: 12,250,000 ÷ 1,036 ≈ 11,824 cfh.
Clocking a gas meter: cfh = cubic feet measured × 3,600 ÷ seconds timed.
No. 2 oil: about 138,500 Btu per gallon. Input (Btu/hr) = gallons per hour × 138,500.
Propane: about 91,452 Btu per gallon.
1 therm = 100,000 Btu. 1 MMBtu = 1,000,000 Btu = 10 therms.
Fuel cost per MMBtu = price per unit ÷ MMBtu per unit. Example: gas at $12 per Mcf ÷ 1.036 MMBtu per Mcf ≈ $11.58 per MMBtu.
Combustion
Excess air (%) ≈ O2 ÷ (20.9 − O2) × 100, with O2 measured on a dry basis. Example: 3% O2 → 3 ÷ 17.9 × 100 ≈ 17% excess air. 6% O2 ≈ 40%.
U.S. Department of Energy rule of thumb: about 1% efficiency gain for every 15% reduction in excess air. Example: 40% down to 17% ≈ 1.5 points.
Turndown ratio = maximum firing rate ÷ minimum firing rate.
Hot water
Btu/hr = 500 × gpm × temperature rise (°F). The 500 comes from 8.33 lb per gallon × 60 minutes. Example: 100 gpm with a 20°F rise = 1,000,000 Btu/hr.
Combustion air
Common rule of thumb: 10 cfm per BHP, about 8 for combustion and 2 for ventilation. More in Boiler Room Combustion Air.
Pressure and temperature
1 psi ≈ 27.7 inches of water column (in. w.c.); 1 in. w.c. ≈ 0.036 psi.
°F = °C × 1.8 + 32; °C = (°F − 32) ÷ 1.8.
Formulas get you close; an analyzer on the stack gives the real numbers. Request a combustion check or email service@combustionspecialists.com.
