How to convert fuel use into heat output
Knowing the fuel and the hourly burn rate is enough to estimate what a combustion system actually delivers. It is the calculation behind checking whether a boiler is correctly sized, comparing fuels on an equal footing, and filling in the energy figures on a utility or compliance report.
The structure is simple: total heat input = heating value × fuel use, and multiplying by combustion efficiency gives the useful output that reaches water or process. One therm is 100,000 BTU, and one boiler horsepower is defined as 33,475 BTU/hr of output.
Burning 10,000 ft³/hr of natural gas at 85% efficiency gives 10.3 MMBTU/hr of input, 103 therms/hr, about 8.76 MMBTU/hr of useful output and roughly 262 boiler horsepower. Swap in No. 2 fuel oil and the same duty needs about 74 gal/hr, which is the kind of comparison that drives fuel switching studies.
Listed heating values are representative HHV figures in line with EIA published values, current as of 2026. Delivered gas heat content and oil grades vary, so rerun the numbers with your supplier's certificate for billing work, and use a measured efficiency rather than the nameplate figure.
Frequently asked questions
The listed figures are higher heating values (HHV), which include the latent heat released when flue gas water vapor condenses. US fuel billing and EIA statistics use HHV, but unless the appliance is condensing, the heat you actually recover is closer to the lower heating value.
One boiler horsepower is defined as 33,475 BTU/hr of output, the heat needed to evaporate 34.5 lb of water per hour from and at 212°F. It lets you compare boilers running at different pressures on one scale.