Biomass Boiler Calculator

Fuel Input Rating divides Heating Load by Boiler Efficiency, so at the default 80% efficiency a 500,000 BTU/hr heating load requires 625,000 BTU/hr of fuel energy input -- the extra 125,000 BTU/hr is lost to combustion inefficiency (flue gas heat loss, incomplete combustion, and similar losses) rather than reaching the building. The boiler's rated heat OUTPUT capacity should still match your Heating Load directly; Fuel Input Rating is the fuel-side figure, not the output-side one.

Pellets carry roughly 7,750 BTU per pound in this calculator, noticeably more than wood chips' roughly 5,200 BTU per pound, because pellets are dried down to about 6% moisture while chips are typically left around 35% moisture. Water in the fuel has to be boiled off before the remaining wood can release its heat, so wetter fuel delivers less usable energy per pound and Fuel Consumption rises to compensate.

It's a real coincidence in the constants, not a display bug -- Annual Fuel multiplies Fuel Consumption by 2,000 assumed heating hours per year, then divides by 2,000 pounds per ton, and those two 2,000s cancel out exactly. The units are genuinely different (pounds per hour versus tons per year), they just happen to land on the same numeric value given this calculator's fixed 2,000-hour heating season assumption.

It's a rough Northern-US average, not a figure calculated from your actual climate or building. A milder climate or a shorter heating season will use meaningfully fewer than 2,000 hours, while a harsher climate or continuous process heat load could exceed it -- treat Annual Fuel as a planning-level estimate and adjust it using your own degree-day data or historical fuel bills for a real budget.

Formula-wise, they're tied -- Fuel Input Rating is Heating Load divided by Boiler Efficiency, and each of those two inputs pulls its own equal-strength weight on the ratio: nudge Heating Load up 10% and Fuel Input Rating climbs by roughly 10%; nudge Boiler Efficiency up 10% of its own value and Fuel Input Rating drops by a comparable share. Heating Load's bigger real-world swings come from its much wider declared range (10,000 to 50,000,000 BTU/hr, versus Boiler Efficiency's comparatively narrow 50% to 95% band), not from the formula weighting it more -- a wider allowed range, not a different sensitivity, is why changing Heating Load typically moves the result further in absolute terms.