Boiler or Furnace Output from Input

Every fuel-fired appliance carries at least two capacities on its plate, and confusing them is a rite of passage. Input is the fuel burned — 100,000 BTU/hr of gas — while output is what actually reaches the water or the ductwork. An 80 % unit delivers 80,000 BTU/hr; the other 20,000 goes up the flue and out the jacket. A condensing boiler pushing 95 % delivers 95,000 from the same burner, which is why replacing a 1980s atmospheric boiler often lets you drop a size and still overheat the house.

Know which efficiency you are quoting. Combustion efficiency is a steady-state flue measurement; AFUE is a seasonal average that penalises cycling, jacket loss and off-cycle draft, and it always reads lower — a boiler tuned to 86 % combustion might carry an 84 % AFUE label. Some manufacturers publish a third figure, net IBR output, which is output derated a further 15 % for piping and pickup loss. The trap in retrofits is sizing on input: swap a 100,000 BTU/hr input atmospheric for a 100,000 input condensing and you have quietly added 15 % capacity to a house that was already oversized. Run this page in reverse when commissioning — measure output with the water-side ΔT and flow, divide by the metered gas input, and you have the real efficiency rather than the brochure's.