A furnace’s input rating is not its heat output

Furnaces are sold by INPUT, but only the output heats your home: output = input × AFUE. A 100,000 BTU/h, 95% AFUE furnace delivers 95,000 BTU/h. Size by output.

A flow diagram of a gas furnace. A 100,000 BTU/h input of fuel enters from the left; the furnace box is labeled 95% AFUE; 95,000 BTU/h of useful heat leaves to the right as the output, and 5,000 BTU/h goes up the flue. Output equals input times AFUE, so the input rating on the box is not the heat delivered — a furnace must be sized by its output. — Source: AHRI / DOE AFUE

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A flow diagram of a gas furnace. A 100,000 BTU/h input of fuel enters from the left; the furnace box is labeled 95% AFUE; 95,000 BTU/h of useful heat leaves to the right as the output, and 5,000 BTU/h goes up the flue. Output equals input times AFUE, so the input rating on the box is not the heat delivered — a furnace must be sized by its output.

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    <img src="https://remodelcalculators.com/api/figure-embed/furnace-input-vs-output" alt="A flow diagram of a gas furnace. A 100,000 BTU/h input of fuel enters from the left; the furnace box is labeled 95% AFUE; 95,000 BTU/h of useful heat leaves to the right as the output, and 5,000 BTU/h goes up the flue. Output equals input times AFUE, so the input rating on the box is not the heat delivered — a furnace must be sized by its output." width="800" height="440" style="width:100%;height:auto;border:0" loading="lazy">
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    Diagram: <a href="https://remodelcalculators.com/construction-diagrams/furnace-input-vs-output" target="_blank" rel="noopener">A furnace’s input rating is not its heat output</a> — courtesy of <a href="https://remodelcalculators.com" target="_blank" rel="noopener">RemodelCalculators.com</a>
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