AFUE — the share of furnace fuel that becomes heat

AFUE is the share of fuel that becomes heat: an 80% furnace loses 20% up the flue, a 98% condensing furnace only 2%. Higher AFUE lowers bills — size by output either way.

Three stacked bars for 80%, 95%, and 98% AFUE furnaces, each burning the same fuel. The green portion is the heat delivered and the gray portion is the loss up the flue: 80% AFUE loses 20%, 95% loses 5%, and a 98% condensing furnace loses only 2%. Higher AFUE means lower fuel bills, not a bigger furnace. — Source: AHRI / DOE AFUE

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Three stacked bars for 80%, 95%, and 98% AFUE furnaces, each burning the same fuel. The green portion is the heat delivered and the gray portion is the loss up the flue: 80% AFUE loses 20%, 95% loses 5%, and a 98% condensing furnace loses only 2%. Higher AFUE means lower fuel bills, not a bigger furnace.

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    <img src="https://remodelcalculators.com/api/figure-embed/furnace-afue-ladder" alt="Three stacked bars for 80%, 95%, and 98% AFUE furnaces, each burning the same fuel. The green portion is the heat delivered and the gray portion is the loss up the flue: 80% AFUE loses 20%, 95% loses 5%, and a 98% condensing furnace loses only 2%. Higher AFUE means lower fuel bills, not a bigger furnace." 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-afue-ladder" target="_blank" rel="noopener">AFUE — the share of furnace fuel that becomes heat</a> — courtesy of <a href="https://remodelcalculators.com" target="_blank" rel="noopener">RemodelCalculators.com</a>
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