Rated versus delivered bathroom fan airflow — the same fan moves less through a real duct

Fans are rated at 0.1 in. of static, but a real duct sits near 0.25 in., where a typical fan delivers only ~70% of its rating.

Two side-by-side panels of the same fan. The left green panel is the lab rating: a short, straight test duct at 0.1 inch of static pressure, where a thick airflow arrow shows the full rated 50 CFM. The right amber panel is a real install: a long duct with elbows and a wall cap at about 0.25 inch of static, where a thinner airflow arrow shows only about 30 CFM — roughly 70 percent of the rating. — Source: DOE / Building America; ENERGY STAR ≥70% at 0.25 in.

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Two side-by-side panels of the same fan. The left green panel is the lab rating: a short, straight test duct at 0.1 inch of static pressure, where a thick airflow arrow shows the full rated 50 CFM. The right amber panel is a real install: a long duct with elbows and a wall cap at about 0.25 inch of static, where a thinner airflow arrow shows only about 30 CFM — roughly 70 percent of the rating.

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    <img src="https://remodelcalculators.com/api/figure-embed/bath-fan-rated-vs-delivered" alt="Two side-by-side panels of the same fan. The left green panel is the lab rating: a short, straight test duct at 0.1 inch of static pressure, where a thick airflow arrow shows the full rated 50 CFM. The right amber panel is a real install: a long duct with elbows and a wall cap at about 0.25 inch of static, where a thinner airflow arrow shows only about 30 CFM — roughly 70 percent of the rating." 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/bath-fan-rated-vs-delivered" target="_blank" rel="noopener">Rated versus delivered bathroom fan airflow — the same fan moves less through a real duct</a> — courtesy of <a href="https://remodelcalculators.com" target="_blank" rel="noopener">RemodelCalculators.com</a>
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