In an effort to reduce the number of dryer fires (over 15,500 per year in the US alone) most building code has strict guidelines for properly venting a dryer. Every foot of duct run reduces airflow efficiency and small radius elbows have an even greater impact.
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While some municipalities are working through the process of adopting relevant code, they often accept engineering reports clarifying the improvement in airflow efficiency to allow an exception when non-specific elbow penalties are in place.
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ASHRAE Fundamentals Handbook has a methodology for calculating the friction loss in ducts and fittings. Based on these formulas Dryer-Ells can be measured the same as straight pipe. This permits use of the maximum allowed run length.
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Underwritings Laboratories prepared a fact find report which shows, among other findings, that Dryer-Ells are 500% more airflow efficient than sectioned, small radius elbows.
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