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ASHRAE updates duct size calculator

USA: A new ASHRAE duct sizing calculator enables air distribution system designers to accurately size ducts, especially flex ducts, under varying amounts of compression.

Developed by ASHRAE and the Air Distribution Institute (ADI), the Duct Size Calculator is a quick reference tool for approximating duct sizes and equivalent sizes of sheet metal duct vs flexible duct. The calculator uses information from ASHRAE Research Project 1333, HVAC Duct Efficiency Measures.

“While the calculator resembles a wheel similar to those used during the days of slide rules, it incorporates three new fields for equivalent duct sizing,” Chris Van Rite, developer of the calculator, said. “These new fields help demonstrate the significant loss of airflow due to improper installation of flexible ducts.”

The calculator includes fields for 4, 15 and 30% compression in flexible ducts. Van Rite notes that the calculations used to create these size references are based on straight line compression as performed in the laboratory on a flat surface. Field installed flexible ducts with bends, kinks and excessive lengths will have additional resistance, which will result in diminished airflow.

“The use of this tool allows duct designers to account for less than optimum installation and gives a more accurate design to installed performance correlation,” Van Rite said.

The Duct Size Calculator features imperial units on one side and SI units on the other.

The cost of the calculator is $34, ASHRAE members ($40, non-members). To order, visit

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