The duct work must be smooth on the inside with no projections from sheet metal screws or other obstructions which would collect lint. Inspection doors must be installed at strategic points in the duct work for periodic lint removal.
Use smooth sweeping elbows instead of sharp 90º elbows. The radius of an elbow should be at least
When the exhaust duct goes through a combustible wall on a roof, leave a
When adding ducts, the duct that leaves the building must be protected from the weather. For horizontal ducting use a 90º turned downward . For vertical duct work use a 180º turned downward. In either case, allow at least twice the diameter of the duct between the duct opening and nearest obstruction. (If the duct is rectangular, leave twice the smallest face dimension.)
IMPORTANT: DO NOT use screen or caps on the outside opening of the exhaust duct work. Screens will clog with lint and caps will restrict the exhaust airflow.
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