Up-Blast Stack Cap combination non-back draft damper is applied at the terminating point of a vertical exhaust stack. The Cap has two damper blades that open automatically when the fan is turned on. When the fan is at operating speed, rain and snow are pushed away from the open duct. The damper blades remain at a slight angle when they are open. At the moment the fan is turned off, the blades drop and close off the open duct. This stops rain, snow or outside air from entering the duct system.Most state and local municipalities “require” an Up-Blast Stack Cap as the terminating point of an exhaust duct that is directed to the atmosphere. It is also recommended to terminate the exhaust duct 6 feet above the roofline in a vertical fashion. This prevents the contaminant from being drawn into the heating or air conditioning units, open windows, make up air units and other open vents in the roof. For additional information, please refer to NFPA 33 and NFPA 91. The Code booklets can be obtained from the National Fire Protection Agency, Quincy MA, (617) 770-3000 or www.nfpa.org.Allow 4 to 10 working days for fabrication.Other sizes available, call for pricing.Collar sized to slip OVER spiral pipe.Click here for Tech Data on Up-Blast Stack Caps