sprinklernotes ESFR Sprinklers and Obstructions Below the Deflectors You have asked the following question: “We have a facility with an existing ESFR sprinkler system (10 ft. by 9 ft. spacing) with new ductwork to be installed 20 ft. below the ESFR sprinklers. Duct width is 36 in. narrowing to 24 in. Would this be considered an obstruction? If so, can the ductwork be positioned in relation to the sprinklers so as not to be an obstruction?” In response to your question, we have reviewed NFPA 13, 2019 edition as the applicable standard. Our informal interpretation is the ductwork would be considered an obstruction.
Items that are greater than 2 ft. in width are considered obstructions per Section 14.2.11. Continuous obstructions for ESFR sprinkler system are required to comply with Section 14.2.11.3, and ductwork is considered a continuous obstruction. Section 14.2.11.3.1 provides five options for how to install ESFR sprinklers around continuous obstructions and they are: (1) Sprinklers shall be installed below continuous obstructions, or they shall be arranged to comply with Table 14.2.11.1.1 for horizontal obstructions entirely below the elevation of sprinklers that restrict sprinkler discharge pattern for two or more adjacent sprinklers such as ducts, lights, pipes, and conveyors. (2) Additional sprinklers shall not be required where the obstruction is 2 in. (50 mm) or less in width and is located a minimum of 2 ft (600 mm) below the elevation of the sprinkler deflector or is positioned a minimum of 1 ft (300 mm) horizontally from the sprinkler. (3) Additional sprinklers shall not be required where the obstruction is 1 ft (300 mm) or less in width and located a minimum of 1 ft (300 mm) horizontally from the sprinkler. (4) Additional sprinklers shall not be required where the obstruction is 2 ft (600 mm) or less in width and located a minimum of 2 ft (600 mm) horizontally from the sprinkler. (5) Ceiling sprinklers shall not be required to comply with Table 14.2.11.1.1 where a row of sprinklers is installed under the obstruction. As long as sprinklers are installed to the requirements of one of these five options the arrangement should be acceptable. Item #4 is the one that addresses your issue. The portions of the duct that are no more than 24 in. can be ignored as long as there is a horizontal separation of at least 24 in. horizontal between the nearest ESFR sprinkler and the edge of the duct. When wider than 24 in., you must have a sprinkler beneath it. Unfortunately, this applies regardless of distance that the duct is below the ceiling. The only exception is when the bottom of the obstruction is within 24 in of the floor (see 9.5.5.3.1.5). Having said this, if the ducts are in a portion of the building that is not storage or there are no combustibles beneath the ducts (including the area within 2 ft of the sides of the duct), there is a reasonable argument (due to the vertical separation AND that it is not protecting a fuel package that creates a challenging fire like storage) to treat this obstruction using the spray sprinkler rules based on a 4 ft wide obstruction. Obviously, this requires approval of the AHJ
