ESFR System and a New HVAC Duct
A 180,000 ft² distribution warehouse in Sparks has a 32-ft ceiling and an ESFR system with K=25.2 pendent sprinklers at 31.5 ft, designed for 24-ft storage of cartoned unexpanded Group A plastics. The rack layout includes 6-in. transverse and 12-in. longitudinal flue spaces. The owner now installs a 4-ft-wide, 24-in.-deep HVAC duct that runs 6 ft below the ceiling, parallel to the branch lines and directly over a main storage aisle. The duct is supported from roof joists; its bottom is about 24 in. above the top of storage. The contractor proposes to leave the ESFR sprinklers as-is because the duct is 'above the storage' and the system was originally hydraulic-calculated with a safety margin. The fire marshal has asked for a written evaluation before accepting the tenant improvement. No sprinklers are currently installed below the duct.
1. Does the duct violate ESFR installation criteria? Explain the reasoning, not just the conclusion.
Read the worked answer
ESFR criteria are stricter than standard-spray criteria because ESFR depends on a strong, unobstructed ceiling-level discharge to suppress a high-challenge fire. A continuous obstruction within the defined zone below the deflector can block the spray pattern and create a shadow over the storage aisle. Here the deflectors are at 31.5 ft and the duct bottom is at roughly 29.5 ft, so the duct is about 24 in. below the deflectors and 4 ft wide, running over a main aisle. That is not a minor isolated obstruction; it is a linear obstruction that can prevent the required coverage. The fact that the duct is above the top of storage does not cure the problem, because the obstruction is below the sprinkler deflectors. The original hydraulic safety margin also does not cure it: obstruction compliance is a layout requirement, not a pressure-margin issue. The contractor must evaluate the duct against the ESFR obstruction rules and the manufacturer's listing.
2. What options are available to resolve the obstruction, and what are the trade-offs?
Read the worked answer
Three options merit evaluation. First, relocate the duct: raise it closer to the ceiling or route it outside the storage aisle. This is often the cleanest solution because it preserves the ESFR design, but structural and coordination constraints may make it impractical. Second, add sprinklers below the duct. That can be acceptable only if the added heads are listed and arranged for the application, the deflector orientation and clearance are correct, and the hydraulic calculation is revised to include the added demand and the changed flow split. Adding heads below a duct is not a field judgment; it requires engineering and often a fire marshal submittal. Third, convert the area to a CMDA design with in-rack sprinklers or a different ceiling-level design. That may be more robust but far more expensive and disruptive. The trade-off is cost and schedule versus preserving the original ESFR basis; the duct cannot simply remain with no analysis.
3. What should the written evaluation and submittal include before the fire marshal accepts the tenant improvement?
Read the worked answer
The evaluation should include the duct geometry relative to the deflectors, the ESFR obstruction criteria used, the manufacturer's listing limitations, and a marked-up plan showing the duct and all sprinklers. If sprinklers are added below the duct, the submittal should include the new head locations, K-factors, temperature ratings, deflector orientation, and a revised hydraulic calculation showing the added demand and the available supply. It should also confirm that storage height, flue spaces, and clearance to deflectors remain compliant. The contractor should include the acceptance test plan and a statement that the system remains an ESFR system or, if converted, clearly identify the new design basis. A letter saying 'the system has margin' is not sufficient; the fire marshal needs a layout and hydraulic basis for accepting the obstruction.