ESFR Swap in an Existing Warehouse With an Overhead Duct
An existing 180,000 sq ft single-story warehouse has a K-8.0 CMDA wet system designed for Ordinary Hazard Group 2 at 0.20 gpm/sq ft over 1,500 sq ft, with 10 ft by 10 ft spacing, 2½ in. branch lines and a 6 in. main. The deck is 32 ft above the slab. The new owner wants to rack Class IV commodity to 24 ft in 8 ft aisles and has asked for ESFR so that in-rack sprinklers can be avoided. The contractor proposes to swap the heads for K-14.0 ESFR at 12 sprinklers and 52 psi, keeping the same 10 ft by 10 ft layout and the same piping, with deflectors 14 in. below the deck, arguing that "ESFR is 12 heads at 52 psi no matter what the commodity is, so no new hydraulic calculation is needed." A 48 in. wide, 24 in. deep HVAC duct runs the length of the building with its top at the deck, directly over one of the proposed rack aisles.
1. Is the proposed K-14 ESFR design valid for this building as configured? Identify the blocking issues in order of severity.
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1. Ceiling height versus the ESFR design tables. K-14 ESFR has design rows for this commodity family up to a 30 ft ceiling. At a 32 ft deck there is no K-14 design for Class IV at 24 ft storage, so the swap as proposed has no code basis regardless of the piping. Options are to drop the deck (or add a false ceiling) to 30 ft, use K-25.2 ESFR at its listed pressure, or go back to CMDA - and CMDA for Class IV racks above 12 ft requires in-rack sprinklers, which is exactly what the owner was trying to avoid.
2. The duct. ESFR deflectors must sit 3 to 18 in. below the ceiling, and ESFR does not tolerate obstructions in the spray path the way CMDA does. The duct bottom is at 28 ft, only about 32 in. below the 30 ft 10 in. deflector plane and inside the prohibited zone above the rack aisle. You cannot fix this by adding a head under the duct; the duct must be raised, rerouted over a non-rack aisle, or the ESFR layout abandoned locally.
3. Branch line and main sizing. A 12-sprinkler ESFR design at roughly 101 gpm per head moves far more water than the existing 2½ in. branch lines and 6 in. main were sized for. Verify branch line sizing against the ESFR requirements and confirm the 6 in. main and supply can carry the flow before promising the owner anything.
4. Spacing. 10 ft by 10 ft is within ESFR maximum spacing, so that part is fine - which is why the contractor's argument sounds plausible and is still wrong.
2. Assume the ceiling height, duct, and piping issues are resolved with K-25.2 heads. Quantify the hydraulic change and describe what must be re-verified on the supply side.
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Old demand. OH2 at 0.20 gpm/sq ft over 1,500 sq ft = 300 gpm, plus 250 gpm hose allowance = about 550 gpm at the base of the riser.
New demand. A 12-sprinkler ESFR design at the listed pressure for K-25.2 (roughly 35 psi) gives about 149 gpm per head, or roughly 1,790 gpm, plus 250 gpm hose allowance for roughly 2,040 gpm. Even a K-14 arrangement at 52 psi lands near 1,210 gpm plus hose, over 1,460 gpm. Either way the demand roughly triples.
What to re-verify:
- The water supply curve at the new flow. A test that looked healthy at 600 gpm tells you little at 1,800 gpm; residual pressure and the shape of the curve both matter, and a booster pump may now be required where none was.
- Friction through the existing 6 in. main over its actual length, plus the riser, backflow preventer, and underground. At these flows, the backflow preventer alone can eat 10 to 15 psi.
- Velocity and available pressure at the test header, and whether the existing fire pump (if any) is rated for the new flow.
- That the 12-sprinkler remote area is correctly located relative to the most demanding aisle, ceiling pockets, and the duct work that remains.
Present the two demands side by side on a semilog supply-demand plot for the AHJ; a tripled demand is the whole story of this project and it should be visible at a glance.
3. The owner insists on keeping the duct where it is. What technical options do you present, and what do you refuse to sign?
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Options worth pricing:
- Raise the duct so its bottom clears the ESFR obstruction zone below the deflector plane; this usually means moving it to a structural bay that is not over a rack aisle.
- Reroute the duct along a main aisle or above a non-storage area - ESFR obstruction rules are about what is under the spray, not about duct length.
- Lower the deck locally to 30 ft over the affected bay to restore a valid K-14 design row, if the structure allows.
- Convert the affected bay to a CMDA design with in-rack sprinklers at the rack tiers, accepting the added cost and the rack-integration work.
What not to accept:
- "We'll just add a sprinkler under the duct." ESFR design assumes an unobstructed spray; a head under a duct does not restore the design basis for the racks it shields.
- Ignoring the duct because it is "only over one aisle" - the aisle is where the rack commodity is, so that is precisely where the obstruction matters.
- Filing a K-14 design at 32 ft and relying on the AHJ not noticing the table limit.
The defensible deliverable is a layout that has a design row in the adopted standard for the actual ceiling height, storage height, and commodity, with every obstruction dimensioned against the deflector plane.