Occupancy Change in a Shell Building: The 'Existing Sprinklers, Unchanged' Note
A 24,000 ft² single-story shell building is protected by an existing wet system designed for light hazard: 0.10 gpm/ft² over 1,500 ft² with a 100 gpm hose allowance, a 4 in. riser and a 6 in. private main. A tenant signs for 9,000 ft² of auto repair bays and expects the existing system to serve the space. The demising wall to the remaining light hazard area is 1-hour rated with no openings. The drawings say 'existing sprinklers, unchanged.' The contractor argues that because the ceiling height and the certificate of occupancy are unchanged, no recalculation is required. The only water supply test on file is 22 months old and was run at a hydrant two blocks from the building.
1. Classify the tenant space and decide whether the 'existing sprinklers, unchanged' note can be accepted.
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Classification. Auto repair bays fall in the Ordinary Hazard Group 2 examples - tires, fuel, and flammable liquids put the space well above light hazard.
What that classification carries with it:
- 0.20 gpm/ft² over a 1,500 ft² remote area
- 250 gpm hose allowance, not 100
- 130 ft² maximum per sprinkler, 15 ft maximum spacing, 7 ft 6 in. maximum to walls
Why 'unchanged' fails. A light hazard layout at 15 ft × 15 ft is 225 ft² per sprinkler, which is 73% over the 130 ft² ceiling limit. Spacing and area are separate limits; the tighter one governs. Heads must be added in the bay area no matter how good the water supply is, and adding them changes the branch line and main hydraulics.
What the 1-hour separation does and does not buy. Separated areas of different classification may be designed to their own criteria, so the office side can stay at 0.10/1,500 with a 100 gpm hose allowance - but only if the rated, opening-free wall is shown and maintained. The separation does not allow the OH2 bay area to be calculated with light hazard criteria.
Bottom line: reject the note, require a new calculation for the OH2 area, and recheck the 4 in. riser, 6 in. main, and backflow device against the worst case. That will almost certainly be the bay area.
2. The contractor submits the 22-month-old hydrant test as the water supply basis. How do you handle it?
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Two separate problems, and both have to be answered.
1. Location. A hydrant two blocks away is not the point of connection. Friction loss in the intervening main at 550+ gpm can easily be 5–15 psi, and it appears nowhere in the calculation. You need a test at the hydrant nearest the service, or a calculation that deducts the known main loss.
2. Age. Most AHJs will not accept a supply test older than 12 months, and some require 6. Twenty-two months, with two winters of main changes in between, is not a current picture of the supply.
Also check the record itself: static, residual, and measured flow (pitot and hydrant coefficient), which hydrant was flowing, date and time, and who ran it. A report with static and residual but no measured flow cannot be plotted and is not a flow test.
Acceptable path if a test at the building is impractical: a current test at the nearest accessible hydrant plus a Hazen-Williams calculation of the loss between that point and the riser, submitted with the calculations. Document the assumption so the next reviewer can follow it.
3. Draft the plan review comments you would issue on the resubmittal.
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- Hazard classification: show the 9,000 ft² bay area as Ordinary Hazard Group 2, with the basis (commodity and process) annotated on the plans.
- Hydraulic calculations: provide a calculation for the OH2 area using 0.20 gpm/ft², a 1,500 ft² remote area, and a 250 gpm hose allowance, showing actual pipe sizes through the riser, main, and backflow device.
- Water supply: provide a current test (≤12 months) at or near the service connection with static, residual, and measured flow; if the test point is remote, include the main friction loss deduction.
- Sprinkler layout: revise the bay area to ≤130 ft² per sprinkler and ≤15 ft spacing, with head locations shown on the reflected ceiling plan.
- Separation: show the 1-hour demising wall with no unprotected openings, and note on the drawings that the separation is what permits the office area to remain light hazard.
- Capacity check: confirm the existing 4 in. riser, 6 in. main, and backflow device can carry the new demand, or show the upgrades.
- Alarm and supervisory devices: confirm the flow switch and valve tamper remain appropriate for the modified system, and that the FDC is unchanged and accessible.