Mixed-Occupancy Sprinkler Retrofit in a Triple-Decker
A three-story wood-frame dwelling in Worcester has been converted to a rooming house: the basement is now a shared laundry, floors 1-3 each have four sleeping rooms plus a common kitchenette, and the attic is used for storage. The owner wants to use NFPA 13D to keep costs down and avoid a fire pump. The town water supply provides a static pressure of 62 psi and a residual of 48 psi at 900 gpm through a 2-inch service. The building official has flagged the occupancy change and questioned whether 13D is still permissible. The contractor has already priced a 13D system with a single automatic riser and 13D-compliant 10-minute duration.
1. Is NFPA 13D an acceptable standard for this building, and what should the contractor tell the owner?
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No. 13D is limited to one- and two-family dwellings and manufactured homes. A rooming house with shared kitchenettes and four sleeping rooms per floor is not a dwelling unit under 13D's scope; it is a residential occupancy requiring a full 13 (or a locally accepted equivalent) system. The contractor should tell the owner that the 13D price is not usable, that the building official is correct to flag it, and that a 13 system with 30-minute (or greater, per hazard) duration and appropriate density for the occupancy must be designed. Any contract signed on the 13D basis should be renegotiated in writing before work proceeds.
2. Assuming a 13 system, what water supply concerns arise from the given hydrant data, and what would you do before finalizing hydraulic calculations?
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At 900 gpm the residual is 48 psi, so the available pressure at the required demand is only part of the story: the 2-inch service is small and will produce a steep supply curve. Before finalizing calcs, obtain a current flow test at the actual service tap, not a distant hydrant, and correct for elevation and for the difference between test hydrant and system point. Then compare the demand point (density x area plus hose allowance) against the supply curve. If the margin is thin, consider a larger service, a tank and pump, or a reduced design area only if the standard permits it for the construction type. Do not assume the 62 psi static is usable at the riser.
3. What plan-review issues does the attic storage and the common kitchenettes create?
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Attic storage is a combustible storage hazard and generally requires sprinkler protection; it cannot be omitted simply because it is an attic. The common kitchenettes are cooking areas and need coverage appropriate to the hazard, and the laundry in the basement introduces a separate hazard. The plan must show sprinklers in the attic, in the laundry, and in each kitchenette, with the correct temperature ratings near cooking equipment and proper clearance to storage. The reviewer will also want the occupancy classification stated on the drawings and the design density justified for the most demanding area.