Hydraulic Calculations: When and Why They Are Needed

Fitzemeyer & Tocci
1 minute ago
5 min read

If you have ever asked a Fire Protection engineer when hydraulic calculations are required for existing systems being modified you’ve probably gotten an unsatisfying “it depends” answer as a result. As a fire protection engineer there are several key points to consider, questions of what the renovations entail including is there an addition to the building, is the group use changing, is the hazard changing, are you adding sprinklers to the project? There are other pieces to consider including the current adopted codes for the location of the project; Authority having jurisdiction (AHJ) and insurance requirements and Engineering best practices and Engineering Society joint position statement.

Calculations are required for all new buildings, building additions, any renovation where the group use is changing or the hazard is changing. Additionally, any modification that adds additional pressure drop to the system including adding a new backflow preventer to a previously non-compliant system would require a calculation as well. There is a gray area on small renovations where the majority of the piping is existing to remain where we as professional engineers get this question the most. This situation is where we will dive into when and where they are needed.

Codes

There are several codes that touch on fire protection and the requirements with the primary one being NFPA 13, each state adopts this standard through NFPA 1 and the building code. Older editions NFPA 13 include an annex exception that has been used for small renovations but this has been removed on the newest editions leaving no option to exempt calculations from projects in states with NFPA 13 2019 or newer.

2016 NPFA 13 includes the following: 8.15.20.5.2: Calculations shall be provided to verify that the system design flow rate will be achieved. However the annex of this notes, “It is not the intent of this section to require a full hydraulic analysis of the existing piping system in addition to new sprinkler layout.” This has been used to justify that if sprinklers are only being relocated that calculations are not required since all the piping mains, and branch pipes a existing to remain.

2019 NFPA 13 and 2022 NFPA 13 has removed this and has a dedicated charter to modification of existing systems, chapter 29 in 2019 and chapter 30 in 2022. In that chapter there is a section 29.5.2 and 30.6.2 respectively that clearly states calculations shall be provided to verify that the system design flow rate will be achieved. No annex section is provided clarifying that calculations are not required for relocation.

2025 NFPA 13 adds the following section into the code clarifying the requirements. Section 30.1.5 was added to the 2025 version noting, “Where existing system modifications include added or relocated sprinklers, or flexible sprinkler hose fittings are retroactively installed on existing systems, a hydraulic analysis or revised hydraulic calculations using current water supply data to accommodate the additional friction loss shall be completed as a part of the installation.” This appears to require a full hydraulic calculation for the renovated area, note that the existing area does not need to be included in that calculation but the calculation does need to include the hazards minimum flow and pressure even if the renovated area or zone does not include active sprinklers able to discharge at that rate.

This means that moving forward in jurisdictions that require 2019 and newer NFPA 13 to be followed a full calculation will technically be required for these renovation projects, but who should provide them?

AHJ, Building and Insurance

Inspection to fire sprinkler system — Fire protection system's inspection

The AHJ, everyone’s favorite term, it means everything and nothing at the same time. Who is it referring to exactly? Generally the fire department, the fire marshal, the building inspector, anyone who can enforce their interpretation of the code or design requirement onto the project. There is also building standards from the owner that need to be confirmed during design, some buildings have design standards requiring all tier 1 permit drawings to be calculated.

For the majority of the projects your first stop is the fire department, they generally review and approve sprinkler plans as they dictate where and what key components of the system are including post indicating valves, sprinkler room locations and access, fire department connections and standpipe locations, etc.
The building inspector also over sees approval of the fire protection plans, they normally coordinate with the fire department on this during a permit review.
The AHJ may have additional above and beyond code minimum requirements including that the engineer provide calculations.
Some insurance company like FM global insurance have separate requirements during their reviews that require the engineers to produce calculations.
The Water department may also have requirements including the service entrance components that need to be incorporated into the detail including but not limited to thrust blocking, pipe type, length of piping inside the building before the backflow preventer, etc.

Until all of those requirements have been confirmed its safe to assume that the engineer is providing calculations on the project.

Engineering best practices and Engineering Society joint position statements

The Society of Fire Protection Engineers (SFPE) has a position statement on that aligns with several organizations including ABET, AFFA, AFSA, ASCET, FSSA, NCEES, NFSA, NICET and NSPE that helps clarify what the expectation in the industry is since in the majority of situations a calculation is required.

SFPE notes that Engineering documents should contain the following:

Select type of system(s) and components
Classify the hazard(s), storage arrangements, and commodities to be protected
Establish the design criteria
Determine and confirm the available water supply, including necessary adjustments
Create a conceptual system layout and hydraulic calculations to verify adequacy of proposed water supply arrangements
Set criteria regarding systems structural support, including seismic documentation (as appropriate)
Identify water quality or other environmental factors affecting the proposed systems
Establish zoning with consideration to building elements (e.g., horizontal exits) or other fire protection/life safety systems (e.g., smoke control)

Based on this while calculations are required for the project the engineer creating tier 1 plans will only need to confirm if the available water supply is able to support the fire protection system that is being designed. Depending on what the engineer is comfortable with this could include a preliminary excel sheet showing a large safety factor on the system. This situation could occur in a renovation with an existing fire pump, the lower floors may have plenty of pressure that a sprinkler relocation project may not require a full hydraulic analysis.

Conclusion

Calculations are needed for all projects. Engineers creating tier 1 plans need to provide full calculations for all new building projects, additions, areas with a change of hazard or occupancy/use. For renovations with relocation of sprinklers the engineer needs to confirm the water supply either by a preliminary calculation showing a sufficiently high safety factor or a full hydraulic calculation confirming that water supply. The contractor providing the tier 2 drawings will provide the hydraulic calculations unless the AHJ or owner requires the engineer to provide calculations as well during construction document / tier 1 drawing submission. Finally, the tier 3 as-built drawings are submitted by the contractor confirming the final configuration of the system after all field conditions have been incorporated into the plans.