Source: EN 12845-2:2024 Clause 5.4 (EN p. 32-33) and Table 7 (EN p. 36); TS EN 12845+A2:2026 Clause 6.2.4.2 (EN p. 32) and Clause 7 (EN p. 35-41).
The title of this article in its previous version was "HHS5 to HHS12". That was wrong and has been corrected: neither EN 12845 nor EN 12845-2 has a class HHS6 or above. The set of classes defined by the standards, and the special 12 sprinkler design of Clause 5.4, are set out below.
How far the HHS classes go
TS EN 12845+A2:2026 Clause 6.2.4.2 (EN p. 32): High Hazard, Storage is divided into four categories:
- HHS1, High Hazard Storage Category I
- HHS2, High Hazard Storage Category II
- HHS3, High Hazard Storage Category III
- HHS4, High Hazard Storage Category IV
EN 12845-2:2024 adds HHS5 to this list for ESFR and CMSA designs (Table 7, EN p. 36; Table 10, EN p. 41; Table 16, EN p. 50). There is no class HHS6 or beyond.
The way the class is determined also matters. The HHS category is determined not by the stack height but by the combustibility of the commodity and its packaging (including the pallet). EN 12845 Annex B (normative, EN p. 127) gives a method based on the material factor (M); Annex C (EN p. 132) contains the alphabetical list of stored products with their categories. Height does not determine the class; it enters the design criteria together with the class: EN 12845 Table 4 (EN p. 38) and Table 5 (EN p. 39) give the maximum permitted storage height together with the category and the storage configuration, with design densities in the range of 7.5 mm/min to 30.0 mm/min.
The storage configurations are also defined in EN 12845 Clause 6.3.2 (EN p. 33) and should not be confused with one another:
| Code | Definition |
|---|---|
| ST1 | Free standing or block stacking |
| ST2 | Single row post pallets, aisles at least 2.4 m |
| ST3 | Multiple row (including double row) post pallets |
| ST4 | Palletized rack (beam type pallet racking) |
| ST5 | Solid or slatted shelves of width 1 m or less |
| ST6 | Solid or slatted shelves of width more than 1 m and not exceeding 6 m |
EN 12845-2 uses STC codes in its own text; Clause 6.1 and the note to Table 11 refer to STC4.3 as multiple row racking and the note to Table 21 refers to STC1 and STC2 as solid pile, palletized, rack or bin box arrangements (EN p. 33, 41, 55).
Clause 5.4: the additional requirements of the 12 sprinkler design for HHS5
The title of Clause 5.4 says exactly this: "Additional requirements for protection of HHS5, with 12 sprinkler design" (EN p. 32).
Sprinkler and installation:
- The sprinklers shall be intermediate temperature rated (79 °C to 107 °C) ESFR pendent sprinklers with a nominal k-factor of K360.
- The maximum sprinkler deflector distance below the ceiling shall be 350 mm.
Rack and aisle arrangement:
- Minimum aisle width 2.4 m.
- The rack is divided by solid vertical barriers running from face of rack to face of rack; the barrier is of 10 mm plywood or particleboard, or 0.78 mm sheet metal, or equivalent; the barriers are spaced at a maximum of 5.0 m intervals.
- The barrier extends from a maximum of 100 mm above the floor to the maximum storage height and extends across the longitudinal flue.
- The plan area of storage between vertical barriers and aisles shall not exceed 12 m².
- The commodity is permitted to extend a nominal 100 mm beyond the vertical barrier at the aisle.
Minimum operating pressure (for both solid piles and rack storage):
| Storage height | Maximum ceiling height | Minimum operating pressure |
|---|---|---|
| Up to 7.6 m | 9.1 m | 2.0 bar |
| Up to 10.7 m | 12.2 m | 4.1 bar |
The same two values appear in the K360 column of the HHS5 rows of Table 7 (EN p. 36). The footnote to Table 7 states that those rows apply only to exposed expanded plastics and only where all the requirements (including the vertical barriers) are applied, and that the 12.2 m row is for palletized and solid pile storage in a closed array.
Water demand: calculated from the standard's values
The flow formula is in TS EN 12845+A2:2026 Clause 14.3 (EN p. 104-105): Q = K√P. Table 6 (EN 12845-2, EN p. 34) gives the duration of supply for a 12 sprinkler design as 60 min and the shape of the design area as 4 × 3 (4 sprinklers per range pipe, 3 range pipes).
| Scenario | Flow per sprinkler | Total for 12 sprinklers | Volume for 60 min |
|---|---|---|---|
| HHS5, ceiling 9.1 m, K360 at 2.0 bar | 360 × √2.0 ≈ 509 L/min | ≈ 6 109 L/min | ≈ 367 m³ |
| HHS5, ceiling 12.2 m, K360 at 4.1 bar | 360 × √4.1 ≈ 729 L/min | ≈ 8 747 L/min | ≈ 525 m³ |
These volumes are the demand of the ceiling sprinklers only; pipe friction losses, static height and any in-rack sprinkler demand are calculated separately.
Where HHS5 appears in the other design tables
- Table 10 (EN p. 41), ESFR designs with special requirements: HHS5 at a ceiling of 7.6 m, 12 at 5.2 bar with K200, 12 at 3.6 bar with K240, 9 at 2.2 bar with K320, 9 at 1.7 bar with K360; at a ceiling of 9.1 m, 12 at 6.9 bar with K200, 12 at 4.8 bar with K240, 12 at 3.5 bar with K320, 12 at 2.8 bar with K360; at a ceiling of 12.2 m, 20 at 5.2 bar with K360.
- Tables 13 and 14 (EN p. 47), modular ESFR in-rack design: for HHS5 with a maximum vertical in-rack installation of 9.1 m, the minimum k-factor is K320 and the minimum flow from the most remote in-rack sprinklers is 455 L/min. The footnote to Table 13 states that for HHS5 in double row and multiple row racks the calculation is based on the 6 in-rack sprinklers in the most remote rack plus the 4 most remote in-rack sprinklers in the adjacent rack, 10 sprinklers in total.
- Table 16 (EN p. 50), STC1 storage with CMSA: HHS5 with storage of 5.5 m and a ceiling of 7.9 m, 15 sprinklers at 3.4 bar with K160 upright in a wet system, or 15 sprinklers at 1.5 bar with K240 upright in a wet system.
Is the pre-calculated route closed?
No. TS EN 12845+A2:2026 Clause 13.1 (EN p. 82) makes full calculation mandatory in only two cases: arrangements with intermediate level HHS in-rack sprinklers and gridded or looped arrangements. The pre-calculated route is open for HHP and HHS through Clause 7.3.2 with Table 7 (EN p. 40) and Clause 13.3.5 with Tables 32-35. Clause 7.2.3.4 (EN p. 37) does require in-rack sprinklers and their associated ceiling sprinklers always to be fully calculated, and its NOTE recalls that the minimum pressure at any operating sprinkler is 2.0 bar (Clause 13.4.4).
Water supply and pump arrangement
TS EN 12845+A2:2026 Clause 10.2 (EN p. 57): the characteristic curves of the pump sets shall be compatible and they shall be able to operate in parallel at all possible flows. Where two pumps are installed, each alone shall be able to provide the specified flows and pressures; where three pumps are installed, each shall be able to provide at least 50 % of the specified flow at the specified pressure. Where a superior or duplicate water supply has more than one pump, no more than one of them is driven by an electric motor.
Annex P (EN p. 164) recommends the use of three pump sets of 50 % capacity conforming to EN 17451 for the water supply of ESFR systems.
For the pump room, Clause 10.3 (EN p. 57-58) gives the requirements for fire resistance (at least 60 min), the order of preference for its location, sprinkler protection and minimum temperature (4 °C for electric pump sets, 10 °C for diesel); the text of the standard contains no minimum floor area for a pump room.
Frequently Asked Questions
How far do the HHS classes go?
TS EN 12845+A2:2026 Clause 6.2.4.2 (EN p. 32) divides high hazard storage into four categories: HHS1 (Category I), HHS2 (Category II), HHS3 (Category III) and HHS4 (Category IV). EN 12845-2:2024 adds HHS5 for ESFR and CMSA designs (Table 7, EN p. 36). Neither standard has a class HHS6 or beyond; the HHS class is determined not by the stack height but by the combustibility of the commodity and its packaging (EN 12845 Annex B, EN p. 127).
Which sprinkler is used in the 12 sprinkler design for HHS5?
EN 12845-2:2024 Clause 5.4 (EN p. 32): the sprinklers shall be intermediate temperature rated (79 °C to 107 °C) ESFR pendent sprinklers with a nominal k-factor of K360. The maximum sprinkler deflector distance below the ceiling shall be 350 mm.
What are the rack and aisle requirements in the HHS5 design?
Clause 5.4 (EN p. 32-33): the minimum aisle width is 2.4 m. The rack is divided by solid vertical barriers running from face of rack to face of rack, spaced at a maximum of 5.0 m intervals; the barrier is of 10 mm plywood or particleboard, or 0.78 mm sheet metal, or equivalent. The barrier extends from a maximum of 100 mm above the floor to the maximum storage height and extends across the longitudinal flue. The plan area of storage between vertical barriers and aisles shall not exceed 12 m². The commodity is permitted to extend a nominal 100 mm beyond the vertical barrier at the aisle.
What is the minimum operating pressure of the HHS5 design?
Clause 5.4 (EN p. 33) gives two values, for both solid piles and rack storage: 2.0 bar for a maximum ceiling height of 9.1 m with storage up to 7.6 m, and 4.1 bar for a maximum ceiling height of 12.2 m with storage up to 10.7 m. The same values also appear in the HHS5 rows of Table 7 (EN p. 36).
Is full hydraulic calculation mandatory in a high bay design?
On the EN 12845 side, no. TS EN 12845+A2:2026 Clause 13.1 (EN p. 82) makes full calculation mandatory only for arrangements with intermediate level HHS in-rack sprinklers and for gridded or looped arrangements. The pre-calculated route is open for HHP and HHS through Clause 7.3.2 with Table 7 (EN p. 40) and Clause 13.3.5 with Tables 32-35. Clause 7.2.3.4 (EN p. 37) does require in-rack sprinklers and their associated ceiling sprinklers always to be fully calculated.
What is the duration of supply in the HHS5 design?
EN 12845-2:2024 Table 6 (EN p. 34) ties the duration to the number of ceiling sprinklers in the design: 60 min for 9, 10 and 12 sprinklers; 90 min for 15 and 16 sprinklers; 120 min for 20, 24, 25 and 30 sprinklers. The duration in a 12 sprinkler HHS5 design is therefore 60 minutes. For comparison, EN 12845 Clause 8.1.1 (EN p. 42) gives 90 min for HHS in CMDA systems.
References
- EN 12845-2:2024 Clause 5.4 (EN p. 32-33)
- EN 12845-2:2024 Clause 6.1 (EN p. 33), Table 6 (EN p. 34)
- EN 12845-2:2024 Table 7 and its footnotes (EN p. 36)
- EN 12845-2:2024 Table 10, Table 11 and its note (EN p. 41)
- EN 12845-2:2024 Table 13, Table 14 (EN p. 47)
- EN 12845-2:2024 Table 16 (EN p. 50), note to Table 21 (EN p. 55)
- TS EN 12845+A2:2026 Clause 6.2.4.2 (EN p. 32), Clause 6.3.2 (EN p. 33)
- TS EN 12845+A2:2026 Table 4 (EN p. 38), Table 5 (EN p. 39)
- TS EN 12845+A2:2026 Clause 7.2.3.4 (EN p. 37), Clause 7.3.2, Table 7 (EN p. 40)
- TS EN 12845+A2:2026 Clause 8.1.1 (EN p. 42), Clause 10.2, 10.3 (EN p. 57-58)
- TS EN 12845+A2:2026 Clause 13.1 (EN p. 82), Clause 13.4.4 (EN p. 102), Clause 14.3 (EN p. 104-105)
- TS EN 12845+A2:2026 Annex B (EN p. 127), Annex C (EN p. 132), Annex P (EN p. 164)
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Download MEP Calc on the App StoreTS EN 12845+A2:2026 (EN 12845:2015+A2:2026) Fixed firefighting systems — Automatic sprinkler systems — Design, installation and maintenance; TS EN 12845-2:2025 (EN 12845-2:2024) ESFR and CMSA sprinkler systems. Every figure in this article is taken from the published standard text; clause, table and page references are listed under References. General information only, not a substitute for the standard or for a design review.