The High Hazard Process (HHP) class covers occupancies where the material has a high fire load and high flammability and can develop a rapidly spreading or intense fire. EN 12845 divides this class into four groups and collects the design criteria in a single table. Source: TS EN 12845+A2:2026, Clause 6.2.4.1, EN p.32 and Table 3, EN p.36.
Definition and groups: Clause 6.2.4.1
High Hazard, Process covers occupancies where the material has a high fire load and high flammability and can develop a rapidly spreading or intense fire. HHP is divided into four groups: HHP1, HHP2, HHP3 and HHP4 (EN p.32).
Note: HHP4 hazards are generally protected by deluge systems, and those systems are outside the scope of the standard.
Table 3: Design criteria for LH, OH and HHP (Clause 7.1, EN p.36)
| Hazard class | Design density (mm/min) | Area of operation, wet or pre-action (m²) | Area of operation, dry or alternate (m²) |
|---|---|---|---|
| LH | 2.25 | 84 | Not permitted, OH1 is used |
| OH1 | 5.0 | 72 | 90 |
| OH2 | 5.0 | 144 | 180 |
| OH3 | 5.0 | 216 | 270 |
| OH4 | 5.0 | 360 | Not permitted, HHP1 is used |
| HHP1 | 7.5 | 260 | 325 |
| HHP2 | 10.0 | 260 | 325 |
| HHP3 | 12.5 | 260 | 325 |
| HHP4 | deluge (see NOTE) | - | - |
Note to the table: HHP4 requires special consideration; deluge systems are not within the scope of this standard.
Clause 7.1 (EN p.35) gives the condition under which these values apply: the design density shall be not less than the appropriate value given in this clause with all the roof or ceiling sprinklers in the room concerned or those in the area of operation, whichever is the fewer, plus all in-rack and supplementary sprinklers, operating. For HHS systems, 7.2 applies.
Table A.3: HHP occupancies (EN p.126)
Annex A is normative and states that Tables A.1, A.2 and A.3 contain lists of minimum hazard classification, that they are also to be used as guidance for occupancies not specifically mentioned, and that they are to be read together with 6.2 (EN p.124).
| HHP1 | HHP2 | HHP3 | HHP4 |
|---|---|---|---|
| Oilcloth and linoleum manufacture | Lighter manufacture | Cellulose nitrate manufacture | Firework manufacture |
| Resin, lamp black and turpentine manufacture | Tar distillation | Rubber tyres for cars and lorries | |
| Rubber substitute manufacture | Garages for buses, empty lorries and railway wagons | Manufacture of foamed plastics, foam rubber and foam rubber products with material factor m3 (see Table B.1), excluding m4 (see Table B.1) | |
| Wood wool manufacture | Candle and paraffin wax manufacturers | ||
| Match manufacturers | Paper machine halls | ||
| Solvent paint spraying shops | Carpet factories with rubber and foamed plastics | ||
| Refrigerator factories | Saw mills | ||
| Printing works | Chipboard manufacture (see NOTE) | ||
| Cable factories for PP/PE/PS or similar with burning characteristics other than OH3 | Paint, colour and varnish manufacture | ||
| Injection moulding (plastics) for PP/PE/PS or similar with burning characteristics other than OH3 | |||
| Plastics factories and plastic products (excluding foamed plastics) for PP/PE/PS or similar with burning characteristics other than OH3 | |||
| Rubber goods factories, synthetic fibre factories (excluding acrylic) | |||
| Rope factories | |||
| Carpet factories with unexpanded plastics | |||
| Shoe factories with plastics and rubber |
Note to the table: additional object protection may be required.
Three frequent confusions are resolved explicitly by this table: match manufacture is in HHP1, not HHP2. Tar distillation is in HHP2, not HHP4. Cellulose nitrate manufacture is in HHP3, not HHP4. Chipboard manufacture is in HHP2.
Pre-calculated systems: Table 7 (Clause 7.3.2, EN p.40)
Clause 7.3.2.1: the water supply shall be able to provide at the highest design point the appropriate flow and pressure specified in Table 7, or as modified by 7.3.2.2 to 7.3.2.5. The total requirement for the running pressure at the control valve set is the sum of the pressure at the design point, the pressure equivalent of the height difference between the design point and the control valve set, and the pressure loss for the flow in the pipework from the control valve set to the design point.
The first three density rows of Table 7 relevant to HHP (EN p.40):
| Section | Design density (mm/min) | Maximum demand flow, wet or pre-action (l/min) | Maximum demand flow, dry or alternate (l/min) |
|---|---|---|---|
| (1) Pipe sizes of Tables 32 and 33, K80 | 7.5 | 2 300 | 2 900 |
| (1) | 10.0 | 3 050 | 3 800 |
| (4) Pipe sizes of Tables 35 and 34, K115 | 12.5 | 3 800 | 4 800 |
Table 7 also gives the pressure at the highest design point (pd) for areas of operation per sprinkler of 6, 7, 8 and 9 m². For example, in section (1) for 7.5 mm/min, 1.80 bar for 8 m² and 2.25 bar for 9 m²; for 10.0 mm/min, 3.15 and 3.90 bar respectively. In section (3) (pipe sizes of Tables 35 and 34, K80) the same densities give 0.70 and 0.90 bar and 1.25 and 1.60 bar. Note to the table: where there are sprinklers in the array higher than the design point, the static head from the design point to the highest sprinklers should be added to pd.
Flow correction where the area changes
| Case | Provision | Clause |
|---|---|---|
| Where the area is smaller than the area of operation | The flow in Table 7 may be reduced proportionally, but the pressure at the highest design point for the area shall be equal to the value in the table or be determined by hydraulic calculation | 7.3.2.2 |
| Where there are fewer than 48 sprinklers | The flow in Table 7 and the appropriate pressure shall be available at the point where the sprinklers enter the HHP or HHS area, at the level of the highest sprinklers | 7.3.2.3 |
| Where the area of operation is larger than the HHP/HHS area and adjoins OH | The total flow is the sum of the flow for the HHP or HHS part, reduced proportionally in accordance with 7.3.2.2, and the flow for the OH part calculated on the basis of a design density of 5 mm/min | 7.3.2.4 |
| Where the area of operation is fed from more than one distribution pipe | The pressure at the level of the highest sprinklers of the design points shall be as in Table 7 or be determined by hydraulic calculation; the flow for each distribution pipe is determined proportionally | 7.3.2.5 |
Clauses 7.3.2.6 and 7.3.2.7 (EN p.41) express the proportional correction as a formula: Q2 = Q1 × a2 / a1, where Q1 is the flow in Table 7, a1 the area of operation for the design density (see Table 4) and a2 the required area of operation. The pressure at the design point remains unchanged.
Water supply duration and volume
Clause 8.1.1 (EN p.42) gives the minimum supply durations: LH 30 min, OH 60 min, HHP 90 min, HHS 90 min.
Table 10: Minimum water volume for pre-calculated HHP and HHS systems (Clause 9.3.2.2, EN p.48)
| Design density not exceeded (mm/min) | Wet systems (m³) | Dry systems (m³) |
|---|---|---|
| 7.5 | 225 | 280 |
| 10.0 | 275 | 345 |
| 12.5 | 350 | 440 |
| 15.0 | 425 | 530 |
| 17.5 | 450 | 560 |
| 20.0 | 575 | 720 |
| 22.5 | 650 | 815 |
| 25.0 | 725 | 905 |
| 27.5 | 800 | 1 000 |
| 30.0 | 875 | 1 090 |
The specified water volume is dedicated to the use of the sprinkler system alone.
For calculated systems, Clause 9.3.2.3 (EN p.48) takes a different route: the minimum effective water volume is calculated by multiplying the maximum demand flow (Qmax) by the duration specified in 8.1.1.
It should be noted that these two routes do not give the same result. Arithmetic check: for HHP1 the wet maximum demand flow in Table 7 is 2 300 l/min; multiplied by the 90 minute duration of 8.1.1 this gives 2 300 × 90 = 207 000 L, that is 207 m³. However, for the same density Table 10 requires 225 m³ in a pre-calculated system. In pre-calculated systems Table 10 is binding. On the dry side, 2 900 × 90 = 261 000 L = 261 m³, whereas Table 10 requires 280 m³.
The same check for HHP2: 3 050 × 90 = 274 500 L = 274.5 m³, and the wet value in Table 10 is 275 m³.
Sprinkler selection and pressure
- Table 37a (Clause 14.2.1, EN p.104): for HHP and HHS ceiling or roof sprinklers, conventional or spray type with K 80, 115 or 160 where the design density is up to 10 mm/min, and conventional or spray type with K 115 or 160 above 10 mm/min.
- Clause 14.2.2 (EN p.104): ceiling, flush, recessed and concealed type sprinklers shall not be installed in OH4, HHP or HHS areas.
- Clause 14.2.3 (EN p.104): sidewall sprinklers shall not be installed in HH installations; the exception is the protection of corridors, cable ducts and columns.
- Clause 13.4.4 (EN p.102): the minimum discharge pressure in HHP and HHS, excluding in-rack sprinklers, is 0.50 bar.
- Clause 14.3 (EN p.104-105): the water flow from a sprinkler is calculated with the relationship Q = K × √P, where Q is in l/min and P in bar, and K is the constant given in Table 37a.
Layout and installation size
- Table 19 (Clause 12.2, EN p.70): in HHP and HHS the maximum coverage area per sprinkler is 9.0 m², with S and D not more than 3.7 m in a standard arrangement and S and D not more than 3.7 m in a staggered arrangement.
- Clause 12.1.2 (EN p.70): in HHP and HHS a clear space of at least 1.0 m is maintained below the deflector of the roof and ceiling sprinklers.
- Table 17 (Clause 11.1.3, EN p.67): for HH the maximum protected area per control valve set, including the OH and LH sprinklers within it, is 9 000 m².
- Clause 6.1 (EN p.31): where there are areas with different hazard classification in open communication, the higher design criteria are extended into the area with the lower classification by at least two rows of sprinklers.
- Clause 20.1.4 (EN p.118-119): in HHP and HHS installations the number of spare sprinklers per system shall not be less than 36. The spare sprinklers, together with sprinkler spanners, are kept in a cabinet in a conspicuous and readily accessible position where the ambient temperature does not exceed 27 °C.
The distinction between HHP and HHS
HHP is a process hazard; HHS is a storage hazard and covers the storage of goods where the limits given in Clause 6.2.3 are exceeded (Clause 6.2.4.2, EN p.32). The classification routes for the two are different:
- The HHP classification follows from the type of occupancy: Clause 6.2.4.1 and Annex A, Table A.3.
- The HHS category (Category I-IV) follows from the material of the goods stored and from the storage configuration: the methodology of Annex B (EN p.127) and the alphabetical product list of Annex C (EN p.132).
Annex B is not used to determine an HHP group; it determines Category I-IV through the material factor (M1-M4). The only exception is the reference to Table B.1 in the HHP3 cell of Table A.3: the manufacture of foamed plastics, foam rubber and foam rubber products falls into HHP3 for those with material factor m3, while those with m4 are excluded.
Field note: in mixed premises (for example a factory with an HHP process in one part and HHS storage in another) the two classifications are carried out separately, and the two row extension rule of Clause 6.1 and the mixed flow calculation of 7.3.2.4 are applied together.
Frequently Asked Questions
What are the design density and area of operation for the HHP groups?
TS EN 12845+A2:2026 Table 3 (Clause 7.1, EN p.36): HHP1 7.5 mm/min, HHP2 10.0 mm/min, HHP3 12.5 mm/min; for all three the area of operation is 260 m² in a wet or pre-action system and 325 m² in a dry or alternate system. For HHP4 the table says deluge, and the note states that special consideration is required and that deluge systems are not within the scope of this standard.
Which occupancy falls into which HHP group?
Table A.3 (EN p.126). Examples: in the HHP1 column, oilcloth and linoleum manufacture, resin, lamp black and turpentine manufacture, rubber substitute manufacture, wood wool manufacture, match manufacturers, solvent paint spraying shops, refrigerator factories, printing works, rope factories; in the HHP2 column, lighter manufacture, tar distillation, garages for buses and empty lorries and railway wagons, candle and paraffin wax manufacturers, paper machine halls, carpet factories with rubber and foamed plastics, saw mills, chipboard manufacture, paint, colour and varnish manufacture; in the HHP3 column, cellulose nitrate manufacture, rubber tyres for cars and lorries, manufacture of foamed plastics, foam rubber and foam rubber products with material factor m3; in the HHP4 column, only firework manufacture.
How is the minimum water volume for HHP found?
In pre-calculated systems Table 10 (Clause 9.3.2.2, EN p.48) is used: for 7.5 mm/min, wet 225 m³ and dry 280 m³; for 10.0 mm/min, 275 and 345 m³; for 12.5 mm/min, 350 and 440 m³. In calculated systems, in accordance with Clause 9.3.2.3, the maximum demand flow Qmax is multiplied by the duration in 8.1.1; for HHP that duration is 90 minutes.
Which K factors may be used for HHP ceiling sprinklers?
Table 37a (Clause 14.2.1, EN p.104): for HHP and HHS ceiling or roof sprinklers, conventional or spray type with K 80, 115 or 160 where the design density is up to 10 mm/min, and conventional or spray type with K 115 or 160 above 10 mm/min. K factors above K160 are not covered by EN 12845-1.
What is the minimum discharge pressure at HHP sprinklers?
Clause 13.4.4 (EN p.102): 0.50 bar in HHP and HHS, excluding in-rack sprinklers. This value is compared with the pressure required to give the density specified in 13.4.1, and the higher of the two applies.
References
- TS EN 12845+A2:2026 Clause 6.1 (EN p.31)
- TS EN 12845+A2:2026 Clauses 6.2.4.1, 6.2.4.2 (EN p.32)
- TS EN 12845+A2:2026 Clause 7.1 (EN p.35), Table 3 (EN p.36)
- TS EN 12845+A2:2026 Clause 7.3.2.1, Table 7 (EN p.40)
- TS EN 12845+A2:2026 Clauses 7.3.2.2 - 7.3.2.7 (EN p.41)
- TS EN 12845+A2:2026 Clause 8.1.1 (EN p.42)
- TS EN 12845+A2:2026 Clause 9.3.2.2, Table 10, Clause 9.3.2.3 (EN p.48)
- TS EN 12845+A2:2026 Clause 11.1.3, Table 17 (EN p.67)
- TS EN 12845+A2:2026 Clauses 12.1.2, 12.2, Table 19 (EN p.70)
- TS EN 12845+A2:2026 Clause 13.4.4 (EN p.102)
- TS EN 12845+A2:2026 Clauses 14.2.1, 14.2.2, 14.2.3, Table 37a, Clause 14.3 (EN p.104-105)
- TS EN 12845+A2:2026 Clause 20.1.4 (EN p.118-119)
- TS EN 12845+A2:2026 Annex A introduction (EN p.124), Table A.3 (EN p.126)
- TS EN 12845+A2:2026 Annex B (EN p.127), Table B.1 (EN p.129), Annex C (EN p.132)
SprinkCalc — Fire Sprinkler Design Across Three Standards
SprinkCalc covers hazard classification, design density and area, K-factor selection, water demand and hydraulic calculations for NFPA 13, FM Global and BS EN 12845 in a single iOS app, and exports a professional PDF report.
Download SprinkCalc on the App Store
MEP Calc — 86+ Engineering Calculators
MEP Calc bundles 86+ engineering modules in one iOS app: 21 fire calculations plus heating, cooling, HVAC, plumbing, steam and natural gas.
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.