One of the most significant provisions of EN 12845-2:2024 is quiet but critical: ESFR and CMSA sprinklers may not be used in non-storage occupancies of hazard classes FH3, FH4 and FH5. In other words, you cannot rely on these technologies in manufacturing and process areas of those classes. This article explains the FH classification, the likely technical reasoning behind the prohibition, and the strict conditions that apply in FH1 and FH2.
1. Groundwork: ESFR, CMSA and CMDA
- ESFR (Early Suppression Fast Response): fast-response ceiling sprinklers to EN 12259-13. In high-risk storage, particularly plastics, they suppress the heat release rate of the fire very rapidly.
- CMSA (Control Mode Specific Application): sprinklers to prEN 12259-15. They operate in control mode, limiting fire growth, with final extinguishment completed by other means.
- CMDA (Control Mode Density/Area): conventional sprinklers to EN 12259-1. The most widely used approach, based on a minimum density in mm/min over a design area.
2. The New FH (Fire Hazard) Classification
The FH classes are set out in Annex A of EN 12845-2:2024, which is taken from the draft prEN 12845-1. The current EN 12845:2015 (including A1 and A2) still uses the LH/OH/HHP classes. The conversion is given in Table A.1 (example occupancies from EN 12845 Annex A, Tables A.1–A.3):
| Former class (EN 12845:2015) | New class (FH) | Scope |
|---|---|---|
| LH (Light Hazard), OH 1 | FH 1 | LH: certain areas of schools and offices, prisons. OH1: hospitals, hotels, restaurants, libraries, computer rooms, cement works. |
| OH 2, OH 3 | FH 2 | OH2: laboratories, laundries, car parks, museums, abattoirs, bakeries, metal working. OH3: glass factories, electronics factories, paper factories, department stores and shopping centres, woodworking. |
| OH 4, HHP 1 | FH 3 | OH4: cinemas and theatres, concert halls, exhibition halls, waste paper processing. HHP1: cable factories, plastics injection moulding, printing works, rubber goods, paint application shops with solvent. |
| HHP 2 | FH 4 | Paint, colour and varnish manufacture, paper machine halls, chipboard manufacturing, bus and lorry depots, candle wax and paraffin manufacture. |
| HHP 3 | FH 5 | Cellulose nitrate manufacture, rubber tyres for cars and lorries, foam plastics and foam rubber manufacture. |
HHP4 of EN 12845 (firework manufacture) does not appear in Table A.1.
3. The ESFR/CMSA Prohibition in FH3, FH4 and FH5 (Clause 6.2)
Clause 6.2 of EN 12845-2:2024 categorically prohibits the ESFR and CMSA concepts in FH3–FH5 (non-storage) occupancies. The standard does not state its reasons; the following is an engineering interpretation:
- Storage-specific technology: ESFR and CMSA were developed for high-ceiling warehouse fires. In a process area the fire spreads horizontally across surfaces and along machinery lines rather than up vertical flue spaces — a spread mechanism outside the storage fire tests on which the ESFR and CMSA design tables are based.
- Obstruction risk: machinery, large pipework, cable trays, ductwork and platforms in a process area seriously obstruct the discharge pattern. ESFR and CMSA are very sensitive to obstruction, so the likelihood that the system fails to suppress or control the fire is high.
- Over-activation: fast-response ESFR sprinklers can react to a widespread hot gas layer, so many sprinklers remote from the seat of the fire may open. The number of sprinklers assumed in the hydraulic design is then exceeded and the pressure drops.
4. Permitted in FH1 and FH2 — but on HHS1–HHS3 Criteria
Outside storage, ESFR and CMSA may only be used in FH1 and FH2. Even then Clause 6.2 imposes a deliberate margin: irrespective of the actual fire load and storage category in the space, the design must follow the high hazard storage criteria for HHS1–HHS3 (clause 6.6 or 6.7). The standard gives no reason; a plausible one is to cover the local accumulations of material and irregular stacking that can arise in a non-storage area. In practice this means, for example, a Table 7 design with at least 12 sprinklers operating at the end pressures required for the chosen K-factor, together with all ESFR/CMSA installation requirements: spacing, obstructions and roof slope.
5. Comparative Summary: ESFR vs CMSA vs CMDA
| Criterion | ESFR | CMSA | CMDA |
|---|---|---|---|
| Operating mode | Suppression — rapidly suppresses the heat release rate. | Control — limits the rate of heat release. | Conventional control — density and area design. |
| Permitted classes (FH) | FH1–FH2 only. Prohibited in FH3, FH4, FH5. | FH1–FH2 only. Prohibited in FH3, FH4, FH5. | Permitted across FH1–FH5. |
| Process limitations | Very sensitive to obstruction and air movement; high risk of failure. | Affected by obstruction; a dry system option exists. | Robust; tolerant of process obstructions. |
| Design approach | K-factor, end pressure, mostly 12 sprinklers (9 to 20 in Tables 10–11). | K-factor, minimum end pressure, defined operating count (12 to 36). | Design area multiplied by density. |
Frequently Asked Questions
In which hazard classes are ESFR and CMSA prohibited under EN 12845-2?
Under Clause 6.2 of EN 12845-2:2024, ESFR and CMSA protection concepts may not be applied in FH3, FH4 or FH5 occupancies. The standard gives no reason; from an engineering standpoint, in these process-dominated classes the fire can spread horizontally across surfaces and machinery lines rather than through storage flue spaces, which lies outside the storage tests behind the ESFR and CMSA tables. The density/area (CMDA) design of EN 12845 is used instead.
Is ESFR prohibited in high hazard storage?
No, and this is the most common misreading of the clause. ESFR and CMSA were developed specifically for high hazard storage, and EN 12845-2 protects storage classes HHS1 to HHS5 with them. The prohibition applies to non-storage process occupancies, which is why the official title of Clause 6.2 is Protection of non-storage occupancies with ESFR and CMSA.
How do the new FH classes map onto the former EN 12845 classes?
Under EN 12845-2 Table A.1, LH and OH1 become FH1, OH2 and OH3 become FH2, OH4 and HHP1 become FH3, HHP2 becomes FH4, and HHP3 becomes FH5. The FH classes are taken from the draft prEN 12845-1.
Why do ESFR and CMSA fail in process areas?
The standard gives no reason, but three engineering reasons stand out. First, the technology was developed for high-ceiling storage fires, whereas process fires can spread horizontally. Second, machinery, pipework, cable trays and ductwork obstruct the discharge pattern. Third, fast-response sprinklers can over-activate under a widespread hot gas layer, opening many sprinklers remote from the fire and reducing the pressure.
If ESFR or CMSA is used in FH1 or FH2, how is it designed?
It is permitted, but Clause 6.2 imposes a strict margin: regardless of the actual fire load and storage category, the design must follow the high hazard storage criteria for HHS1 to HHS3 (clause 6.6 or 6.7). In practice that means, for example, at least 12 sprinklers operating together at the end pressures required for the selected K-factor under Table 7, plus all ESFR/CMSA installation requirements: spacing, obstructions and roof slope.
What sprinkler approach should be used in FH3 to FH5 areas?
For medium and high hazard industrial processes, use the conventional CMDA density and area design of EN 12845, supplemented by a purpose-designed deluge system where the risk demands it. CMDA may be used across all classes from FH1 to FH5.
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Download MEP Calc on the App StoreTS EN 12845-2:2025 (EN 12845-2:2024), Fixed Firefighting Systems — Automatic Sprinkler Systems — Part 2: Design and Installation of ESFR and CMSA Sprinklers (Clause 6.2, Clauses 6.6 and 6.7, Annex A Table A.1). TS EN 12845:2015+A2:2026, Design, Installation and Maintenance. Values are for information; the full text of the current standard governs any actual design.