One of the most significant changes in EN 12845-2:2024 is quiet but critical: ESFR and CMSA sprinklers may no longer be used in FH3, FH4 and FH5 hazard classes. In other words, you cannot rely on these technologies in manufacturing and process areas. This article explains the new FH classification, the 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 an assumed area of operation.
2. The New FH (Fire Hazard) Classification
The latest revision of EN 12845 converted the hazard classes into an FH matrix aligned with modern risks. The former-to-new mapping:
| Former class (EN 12845:2015) | New class (FH) | Scope |
|---|---|---|
| LH (Light Hazard) | FH 1 | Low fire load and combustibility (schools, prisons, office areas). |
| OH 1 – OH 2 | FH 2 | Moderate fire load, industrial production and commerce (glass, ceramics, abattoirs). |
| OH 3 – OH 4 | FH 3 | Higher fire load ordinary hazard processes (paper mills, retail stores). |
| HHP 1 – HHP 2 | FH 4 | Production where fire spreads very rapidly (paint works, cable factories). |
| HHP 3 | FH 5 | The most severe processes (plastic injection moulding, foamed plastics, rubber). |
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 process) volumes. The reasons are:
- 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 flues — a spread mechanism these sprinklers were never tested against.
- Risk of system failure: machinery, large pipework, cable trays, ductwork and platforms in a process area seriously obstruct the discharge pattern. With those obstructions the probability of total system failure is very high.
- Over-activation: fast-response ESFR heads react to sudden hot gas surges, so a large number of sprinklers remote from the seat of the fire can open simultaneously. Pressure collapses, the system falls outside its hydraulic envelope and the fire is not suppressed.
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 in the space, the design must follow the criteria for the highest storage classes, HHS1–HHS3. Why? To absorb the local accumulations of material, irregular stacking and ceiling-height-dependent risks that can arise in a non-storage area. In practice this means a design in which at least 12 sprinklers operate simultaneously at the end pressures required for the chosen K-factor, together with full compliance with the installation, bracing, spacing and roof slope rules.
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, head pressure, 12 sprinklers. | K-factor, minimum head pressure, defined operating count. | Area of operation 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. In these process-dominated classes the fire spreads horizontally across surfaces and machinery lines rather than through storage flues, and ESFR and CMSA have not been tested against that spread mechanism. Conventional CMDA or deluge systems are 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?
LH becomes FH1, OH1 and OH2 become FH2, OH3 and OH4 become FH3, HHP1 and HHP2 become FH4, and HHP3 becomes FH5. The FH matrix was introduced to classify hazard more precisely and in line with modern fire risks, particularly the plastics content of stored and processed goods.
Why do ESFR and CMSA fail in process areas?
There are three main reasons. First, the technology was developed for high-ceiling storage fires, whereas process fires spread horizontally. Second, machinery, pipework, cable trays and ductwork obstruct the discharge pattern, and with those obstructions the risk of total system failure is high. Third, fast-response heads can over-activate in sudden hot gas surges, opening many sprinklers remote from the fire, collapsing pressure and taking the system outside its hydraulic design envelope.
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 present, the design must follow the criteria for the highest storage classes, HHS1 to HHS3. In practice that means at least 12 sprinklers operating simultaneously at the end pressures required for the selected K-factor, plus full compliance with the installation, spacing, bracing and roof slope rules.
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 defined in EN 12845-1, or a purpose-designed deluge system where flame spread is extremely rapid. CMDA may be used freely across all classes from FH1 to FH5.
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Download MEP Calc on the App StoreTS 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.