EN 12845-2 ESFR/CMSA Design and Minimum Operating Pressures (FH1/FH2)

The previous article covered why ESFR and CMSA are prohibited in FH3 to FH5. This one moves to the numbers for the classes where they are permitted, FH1 and FH2: the K-factor, ceiling and storage height, and minimum operating pressure tables of EN 12845-2:2024, together with the wet and dry pipe options and the special-condition design route.

1. FH Classification and Applicability

The governing rule (EN 12845-2 clause 6.2): in non-storage occupancies ESFR and CMSA may be used only in FH1 and FH2, and they must be designed as for HHS1–HHS3 (clause 6.6 or 6.7) regardless of the actual commodity category stored. They cannot be applied in FH3 to FH5. The conversion from the former EN 12845 classes is given in Annex A, Table A.1:

2. ESFR Ceiling-Only Design: K-Factors and Minimum Operating Pressures (Table 7)

ESFR may only be used on wet pipe systems. The design is set by ceiling or roof height, storage height, K-factor and minimum operating pressure — and interpolation and extrapolation are prohibited (clause 6.1). The HHS1–HHS3 values below are taken from Table 7 and are based on at least 12 ceiling sprinklers operating. Upright ESFR is permitted only with K200 and K240, up to a 10.7 m ceiling; the upright pressures are the same as the pendent K200/K240 values.

The 15.2 m and 16.8 m rows (K400/K480) exclude multiple-row racks, and for K400 the ceiling-to-deflector distance is limited to 325 mm (Table 7, footnotes c and d). US equivalents of the K-factors: K160 (K11.2), K200 (K14.0), K240 (K16.8), K280 (K19.6), K320 (K22.4), K360 (K25.2), K400 (K28.0), K480 (K33.6).

3. Special-Condition ESFR Design (9 or 10 Sprinklers)

Where all the additional requirements of clause 6.6.1.2 are met — the horizontal airflow rule (5.2.6.1) is applied without exception, obstructions above racks follow the approach of 5.3.3.2.2, the design area is at least 71 m², and the ceiling is non-combustible and preferably unobstructed — Table 10 allows the calculation at certain heights to be based on 9 sprinklers instead of 12 (the HHS4 and HHS5 rows also contain 10, 15 and 20 sprinkler designs). This gives a substantial saving in tank volume and pump capacity. HHS1–HHS3 values:

4. CMSA Ceiling-Only Design Criteria

CMSA operates in control mode and, unlike ESFR, may also be used on dry pipe systems — where the number of operating sprinklers increases to account for water delivery delay. The values below are for STC1 storage of HHS1/HHS2 commodities (Table 15):

Note: under clause 6.1, HHS3 protection is used for HHS1 and HHS2 unless a table specifies otherwise, and clause 6.2 ties non-storage FH1/FH2 areas to an HHS1–HHS3 design. The HHS3 values are in Table 16; for example, 7.6 m storage under a 9.1 m ceiling requires 15 K240 upright sprinklers at 1.5 bar. For 7.6 m storage the HHS3 table has no K160/K240 upright option for a 10.7 m ceiling.

5. Critical Hydraulic and Layout Limits

• Water supply duration (Table 6): depends on the number of ceiling sprinklers in the design — 60 minutes for 9, 10 and 12 sprinklers, 90 minutes for 15 and 16, and 120 minutes for 20 to 30. CMSA protection of roll paper requires 120 minutes and of rubber tyres 180 minutes. Combined ceiling and in-rack designs (clause 6.6.2.1.1) require at least 60 minutes.
• Roof or ceiling slope (clause 5.2.1): a maximum of 10° (17.6 %). Beyond that there are three options: a flat, continuous false ceiling over the affected area and two sprinkler lines beyond it in every direction; for rack-only storage, in-rack sprinklers to 6.6.2.1 plus an additional in-rack level at the top of storage; or a design to EN 12845.
• Sprinkler spacing (clauses 5.1.3.2 / 5.1.3.3): for ESFR the minimum distance between sprinklers is 2.4 m, the maximum 3.7 m for ceilings up to 9.1 m and 3.1 m above 9.1 m, with 6–9.3 m² per sprinkler. For CMSA the minimum is 2.4 m and the maximum 3.7 m (3.1 m for combustible obstructed construction), with 7.5–9.3 m² per sprinkler (12 m² in certain STC1 cases).
• Favourable area (clause 6.4): a favourable design area calculation is not required, and no pipe diameter reduction to limit the favourable area demand is allowed.
• Aisle width (clause 6.1): loading aisles at least 1.2 m wide; the 15.2 m and 16.8 m rows of Table 7 require 1.8 m or 2.4 m, and exposed expanded plastics (HHS5) require 2.4 m.

Frequently Asked Questions

Can you interpolate within the ESFR design tables?

No. Interpolation between values and extrapolation beyond them are both prohibited in the EN 12845-2 ESFR tables. Ceiling height, storage height, K-factor and minimum operating pressure must be taken directly from the table. For an intermediate case, use the next row up (higher ceiling): the criteria for a given ceiling height may be applied to a lower ceiling provided no other parameter changes (clause 6.1).

Are ESFR and CMSA used on wet or dry systems?

ESFR may only be used on wet pipe systems (clause 6.5). CMSA can be used on both wet and dry pipe systems, but on a dry system the number of operating sprinklers is increased to allow for water delivery delay - for example a K160 upright requires 15 operating sprinklers wet and 25 dry.

What are typical minimum ESFR operating pressures in FH1 and FH2?

On an HHS1-HHS3 ceiling design, for a 9.1 m ceiling with 7.6 m storage the values are K200 at 3.4 bar, K240 at 2.4 bar, K320 at 1.7 bar and K360 at 1.0 bar. At a 10.7 m ceiling the pressures rise, with K200 reaching 5.2 bar. Upright ESFR is limited to K200/K240 up to a 10.7 m ceiling; from 12.2 m only pendent sprinklers are used, and at 15.2 m and 16.8 m only K400/K480 options exist. Table 7 is based on at least 12 ceiling sprinklers operating together.

When can an ESFR design use 9 sprinklers instead of 12?

Through the special-requirements design of clause 6.6.1.2: the ceiling must be non-combustible, the horizontal airflow rule (5.2.6.1) and the above-rack obstruction rule (5.3.3.2.2) must be applied, and the design area must be at least 71 m2. Table 10 then allows the hydraulic calculation at certain heights to be based on 9 sprinklers rather than 12 - for example K240 at a 7.6 m ceiling with 9 sprinklers at 1.7 bar. This produces a significant saving in tank volume and pump capacity.

What are the roof slope and duration limits for ESFR and CMSA systems?

The maximum roof or ceiling slope is 10 degrees (17.6 %); beyond that a flat false ceiling, an in-rack solution or a design to EN 12845 is required (clause 5.2.1). The water supply duration depends on the number of sprinklers in the design (Table 6): 60 minutes for 12 sprinklers, 90 minutes for 15 to 16, and 120 minutes for 20 or more. The minimum distance between ESFR sprinklers is 2.4 m.

Why must ESFR and CMSA be designed to HHS1-HHS3 even in a light hazard building?

Clause 6.2 requires it, independent of the storage category. The standard does not state its reasoning, but a non-storage area can still accumulate material locally in ways the designer cannot control - irregular stacking, temporary storage, packaging left in place. Requiring these storage class criteria builds in a margin so that the installed system remains valid if the space is used more heavily than the nominal hazard class suggests.

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