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: ESFR and CMSA may be used only in FH1 and FH2, and they must be sized on an HHS1–HHS3 basis regardless of the actual commodity category stored. They cannot be applied in FH3 to FH5.
| Former class | FH class | ESFR / CMSA applicability |
|---|---|---|
| LH | FH 1 | Applicable — sized on HHS1–HHS3 design criteria. |
| OH1–OH2 | FH 2 | Applicable — sized on HHS1–HHS3 design criteria. |
| OH3–OH4 | FH 3 | Not applicable. |
| HHP1–HHP2 | FH 4 | Not applicable. |
| HHP3 | FH 5 | Not applicable. |
2. ESFR Ceiling Sprinklers: K-Factors and Minimum Operating Pressures
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. The HHS1–HHS3 based table below is generally calculated on the assumption that the 12 most remote ceiling sprinklers operate simultaneously:
| Max. ceiling (m) | Max. storage (m) | Orientation | K-factor and minimum pressure |
|---|---|---|---|
| 7.6 m | 6.1 m | Pendent / upright | K200: 3.4 bar · K240: 2.4 bar · K320: 1.7 bar · K360: 1.0 bar |
| 9.1 m | 7.6 m | Pendent / upright | K200: 3.4 bar · K240: 2.4 bar · K320: 1.7 bar · K360: 1.0 bar |
| 10.7 m | 9.1 m | Pendent / upright | K200: 5.2 bar · K240: 3.6 bar · K320: 2.4 bar · K360: 1.4 bar |
| 12.2 m | 10.7 m | Pendent only | K240: 3.6 bar · K320: 2.8 bar · K360: 1.7 bar |
| 13.7 m | 12.2 m | Pendent only | K320: 2.8 bar · K360: 2.8 bar |
| 15.2 m | 13.3 m | Pendent only | K400: 2.8 bar · K480: 3.8 bar (minimum 1.8 m aisle) |
| 16.8 m | 14.9 m | Pendent only | K400: 5.5 bar (minimum 2.4 m aisle) · K480: 3.8 bar (minimum 1.8 m aisle) |
3. Special-Condition ESFR Design (9 or 10 Sprinklers)
Where the ceiling is of non-combustible construction and there is no obstruction in front of the sprinkler umbrella pattern, the calculation at certain heights may be based on 9 or 10 sprinklers instead of 12 — a substantial saving in tank volume and pump capacity:
| Max. ceiling | Max. storage | K200 | K240 | K320 | K360 |
|---|---|---|---|---|---|
| 7.6 m | 6.1 m | 9 sp @ 2.4 bar | 9 sp @ 1.7 bar | 9 sp @ 1.4 bar | 9 sp @ 1.4 bar |
| 9.1 m | 7.6 m | 12 sp @ 3.5 bar | 12 sp @ 2.4 bar | 9 sp @ 1.4 bar | 9 sp @ 1.4 bar |
| 10.7 m | 9.1 m | Not applicable | Not applicable | 12 sp @ 2.0 bar | 12 sp @ 1.6 bar |
| 12.2 m | 10.7 m | 12 sp @ 5.2 bar | 12 sp @ 3.6 bar | 9 sp @ 3.5 bar | 9 sp @ 2.8 bar |
4. CMSA Ceiling 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 table below is for STC1 block storage in FH1/FH2 on an HHS1/HHS2 basis:
| Max. storage (m) | Max. ceiling (m) | K-factor and orientation | System | Operating sp. | Min. pressure |
|---|---|---|---|---|---|
| 7.6 m | 9.1 m | K280 pendent | Wet | 15 | 1.1 bar |
| 7.6 m | 9.1 m | K360 pendent | Wet | 15 | 0.7 bar |
| 7.6 m | 9.1 m | K360 upright | Wet | 12 | 1.4 bar |
| 7.6 m | 10.7 m | K160 upright | Wet | 15 | 1.7 bar |
| 7.6 m | 10.7 m | K160 upright | Dry | 25 | 1.7 bar |
| 7.6 m | 10.7 m | K240 upright | Wet | 15 | 1.0 bar |
| 7.6 m | 10.7 m | K240 upright | Dry | 25 | 1.0 bar |
| 7.6 m | 12.2 m | K360 pendent | Wet | 15 | 1.6 bar |
5. Critical Hydraulic and Layout Limits
- System duration: the minimum water supply duration is 90 minutes; in combined ceiling and in-rack designs it may not fall below 60 minutes.
- Roof or ceiling slope: a maximum of 10 degrees. Beyond that, a flat suspended ceiling or a combination of ceiling and in-rack sprinklers becomes mandatory.
- Sprinkler spacing: for ESFR the minimum spacing between heads is 2.4 m. Maximum spacing is 3.7 m for ceilings up to 9.1 m, reducing to 3.1 m above 9.1 m.
- Pipe reduction: reduction in the direction of flow is permitted, but the minimum feed diameters for critical branch lines and ceiling sprinklers must be respected.
- Aisle width: a minimum of 1.2 m for HHS3, increasing to 1.8 m or 2.4 m for HHS4 and for the special high-level designs above 15.2 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, and any intermediate case is rounded up to the next safe row.
Are ESFR and CMSA used on wet or dry systems?
ESFR may only be used on wet pipe systems. 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. Above 12.2 m only pendent heads and larger K-factors are permitted. The calculation is normally based on the 12 most remote ceiling sprinklers operating together.
When can an ESFR design use 9 sprinklers instead of 12?
Through the special-condition design route, provided the ceiling is of non-combustible construction and there is no obstruction in front of the sprinkler umbrella pattern. At certain heights the hydraulic calculation may then be based on 9 or 10 heads 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; beyond that a flat suspended ceiling or a ceiling plus in-rack combination is required. The minimum water supply duration is 90 minutes, and may not fall below 60 minutes in a combined ceiling and in-rack design. The minimum spacing between ESFR heads is 2.4 m.
Why must ESFR and CMSA be designed to HHS1-HHS3 even in a light hazard building?
Because a non-storage area can still accumulate material locally in ways the designer cannot control - irregular stacking, temporary storage, packaging left in place. Requiring the highest storage class criteria builds in a deliberate margin so that the installed system remains valid if the space is used more heavily than the nominal hazard class suggests.
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 12845-2:2024, Fixed Firefighting Systems — Automatic Sprinkler Systems — Part 2: Design and Installation of ESFR and CMSA Sprinklers. 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.