The 12 bar ceiling, town main service connection rules, backflow prevention and what zoning means on a tall building.
Called to commission the sprinklers in a 28-storey mixed-use building, the job had already gone wrong. Pump closed valve pressure was 14.8 bar; the gauge on the basement wet alarm valve swung around 13 bar; the sprinkler manufacturer had withdrawn its certificate and the insurer would not accept the system. The cause was obvious: the design had run a 90 m vertical riser as a single zone. The simplest rule in EN 12845 — do not exceed 12 bar at the equipment — is the one most often broken on site. This article opens the water supply clauses from a site perspective.
What the clause says
The water supply must automatically provide the pressure and flow conditions the system requires. The minimum durations by hazard class are clear:
- LH: 30 minutes
- OH: 60 minutes
- HHP and HHS: 90 minutes
For a town main and inexhaustible sources, that duration is treated as indirectly satisfied; with a tank and pump, the tank is sized to meet it.
The continuity clause is the least read but the one insurance surveyors ask about most: the water supply must be protected against freezing, drought, flooding and anything else that reduces flow. Salty or dirty water is not permitted in a sprinkler main except under specified conditions. The feed pipe and control valve set are kept at a minimum of 4 °C.
The 12 bar ceiling
Outside test conditions, static pressure must not exceed 12 bar at:
- Sprinkler heads
- Multiple jet control devices
- Water flow detectors
- Dry and pre-action alarm valves
- Accelerators and exhausters
- Water motor alarms
- Zone control valves
The calculation must include the increase arising from the pump's closed valve pressure and driver speed tolerance. Field practice: a pump rated at 10 bar can reach 20–30 % above that at closed valve, so a static peak of 12–13 bar is normal. Treating it like the duty point pressure is a mistake.
Where the vertical difference between the highest and lowest sprinkler does not exceed 45 m, pressure may exceed 12 bar at the pump discharge, wet alarm valve, stop valve and mechanical pipe connections — but only at those positions, never at the sprinkler head itself.
Tall buildings: the 45 m rule and zoning
The one exception for high-rise buildings: where the level difference between the highest and lowest sprinkler exceeds 45 m, the pump discharge, riser and distribution pipework may exceed 12 bar — provided all equipment exposed to that pressure is rated for it. For sprinkler heads and the listed equipment, the 12 bar limit never flexes.
Field practice is clear: above 45 m, floor-group zoning is used. Since 1 m of water column is about 0.098 bar, a 90 m height gives roughly 8.8 bar of hydrostatic pressure alone. Add the closed valve condition and holding 12 bar on a single zone is physically impossible. A typical arrangement:
| Building height | Zones | Level difference per zone | Typical solution |
|---|---|---|---|
| 0–45 m | 1 | ≤ 45 m | Single pump, direct distribution |
| 45–90 m | 2 | about 40 m | Pressure reducing valves or an intermediate tank with a booster on the upper zone |
| 90–150 m | 3 | about 40 m | Intermediate tanks in sequence |
| Above 150 m | 4 or more | about 40 m | An independent pump station per zone |
A pressure reducing valve alone is not a solution; if it fails, the lower zone again exceeds 12 bar. The common field interpretation: a reducing valve plus a relief valve, or preferably a mechanically separated intermediate tank with its own pump.
The service connection
How much water may be drawn from the sprinkler main for other services — hose reels, hydrants, industrial use — is limited. The key conditions:
- The connection is made upstream of and as close as practicable to the control valve set, with a separate stop valve.
- It is permitted only where the sprinkler system is not protecting a high-rise building.
- It is permitted only where the sprinkler system is not protecting a multi-storey building.
- Sprinkler pumps must be separate from hydrant pumps, except under the combined supply provisions.
The permitted sizes:
| Water supply type | Permitted connection |
|---|---|
| Town main, main ≥ 100 mm | One connection, ≤ 25 mm, non-industrial use |
| Town main, main ≥ 150 mm | One connection, ≤ 40 mm non-industrial; or ≤ 50 mm for hose reels plus a further 40 mm non-industrial |
| Elevated private reservoir, gravity tank, automatic pump | One connection, ≤ 50 mm, hose reels only |
The note allowing an additional supply arrangement with a check valve for the fire service is built in Turkey as the fire service inlet connection; it is a separate circuit feeding the sprinkler main and falls outside these size limits.
Backflow prevention: when and which class?
EN 12845 requires a backflow prevention device wherever the sprinkler system could create a contamination risk — foam concentrate, antifreeze, dirty process liquid. The standard explicitly prohibits salt and calcium chloride solutions used against freezing.
Where the supply is from a main, the local water authority is the deciding factor. Turkish utilities require an EN 1717 / EN 12729 type RP or EA backflow preventer between the potable main and the fire main. In practice:
- Fluid category 3 (a health risk, not lethal): type EA, a controllable check valve.
- Fluid category 4 (a health risk that can be lethal — antifreeze or foam additive): type RP, a reduced pressure zone device.
- Fluid category 5 (microbiological): an air gap (type AA or AB) — in practice a break tank.
The fluid category depends on what is added to the sprinkler water; with antifreeze or AFFF/F3 foam it does not fall below category 4, and a single check valve in the line is then insufficient.
Allowing for main pressure fluctuation
The water supply test must be carried out at the system's maximum demand flow. On a town main supply, the test result is not a single day's or hour's measurement; the reference is the annual minimum guaranteed pressure-flow curve from the utility. Our working rule:
- Obtain a written guaranteed pressure from the water authority, for both day and night minima.
- Apply at least a 0.5 bar safety margin below the guaranteed value.
- Verify the hydraulic calculation against that reduced value; the system demand curve must sit below it.
- Where the main swings by more than 1 bar hour to hour, do not accept it as a suitable supply on its own; a tank and pump become mandatory as a superior or duplicate supply.
Across much of Turkey, guaranteed main pressure is given in the 1.5–2.5 bar band. For an OH3 shopping centre that is not adequate as a sprinkler supply; a tank with a diesel or electric pump station is the standard arrangement.
Common errors
- A single zone in a tall building: one pump and one riser in a 60 m building. The basement equipment sees 14–15 bar, the sprinkler listing is void and the insurer will not sign.
- Ignoring closed valve pressure: the calculation says 9 bar at the duty point, but shut-off is 12.5 bar — a breach.
- Taking a hose reel off downstream of the sprinkler control valve: the requirement is a stop valve upstream of the control valve set; otherwise operating the hose reel triggers a sprinkler alarm.
- A single check valve on an antifreeze-dosed main: fluid category 4 requires a type RP device; a plain swing check does not qualify.
- Calculating on daytime main pressure: a design built on the 3.5 bar seen during the day fails when it drops to 1.8 bar at night.
- Combining sprinkler and hydrant pumps in a tall building: separate pumps are explicitly required.
Comparison with NFPA
NFPA 13 sets an upper limit of 12.1 bar (175 psi) for sprinkler equipment — practically identical to the EN 12845 figure of 12 bar. Products listed for 17.2 bar (250 psi) exist on the NFPA side, and 16 bar PN rated sprinklers can be used similarly on special European projects, but that is not the default design threshold. NFPA 24 leaves backflow prevention to the local health authority, with double check detector and reduced pressure detector assemblies common. EN 1717 introduces a fluid-category classification, which is stricter and clearer when used alongside EN 12845.
Turkish context
BYKHY regulates sprinkler systems largely by reference to EN 12845. Its high-rise definition — above 21.5 m — triggers zoning far earlier than the EN 12845 45 m threshold, so additional pump and zone logic comes into play under BYKHY first. Water utility fire main specifications require an EA type backflow preventer for potable water protection, moving to RP where the sprinkler main carries a foam additive. In practice, an RP device immediately downstream of the utility's service valve satisfies both the standard and the local authority.
Frequently asked questions
What is the maximum static pressure permitted at sprinkler equipment?
12 bar at sprinklers, flow detectors, dry and pre-action alarm valves, accelerators, water motor alarms and zone control valves. The increase from closed valve conditions is included in that calculation.
How is the 12 bar limit maintained in a tall building?
Where the difference between the highest and lowest sprinkler exceeds 45 m, zoning is required. In practice a new zone starts roughly every 40 m, with pressure reducing valves or an intermediate tank and booster keeping pressure below 12 bar.
What does the standard say about feeding hydrants from the sprinkler main?
The connection may be taken upstream of the control valve set through a separate stop valve, within the tabulated size and purpose limits, provided the system is not protecting a high-rise or multi-storey building. Otherwise separate pumps are mandatory.
Where is a backflow preventer mandatory?
Wherever there is a contamination risk — foam, antifreeze and similar. On a town main supply, an EA or RP device is selected by EN 1717 fluid category, and Turkish utilities typically require it.
Which pressure is used when the main fluctuates?
The annual minimum guaranteed pressure with at least a 0.5 bar safety margin. Where the fluctuation exceeds 1 bar, the main is not acceptable as a supply on its own and a tank with pumps becomes mandatory.
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Download MEP Calc on the App StoreBS EN 12845:2015+A1:2019 Fixed firefighting systems — Automatic sprinkler systems. EN 12845-2:2024 (CMSA & ESFR sprinkler systems). NFPA 13 Standard for the Installation of Sprinkler Systems. Turkish Regulation on Fire Protection of Buildings (BYKHY). FM Global Property Loss Prevention Data Sheet 2-0.