EN 12845 Minimum Sprinkler Discharge Pressure

What the minimum discharge pressure clause actually says, where the most unfavourable sprinkler sits, and why the floors differ for LH, OH, HHP, HHS and in-rack heads.

On a logistics project working under OH3 racking, I once saw a hydraulic report showing 0.28 bar at the end-of-branch sprinklers with the note "the density is satisfied, no problem". There was a problem. EN 12845 treats meeting the density as the minimum requirement, then sets a separate absolute pressure floor beneath it. A K80 sprinkler may well achieve the density at 0.28 bar; the standard does not accept it. For OH the floor is 0.35 bar, and the calculation has to meet it.

This article opens the EN 12845 minimum sprinkler discharge pressure rule alongside field practice: which figure applies to which class, exactly where the "most unfavourable sprinkler" is, why in-rack pressures rise, and where it diverges from NFPA 13.

What the clause says

In summary: when all the sprinklers in the area of operation are discharging simultaneously, the pressure measured at the hydraulically most unfavourable sprinkler must be no less than either the pressure needed to achieve the required density, or the fixed values below — whichever is higher.

Hazard class	Typical K-factor	Absolute minimum pressure
LH (light hazard)	K57	0.70 bar
OH1, OH2, OH3, OH4	K80	0.35 bar
HHP and HHS (excluding in-rack)	K115 / K160	0.50 bar
HHS in-rack — K115	K115	1.00 bar
HHS in-rack — K80	K80	2.00 bar

The first three rows are the numerical floor set by the clause, separate from the density calculation. The last two are stated specifically for in-rack heads; a related note also sets 2.0 bar as the minimum for any operating sprinkler, which corresponds in practice to the K80 in-rack case.

Where the "most unfavourable sprinkler" is

This gets confused on site: the minimum pressure is not the average across the area of operation, but the value measured at the most remote and highest sprinkler within it. Typically:

• The end sprinklers on the branch furthest from the alarm valve.
• In multi-storey buildings or where load levels vary, the sprinkler at the highest point.
• On looped and gridded systems, the sprinkler falling within the "most unfavourable area" definition.

In the hydraulic model you mark that head as the design point. If the calculation shows 0.9 bar somewhere else but 0.32 bar at the design point, an OH report is rejected — because the standard says the weakest pressure in the area cannot fall below the floor.

Absolute minimum versus average

Two separate requirements must both be satisfied:

1. Density requirement: the average density over the area of operation — 5.0 mm/min for OH3, for instance — must be achieved.
2. Absolute pressure floor: pressure at the most unfavourable sprinkler must not fall below the class floor.

Neither overrides the other; the higher of the two governs. A K115 head might achieve the OH3 density at 0.30 bar, but because the floor is 0.35 bar the calculation is lifted to 0.35 bar. In LH the gap is wider still: a K57 head can achieve 2.25 mm/min at roughly 0.15 bar, but the LH floor is 0.70 bar — some 4.5 times the density pressure. That is the practical floor EN 12845 sets to guarantee droplet quality from the small K-factor chosen for light hazard.

Why in-rack pressures are so much higher

HHP and HHS ceiling sprinklers share a 0.5 bar floor, while intermediate in-rack heads sit markedly higher: 1.0 bar for K115 and 2.0 bar for K80. The reason is what an in-rack head has to do — suppress a fire inside the racking by driving droplets down between palletised masses. At low pressure the spray is fine and dispersed, sits on top of the pallet and never reaches the layers below. The standard raises the minimum pressure to guarantee that droplet momentum.

A related note also requires in-rack heads and the ceiling sprinklers associated with them to be fully calculated in every case, observing the 2.0 bar floor for any operating sprinkler. This matters particularly in ST4–ST6 high-bay rack design.

Common field errors

The errors I see most often in hydraulic reports:

• Forgetting the end-of-branch head. The calculation shows "0.5 bar average" and approval is sought — while the design point head sits at 0.28 bar. Check the weakest sprinkler in the area of operation.
• Assuming density is enough. A larger K-factor may achieve the density at low pressure; the absolute floor still binds.
• Choosing a small K-factor to drop the pressure. Achieving LH density at 0.3 bar with a K57 head does not qualify; the sprinkler must be lifted to 0.7 bar, which feeds back into pipe sizing and pump head.
• Calculating in-rack and ceiling heads together. In-rack heads meet a 1.0 or 2.0 bar floor; ceiling heads meet 0.5 bar. A combined calculation must observe both.
• Forgetting static head. In a multi-storey building the floor must be met at the highest sprinkler; the gauge at the valve outlet is misleading.

Comparison with NFPA 13

NFPA 13 is structured differently. It sets a general absolute minimum operating pressure of 7 psi (about 0.48 bar) for standard spray sprinklers, with far higher figures in storage and ESFR applications — for example 50 psi (3.4 bar) for ESFR K14. EN 12845 instead uses fixed values differentiated by hazard class:

Class / application	EN 12845 minimum	NFPA 13 approach
Light hazard	0.70 bar	7 psi (about 0.48 bar) general floor
Ordinary hazard	0.35 bar	7 psi (about 0.48 bar) general floor
Extra hazard process	0.50 bar	Higher values driven by the density/area curve
Storage	0.50 bar at ceiling, 1–2 bar in-rack	Varies by commodity and rack type; 50+ psi for ESFR

The practical consequence: designing the same building to NFPA 13, your end-of-branch pressure for OH must not fall below 7 psi — slightly higher than the European figure — while for light hazard NFPA allows 0.48 bar where EN 12845 lifts you to 0.70 bar. Rather than asking which is more conservative, the correct framing is: which standard did you choose, and the floor follows from it. Where BYKHY applies, EN 12845 is the binding reference.

How to verify the calculation

A short check list before issuing the hydraulic report:

1. Mark the most remote and highest sprinkler in the area of operation as the design point.
2. Read the design point pressure from the output — that value, not the average.
3. Find the floor for the hazard class (LH 0.7, OH 0.35, HHP/HHS 0.5 bar).
4. Run the density calculation separately; the working pressure is the greater of the density pressure and the floor.
5. Where in-rack heads exist, check separately that the 1.0 bar (K115) or 2.0 bar (K80) floor is met.
6. For pump selection, add the 0.5 bar friction allowance on top of this value — the pump curve has to meet it.

Application in Turkey

EN 12845 is the basis for sprinkler design under BYKHY, and fire service compliance letters check these floors. In high-bay logistics facilities (HHS, ST4–ST6) the 0.5 bar ceiling floor and the 1–2 bar in-rack floor are frequently overlooked, forcing a project revision. When K-factor, sprinkler type and area of operation are chosen at design stage, this requirement feeds straight back into pump head — calculating it early avoids surprises at the end of the project.

Frequently Asked Questions

What are the absolute minimum pressures?

0.70 bar for light hazard, 0.35 bar for ordinary hazard, 0.50 bar for high hazard ceiling sprinklers, 1.0 bar for K115 in-rack and 2.0 bar for K80 in-rack.

Is meeting the density enough?

No. The density requirement and the absolute pressure floor are separate, and the higher of the two governs. A K115 head meeting OH3 density at 0.30 bar still has to be lifted to 0.35 bar.

Where is the most unfavourable sprinkler?

The most remote and highest head within the area of operation — not the average. That head is the design point, and the floor must be met there.

Why do in-rack heads need so much more pressure?

Because they must drive droplets down between palletised goods. At low pressure the spray is fine and dispersed, sits on top of the pallet and never penetrates to the layers below.

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