EN 12845 Ceiling Sprinklers: Flush, Recessed and Concealed

Hotel lobbies, shopping centres, hospitals, offices: the architect wants sprinklers hidden and the designer's density and K-factor arithmetic gets tight. A field guide to picking the right one of the four types.

If, while concealed sprinklers are going into the plasterboard ceiling of a five-star hotel lobby, somebody says "the cover plates don't match the RAL, let's have the painter spray them" — you need to stop. That sentence is where site practice leaves the standard and its EN 12259-1 approval behind and starts improvising. This article sorts out which of flush, recessed and concealed goes where, how high a K-factor you can reach, and which type suits which architectural setting.

Definitions

EN 12845 defines four pendent geometries separately, and calling them all "hidden sprinklers" is the main source of confusion:

• Standard spray pendent — the classic sprinkler discharging downwards. The body sits entirely below the ceiling, with the deflector usually 75–150 mm below it.
• Ceiling or flush sprinkler — a pendent whose body is partly above the ceiling, with the heat-sensitive element below the ceiling plane. Minimal metal protrudes.
• Recessed sprinkler — all or part of the heat-sensitive element sits above the ceiling plane, concealed by a two-piece escutcheon; the bulb and deflector remain visible.
• Concealed sprinkler — a recessed sprinkler with a cover plate that releases with heat. Only a circular plate is visible; bulb, deflector and body are entirely hidden.

Deflector position

The general rule is often overlooked: the sprinkler deflector must sit between 0.075 m and 0.15 m below the ceiling, except where ceiling, flush or recessed sprinklers are used. So the 75–150 mm window is mandatory for a standard pendent, while for flush, recessed and concealed types the manufacturer's approved installation template governs — usually leaving the deflector within 0–30 mm of the ceiling. The lower limits are fixed: a sprinkler may not be brought more than 0.3 m below a combustible ceiling, or more than 0.45 m below a non-combustible (A1/A2) ceiling.

Separately, the clear volume required below the deflector is 0.3 m for flat spray sprinklers in LH and OH, 0.5 m for other types, and 1.0 m for HHP and HHS. Because a concealed deflector drops down when the plate releases, take the operating position from the manufacturer's data, not the cover plate line.

Where each type is prohibited

EN 12845 sums up the selection in one sentence: ceiling, flush, recessed and concealed sprinklers shall not be installed in OH4, HHP or HHS areas. In other words:

Hazard class	Pendent	Flush	Recessed	Concealed
LH (hotel rooms, office floors, libraries)	OK	OK	OK	OK
OH1–OH3 (retail, hospitals, restaurants)	OK	OK	OK	OK
OH4 (printing, textiles)	OK	Prohibited	Prohibited	Prohibited
HHP (chemicals, plastics process)	OK	Prohibited	Prohibited	Prohibited
HHS (storage)	OK (K160 and above)	Prohibited	Prohibited	Prohibited

Three further prohibitions apply to drop-down deflector types, meaning concealed heads: where the ceiling slope exceeds 45°, where the atmosphere is corrosive or heavily dust laden, and in or under racking. A concealed sprinkler in a workshop ceiling is a defect waiting to be found; a concealed cover plate above a cold store duct is a poor idea.

K-factor and density compatibility

The permitted K-factors are fixed by hazard class. The critical point: the practical K range for flush, recessed and concealed bodies is limited.

Hazard	Design density (mm/min)	Permitted K	Concealed in practice
LH	2.25	K57	K57 standard from manufacturers
OH1–OH3	5.0	K80 or K115	K80 and K115 available
OH4	5.0	K80 or K115	Prohibited
HHP/HHS (up to 10 mm/min)	≤ 10	K80, K115 or K160	Type prohibited — pendent required

The field consequence: 5.0 mm/min over an average 12 m² area with a K115 sprinkler gives roughly 0.75 bar. The same duty on K80 needs about 1.5 bar, which pushes the pump curve up and feeds through into pipe sizes. When the architect asks for "slim concealed plates", the decision effectively caps you at K80 and multiplies the system pressure. That negotiation belongs in the design office, not on site.

Cover plate temperature logic

A concealed sprinkler operates in two stages:

1. The cover plate separates from the body as its solder releases at a set temperature, typically 57 °C.
2. Once the plate falls, the deflector drops into its operating position; the main glass bulb then bursts at the normal fire temperature, usually 68 °C red or 74 °C yellow.

The sprinkler rating must not exceed the ambient by more than 30 °C, and 68 or 74 °C suits normal environments. The concealed plate must always be rated lower than the main bulb — otherwise the sprinkler tries to operate with the plate still in place, pressure throws the plate off and the spray pattern is disturbed. A 57 °C plate under a 100 °C bulb is the typical pairing.

Choosing by architectural setting

Space	Hazard	Recommended type	Reason
Hotel room, corridor, lobby	LH	Concealed with RAL plate	Design led, should not be visible in the plasterboard
Hotel kitchen service area	OH3	Pendent or recessed	Grease and vapour foul a concealed plate
Shopping centre unit	OH3 by contents	Recessed or concealed	Architectural demand, density held with K115
Mall common area, atrium ceiling	OH2	Pendent, exposed concrete soffit	The soffit is visible; concealment is pointless
Hospital ward and corridor	OH1	Concealed, cleanable plate	Hygiene, appearance, infection control
Hospital laboratory or sterilisation	OH2	Recessed	Steam and cleaning loosen a concealed solder joint
Open-plan office	LH	Concealed or flush	Architectural, low hazard
Office archive or IT store	OH3	Pendent	K115 and above needed; concealed bodies too limited
Car park, plant floor	OH2	Pendent, exposed soffit	No aesthetic concern, mechanical impact risk
Places of worship, museums, historic buildings	LH	Concealed where achievable	Visual sensitivity

Testing and approval

EN 12845 requires sprinklers to comply with EN 12259-1, which covers the body, bulb or solder, deflector, K-factor, temperature rating and K-factor tolerance; the concealed cover plate falls under the same family plus the manufacturer's specific approval (LPCB, VdS, FM or UL).

For a site check, look for three marks on the body or yoke: the K-factor, the nominal temperature (with its colour code) and the approval mark. The concealed plate carries its own nominal temperature stamp, usually 57 or 68. Mixing manufacturers — sprinkler from one, plate from another — voids the approval; the assembly must be listed as a matched set.

The most common field errors

• Painting the concealed cover plate. To match the ceiling colour, someone asks the painter to give it a coat; the extra paint layer insulates the 57 °C solder, the plate does not fall during a fire and the sprinkler does not operate. The fix: order factory-finished plates in the required colour.
• Fitting concealed sprinklers in a prohibited area such as OH4, storage or a kitchen. The prohibition is explicit and an inspection will reject the installation.
• The wrong plate temperature. Ordering a 100 °C plate with a 100 °C bulb: the sprinkler operates while the plate is still on, the water throws it off in a ring and the pattern is wrong.
• Escutcheons set at the wrong depth on recessed heads. Any installation outside the manufacturer's template voids the K-factor approval — sticking an extra escutcheon over new plaster is a classic.
• Concealed sprinklers on a roof steeper than 45°. The drop-down deflector cannot take a proper position on a slope; it is prohibited.
• A concealed cavity deeper than 0.3 m below a combustible ceiling. This exceeds the limit, and a delay between flame and sprinkler becomes inevitable.
• Removing the plate "just to have a look" during ceiling works. Once the solder has disengaged, the same plate cannot be refitted; it is a single-use item.

Comparison with NFPA 13

NFPA 13 defines the same four types. The numerical differences:

• The EN 12845 prohibition in OH4, HHP and HHS has no direct NFPA equivalent; NFPA permits any listed application. But high-K storage types (K200, K360) are not manufactured in concealed bodies anyway, so the effect is much the same.
• EN 12845 sets deflector height at 75–150 mm; NFPA 13 allows 25–305 mm — more flexible on the NFPA side, tighter on the EN side.
• NFPA leaves the cover plate temperature to the manufacturer's listing, typically 57 or 74 °C. EN 12845 with EN 12259-1 follows the same logic.
• NFPA 13 explicitly prohibits painting a concealed cover plate; the EN series has no such sentence, but painting automatically voids the EN 12259-1 approval.

Turkish context

BYKHY treats a sprinkler system as compliant when designed to TS EN 12845 or equivalent, and accepts the TS EN 12259 series for components. So on Turkish projects using concealed, flush or recessed sprinklers, the product schedule must show:

• A CE mark and declaration of conformity to EN 12259-1,
• At least one recognised approval mark — LPCB, VdS, FM or UL,
• A cover plate and sprinkler body matched as a set in the same manufacturer's listing,
• Documentation of the cover plate nominal temperature, usually 57 °C.

Fire brigade approval inspections specifically ask for the RAL code and manufacturer's label on concealed plates in plasterboard ceilings; site painting found on inspection fails acceptance. On hospital projects, health ministry inspectors additionally ask about cleanability of the plate surface.

A short decision matrix

• The architect wants concealment and the hazard is LH or OH1–3: concealed, with a plate rated below the sprinkler bulb.
• Exposed concrete soffit, no aesthetic concern: standard pendent — cheapest, least work, fewest errors.
• Kitchen, humid or corrosive atmosphere: recessed or pendent, not concealed.
• Storage, OH4 or heavy process: flush, recessed and concealed are prohibited — use pendent or upright.
• High density combined with an imposed low K: redo the hydraulic calculation; the pump curve will change.

Frequently Asked Questions

Where are concealed sprinklers prohibited?

In OH4, HHP and HHS areas, and for drop-down types also on ceilings steeper than 45 degrees, in corrosive or dusty atmospheres, and in or under racking.

At what temperature does the cover plate fall?

Typically 57 C. The plate must always be rated below the main bulb, so it releases first and lets the deflector drop into position.

Can the cover plate be painted on site?

No. An extra paint layer insulates the solder, the plate does not fall and the sprinkler never operates. Order factory-finished plates instead.

What K-factor can a concealed head reach?

K57 for LH and K80 or K115 for OH1 to OH3. Higher K bodies are not made in concealed form, and those hazard classes prohibit the type anyway.

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