EN 12845 Sprinkler Thermal Sensitivity and RTI Classes

Working through the standard response A, special and quick classes from a site perspective: why fitting quick-response heads in a warehouse opens the whole ceiling at once.

During commissioning at a food packaging warehouse, a single small pallet fire opened seven ceiling sprinklers at once. The hydraulic calculation had been built on four heads, system pressure fell below 1.4 bar, and a real fire could not have been controlled. The cause was one word overlooked in the specification: although the warehouse was HHS3 rather than OH3, the contractor had fitted quick response (QR) heads on the reasoning that "faster is safer". The quiet warning in the thermal sensitivity clause exists precisely to prevent that.

This article covers what RTI means, the three sensitivity classes EN 12845 defines, why ESFR is necessarily quick response, and which class belongs in which hazard class as a design decision.

What RTI means physically

RTI (Response Time Index) measures the thermal lag of a sprinkler's heat-sensing element — a liquid bulb or fusible link — against a hot gas stream. Its unit is (m·s)^0.5. The meaning: a bulb with a large mass and a small contact area reaches its threshold late; a thin-walled, small-volume bulb reaches it quickly.

The value is derived in a laboratory plunge test: the sprinkler is plunged from room temperature into a tunnel with a constant-velocity (typically 2.5 m/s) air stream at a constant temperature (around 197 °C). The time to bulb operation is recorded and converted to RTI. Two heads of the same nominal temperature — 68 °C, say — do not operate together; a low-RTI head may respond in seconds where a high-RTI head takes tens of seconds.

The EN 12845 note summarises the classification: most sprinklers are graded, in decreasing order of sensitivity, as quick response, special response and standard response A. The numerical limits come from EN 12259-1.

The three classes

Class	RTI limit (m·s)0.5	Typical use
Standard A	RTI > 80	High-hazard storage ceilings, ceiling heads above in-rack systems
Special	50 < RTI ≤ 80	OH1–4, high-hazard process, plant spaces above offices
Quick (QR)	RTI ≤ 50	Light hazard (hotels, hospitals, offices), in-rack heads, ESFR, all residential except dry systems

The standard gives a table stating which class is permitted in which installation. Read from the site:

Class	In-rack	Ceiling above in-rack	Pre-action / dry type A	All others
Standard A	No	Yes	Yes	Yes
Special	No	Yes	Yes	Yes
Quick	Yes	Yes	No	Yes

Two critical rules follow:

• In-rack heads may only be quick response. The head inside the rack must open before the heat column from a burning pallet reaches the ceiling.
• Ceiling sprinklers must be equal to or slower than the in-rack heads. With QR below, standard A or special goes above. Reverse that and the ceiling opens first, wasting water without reaching the fire in the rack — the trigger for the classic cold soldering scenario.
• Quick response is prohibited on pre-action and type A dry systems. Air venting and water delivery can take up to 60 seconds, so a QR head opens more widely while the hot gas layer disperses and the hydraulics deteriorate.

Why ESFR must be quick response

The answer is in the name: Early Suppression Fast Response. ESFR does not "control" a fire; it suppresses it. That means delivering very high flow — K200 to K360, 240 to 500 L/min per head — into the seat of the fire within the first 60 to 90 seconds, before the heat release rate peaks. A slow bulb misses that window entirely.

So every ESFR head certified to EN 12259-13 is approved as quick response at the manufacturing stage. On site you need not say "I chose ESFR without checking RTI" — the product already is. For CMSA heads, check the manufacturer's documentation; storage CMSA heads generally favour quick response, but it varies by product.

The design decision: LH versus HHS

Comparing the two extremes, because most site errors start here.

LH — hotel rooms, offices, places of worship

Choose QR. These are occupied spaces where evacuation time is critical. EN 12845 gives light hazard 2.25 mm/min over 84 m² — a low fire load. Early operation directly affects life safety, and NFPA 13 takes the same position. Fitting standard A here runs against modern hotel regulation.

HHS3 — palletised dry goods warehouse

Choose standard A. Most inspecting engineers miss this. In high-hazard storage the fire load is high, and we want the hot gas layer to spread beneath the ceiling, because the hydraulic calculation is built on a defined area of operation of 260 m². With QR heads, sprinklers open one by one before the heat layer has spread across 50–60 m², calls come from outside the design area, and the system loses nominal pressure by the fourth head. That is why the sensitivity table prefers standard A or special for high-hazard storage ceilings and mandates QR only for in-rack heads.

HHP — chemical process, spray booth

Special or standard A. High-hazard process has rapid growth potential but the area of operation is capped at 260 m². Special is the sweet spot: neither as early as light hazard nor as late as storage.

A real field case

The chain of events at the food packaging warehouse:

1. The specification came out as OH3 (5.0 mm/min over 216 m²) and the contractor priced it on that basis.
2. Before commissioning, palletised cartons were stacked to 5.4 m. It had in fact become HHS3 (12.5 mm/min over 260 m²).
3. The design was never revised, but the fire safety consultant said "use quick response to be safe" and K115 QR heads were fitted.
4. In the hot smoke test a single test pallet burned; seven heads opened within 38 seconds. The four heads assumed in the calculation fell below the 1.5 bar nominal pressure.

The fix took two steps: ceiling heads were changed to K160 standard A, and a separate K115 QR in-rack line was run inside the racking. The hydraulics were recalculated and the pump curve proved adequate. The cost of the specification-to-site mismatch: around 380 head replacements plus pipework changes.

The lesson in one sentence: "faster is always safer" is false — thermal sensitivity class is an equation solved together with design density and area of operation.

Do not confuse RTI with temperature rating

RTI and nominal operating temperature (68, 79, 93 °C and so on) are separate parameters. RTI describes response speed; the nominal temperature describes the operating threshold. A quick response head can come with a 68 °C bulb or a 141 °C one. In hot spaces — cooking areas, foundries, boiler rooms — the high nominal temperature is chosen first, and the sensitivity class second. EN 12845 sets nominal temperature by ambient in one clause and sensitivity separately in another.

Comparison with NFPA 13

NFPA 13 and EN 12845 draw on the same international physics (UL 199, FM 2008 and EN 12259-1 limits are very close). The NFPA classification:

• Fast response: RTI ≤ 50 — the EN 12845 "quick" equivalent
• Special response: 50 < RTI ≤ 80 — "special"
• Standard response: RTI > 80 — "standard A"

NFPA 13 requires quick response in light hazard areas, pointing the same way as the EN sensitivity table. On the storage side, NFPA 13 also requires QR for ESFR and in-rack heads, running exactly parallel to EN 12845-2.

In practice the numerical RTI limits are identical when moving between the two standards; only the terminology changes.

Turkish context

BYKHY accepts TS EN 12845 and NFPA 13 as equivalents for sprinkler design. The problem I meet most often in building inspection is a project issued under an EN 12845 label but written in NFPA terminology — "QR standard spray", say — which can conflict with the sensitivity table.

The items to check:

• Is the hazard class (LH, OH, HHP, HHS) unambiguous in the specification?
• Is the RTI class stated separately for each zone? A line reading only "K80, 68 °C" is incomplete.
• In warehouse and rack areas, are ceiling and in-rack heads listed separately?
• Where there is a pre-action or dry system, has "no QR" been written into the specification?
• Does the product certificate reference EN 12259-1 or EN 12259-13 and state the measured RTI?

Frequently asked questions

What is RTI and what are its units?

The Response Time Index describes the sensing lag of a liquid-filled glass bulb or fusible link, in (m·s)^0.5. A low value means fast response, a high value slow.

Which RTI classes does EN 12845 define?

Three: standard A (RTI > 80), special (50 < RTI ≤ 80) and quick (RTI ≤ 50), with a table setting out which class may be used in which installation.

Why is fitting quick response heads in a warehouse wrong?

Because with QR at high-hazard storage ceilings, many sprinklers open before the hot gas layer has spread across the design area, and the hydraulic design falls short. The table therefore mandates QR for in-rack heads while directing ceilings to standard A or special.

Must ESFR sprinklers be quick response?

Yes. The name itself is Early Suppression Fast Response, and every ESFR head produced to EN 12259-13 is in the quick response class.

How do these classes apply under Turkish regulation?

BYKHY leaves sprinkler design to NFPA 13 or TS EN 12845. Whichever you choose, you must follow that standard's thermal sensitivity table — and terminology differences between the two are a frequent objection at building inspection.

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