RTI (Response Time Index) measures how quickly a sprinkler senses heat. A low RTI means fast response, a high RTI slow response. Its unit is (m·s)0.5.
The physics
A glass bulb or fusible link reaches its operating temperature by absorbing heat from the hot gas around it. That heat transfer depends on the element's mass and surface area. A thin bulb heats quickly, a thick bulb slowly. RTI reduces that behaviour to a single number, measured under a standard temperature and velocity condition.
The practical equivalent: an ESFR bulb is typically 3 mm in diameter against 5 mm for a classic sprinkler. Those two millimetres translate into seconds.
Three classes
| Class | RTI | Use under EN 12845 Table 38 |
|---|---|---|
| Quick (QR) | 50 or less | The only option for in-rack sprinklers; permitted at ceiling level; not permitted in dry or Type A pre-action systems. ESFR sprinklers are in this class. |
| Special | Above 50 and up to 80 | Permitted at ceiling level and in dry/pre-action systems; not permitted in racks. |
| Standard A | Above 80 | Permitted at ceiling level and in dry/pre-action systems; not permitted in racks. |
Choosing sensitivity in a warehouse
Intuition misleads here: "fast response is always better" is not true. EN 12845 clause 14.5.1 requires that, where sprinklers are installed in racks, the ceiling sprinklers have a sensitivity equal to or slower than the in-rack sprinklers, and Table 38 permits only quick response in racks. The reason: if the ceiling sprinklers open before the in-rack ones, too many ceiling sprinklers can open one after another under the hot gas layer. The hydraulic design was made for a defined number of sprinklers; once more open, the system pressure falls and the sprinklers cannot deliver their required flow.
The result is the opposite failure to cold soldering: many sprinklers open, each at low pressure. The note to EN 12845 Table 38 also requires the effect of different sensitivities to be considered when new sprinklers are added to an existing installation, to avoid excessive activations. Quick response sprinklers are not permitted in dry or Type A pre-action systems (Table 38).
QR is mandatory for ESFR. The name expands to Early Suppression Fast Response. ESFR works on early detection and suppression; under the NFPA 13-2025 definition an ESFR sprinkler has a thermal element with an RTI of 50 (m·s)1/2 or less, and ESFR sprinklers made to EN 12259-13 are also quick response. QR is generally preferred for CMSA in storage as well.
In concealed spaces
In a concealed space such as above a suspended ceiling, hot smoke spreads horizontally and can open sprinklers away from the fire. Under EN 12845 clause 5.4 a concealed space is protected as LH where the main hazard is LH and as OH1 in all other cases, and the spacing follows Table 19 (4.6 m maximum for LH, 4.0 m for OH in a standard layout). EN 12845 sets no separate quick response requirement for concealed spaces.
Do not confuse it with the temperature rating
RTI and the temperature rating are two separate properties. The temperature rating says at what temperature the sprinkler opens; RTI says how quickly it reaches that temperature. A 68 °C sprinkler can be made in both QR and Standard A classes. The colour code shows only the temperature; RTI is read from the product certificate.
Turkish context
BYKHY Article 96(5) requires sprinkler system design to TS EN 12845, and Article 96(1) requires components to comply with TS EN 12259, so the EN 12845 and EN 12259-1 thermal sensitivity classification governs for local approval. Where an insurer asks for NFPA 13 as an added layer, its classification is checked separately; mixing the classification language of the two creates problems at inspection.
Frequently Asked Questions
What is the unit of RTI?
(m·s)^0.5, the square root of metre-seconds. A low RTI means fast response and a high RTI slow response.
Can quick response sprinklers be used in a warehouse?
Under EN 12845 Table 38, yes: only quick response is used in racks, and it is permitted at ceiling level too. But ceiling sprinklers must be equal to or slower than the in-rack sprinklers (14.5.1), and quick response is not permitted in dry or Type A pre-action systems. Otherwise many sprinklers can open and the pressure can fall.
Must ESFR sprinklers be quick response?
Yes. ESFR expands to Early Suppression Fast Response; under the NFPA 13-2025 definition its RTI is 50 (m·s)^1/2 or less, and ESFR sprinklers made to EN 12259-13 are also quick response.
Are RTI and the temperature rating the same?
No. The temperature rating says at what temperature the sprinkler opens; RTI says how quickly it reaches that temperature. The colour code shows only the temperature.
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Download MEP Calc on the App StoreEN 12845:2015+A2:2026 · EN 12259-1 · EN 12259-13 · NFPA 13 (2025) · FM Global DS 8-9. Definitions are for information only.