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 | Typical use |
|---|---|---|
| Quick (QR) | 50 or less | ESFR, life safety, concealed spaces |
| Special | Above 50 and up to 80 | Intermediate applications |
| Standard A | Above 80 | Storage ceiling sprinklers, industrial |
Why QR is a mistake in a warehouse
Intuition misleads here: "fast response is always better" is not true. In standard rack storage, the ceiling sprinklers should be slow response. The reason: while the fire's hot gas layer is still gathered over a narrow area, QR sprinklers open one after another. The hydraulic design was made for a defined number of sprinklers; once more open, the system pressure falls and no sprinkler delivers its required flow.
The result is the opposite failure to cold soldering: the fire is not suppressed, and the system is swamped at low pressure. EN 12845 warns about that risk explicitly in a note.
QR is mandatory for ESFR. The name expands to Early Suppression Fast Response. ESFR works on early detection and suppression, and every ESFR head manufactured to EN 12259-13 is in the quick response class. QR is generally preferred for CMSA in storage as well.
QR in concealed spaces
In a concealed space such as above a suspended ceiling, hot smoke spreads horizontally along ventilation ducts; a fire starting at one point can open a sprinkler 5 to 10 m away. That is why QR is preferred there and the sprinkler spacing tightened from 6.0 m to 4.6 m.
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 leaves sprinkler design to NFPA 13 or EN 12845. Whichever standard you choose, you must follow its thermal sensitivity table; 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.
Why are quick response sprinklers not fitted in a warehouse?
While the hot gas layer is still gathered over a narrow area, many sprinklers open at once. Because the hydraulic design was made for a defined number, the pressure falls and none delivers its required flow.
Must ESFR sprinklers be quick response?
Yes. ESFR expands to Early Suppression Fast Response, and every ESFR head manufactured to EN 12259-13 is in the quick response class.
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 StoreBS EN 12845:2015+A1:2019 · EN 12259-1 · EN 12259-13 · NFPA 13 (2025) · FM Global DS 8-9. Definitions are for information only.