Two sprinklers with the same temperature rating and the same K-factor can open at noticeably different times. The difference lies in how quickly the thermal element responds to heat. This is not a catalogue detail: NFPA 13, EN 12845 and FM Global make quick response mandatory in some places, prohibit it in others and restrict mixing the two types in one area. This note collects the rules with their clause numbers and then sets them side by side for typical scenarios.
How response is defined: RTI
Response speed is measured by the RTI (response time index). NFPA 13 (2025), clause 3.3.223.2, treats a thermal element with an RTI of 50 (m·s)1/2 or less as fast response and one of 80 (m·s)1/2 or more as standard response. The quick-response (QR) sprinkler definition (3.3.223.4.16) uses the same limit of 50 and requires the product to be listed as quick-response. ESFR (3.3.223.4.5) and residential sprinklers (3.3.223.4.17) are fast-response types by definition.
The glossary of FM Global DS 2-0 (Appendix A) uses the same limits: an RTI typically of 50 (m·s)0.5 or less is quick response, 80 (m·s)0.5 or more is standard response. Values in between are called special response, although FM notes that FM Approvals Class 2000 and 2008 do not currently recognise that rating. FM also caps the conductivity factor for quick response at 1.0 (m/s)0.5. EN 12845 lists the classes in descending order of sensitivity as quick, special and standard 'A', with a reference to EN 12259-1 (clause 14.5.1); the EN 12845 text itself gives no numerical RTI limits for them.
The point most often confused on site: response speed and temperature rating are separate properties. FM DS 8-9 Table 7 gives both quick-response and standard-response columns for the same 70 °C (160 °F) rating. The physics and test method behind RTI are covered in What is RTI?
NFPA 13 (2025): what is required and what is prohibited
Quick response in light hazard
Under clause 9.4.3.1, light hazard areas use one of the following: quick-response sprinklers, residential sprinklers complying with Chapter 12, quick-response CMSA sprinklers or ESFR sprinklers. The light hazard examples in Annex A.4.3.2 include offices, educational buildings, hospitals and residential occupancies, so a standard-response spray sprinkler is not the default choice there. The exception is 9.4.3.3: where no listed quick-response sprinkler exists in the required temperature range, standard response is permitted.
No mixing within a compartment
Clause 9.4.3.2: where quick-response sprinklers are installed, every sprinkler in that compartment is quick-response. There are three exceptions: a temperature range with no listed quick-response product (9.4.3.3), in-rack sprinklers (9.4.3.4), and, outside light hazard, a single product listed for both standard-response and quick-response protection at different coverage areas, installed in one compartment at both spacings (9.4.3.5).
Where quick response is prohibited
Under the density/area method, clause 19.2.3.2.2.2 prohibits quick-response sprinklers in extra hazard occupancies and in other occupancies with substantial amounts of flammable liquids or combustible dusts. In storage, the control mode density/area (CMDA) criteria are built around standard-response sprinklers: at densities of 8.2 mm/min (0.20 gpm/ft²) or less, K-5.6 (K80) standard-response sprinklers are permitted; up to 13.9 mm/min (0.34 gpm/ft²) a minimum of K-8.0 (K115) is used, and above that a storage-listed standard-response sprinkler of at least K-11.2 (K160) (21.1.2 to 21.1.4). Quick-response spray sprinklers may be used for storage only where listed for that use (21.1.5).
Specific places where quick response is required
- Below cloud ceilings where sprinklers are omitted above: quick-response standard spray or extended coverage sprinklers (9.2.7.2.1).
- Supplemental sprinklers below obstructions: quick-response and ordinary temperature rated unless 7.2.4 or 9.4.2 requires otherwise (9.5.5.3.3.1).
- Combustible concealed spaces of wood joist or wood truss construction with members less than 900 mm (3 ft) on centre and a pitch of 4 in 12 or steeper: quick-response (10.2.7.1.4.1).
- Drop-out ceiling panels are not installed below quick-response sprinklers unless specifically listed for that application (9.3.11.2).
The benefit of quick response
With listed quick-response sprinklers, the design area may be reduced without changing the density (19.2.3.2.3.1) when all of the following apply: wet pipe system, light or ordinary hazard, a maximum ceiling height of 6.1 m (20 ft), no unprotected ceiling pockets over 3.0 m² (32 ft²) and no unprotected areas above cloud ceilings. Even in the reduced area, at least five sprinklers are calculated (19.2.3.2.3.2). A dry system does not get this benefit.
High ceilings and ESFR
In Ordinary Hazard Group 2, standard-response standard-coverage sprinklers are not permitted under high ceilings. NFPA 13-2025 states this limit differently in two places: 30 ft (9.1 m) in Section 10.2.5 and 40 ft (12.2 m) in Section 19.2.3.2.5.1(C). Using the 9.1 m threshold satisfies both. ESFR is always fast response; it is used only in wet pipe systems unless specifically listed for dry or preaction systems (14.2.2), and its temperature rating is ordinary or intermediate (14.2.5). Where an ESFR area adjoins an area with standard-response sprinklers, a noncombustible draft curtain at least 600 mm (2 ft) deep separates them, with a clear aisle of at least 1.2 m (4 ft) centred below it (14.2.4.1, 14.2.4.2; repeated in 20.9.5.3.2 and 20.9.5.3.3). For ESFR limits, try the ESFR suitability checker.
Modifying an existing system
Chapter 30 leaves a practical route for older light hazard systems: for modifications or additions to an existing light hazard system equipped with standard-response sprinklers, new standard-response sprinklers are permitted (30.3.3), and individual standard-response sprinklers may be replaced like for like (30.3.4). If the system is converted to quick-response or residential sprinklers, however, every sprinkler in the compartment is changed (30.3.5).
EN 12845 and EN 12845-2
In EN 12845:2015+A1:2019 the governing rule is Table 38 (clause 14.5.1). It shows where each of the three sensitivity ratings may be used:
| Sensitivity rating | In-rack | Ceiling above in-rack sprinklers | Dry systems / pre-action Type A | All others |
|---|---|---|---|---|
| Standard 'A' | No | Yes | Yes | Yes |
| Special | No | Yes | Yes | Yes |
| Quick | Yes | Yes | No | Yes |
Three conclusions follow. In racks, only quick-response sprinklers are used. Quick response is not allowed in the dry systems and pre-action Type A column. In the "all others" column all three ratings are allowed, so, unlike NFPA 13, EN 12845 places no general ban on standard 'A' sprinklers in light hazard areas such as offices. Clause 14.5.1 also requires ceiling sprinklers above in-rack sprinklers to be of equal or slower response than those in the racks. The table note adds that when new sprinklers are added to an existing installation, the effect of different sensitivities may need to be considered to avoid excessive activations. More on the EN side: EN 12845 sprinkler thermal sensitivity and RTI.
Two further clauses change the choice. Where the additional reliability measures of Annex F are applied, quick-response sprinklers are used; standard 'A' and special response are accepted only in rooms of at least 500 m² in area or at least 5 m in height (F.4). For polypropylene or polyethylene storage bins in palletised racks, ceiling sprinklers are rated special and in-rack sprinklers special or quick (G.8.3).
ESFR and CMSA fall under EN 12845-2:2024. ESFR is used in wet installations only, while CMSA may be wet or dry (clause 4). The ESFR concept has no dry option; where dry pendent ESFR sprinklers are used, the installation is treated as wet (6.5). The response characteristic is given in the design tables of Clause 6; according to the standard's note, ESFR is typically rated 68 to 74 °C and fast response (5.1.1). Dry pipe (tree type) CMSA designs use upright, standard-response, 141 °C ceiling-level CMSA sprinklers, and gridded or looped systems are not used (6.7.2). Between adjacent areas at the same ceiling height with different RTI, a noncombustible draft curtain at least 600 mm deep is installed, with an aisle at least 1.2 m wide, free of stored goods, centred on the midpoint between the two sprinkler types (6.3). Below solid mezzanines of combustible construction or with combustible occupancy beneath, quick-response or ESFR sprinklers are installed; standard response is allowed only where a line of sprinklers sits no more than 0.6 m from the mezzanine edge with all goods inside that line, or where a smoke curtain surrounds the mezzanine (5.2.7).
FM Global: DS 2-0 and DS 8-9
DS 2-0 (October 2021, interim revision April 2025) starts from a ban on mixing: sprinklers with different RTI ratings are not mixed within the same area (2.5.1.2.1). There are two exceptions: the two areas are separated by a solid noncombustible draft curtain at least 0.6 m (2 ft) deep, with a space at least 2.3 m (7-1/2 ft) wide clear of combustibles on the standard-response side (2.5.1.2.5); or adjacent ceilings differ in elevation by at least 0.6 m, the standard-response sprinklers are under the higher ceiling and the same 2.3 m clear space is provided (2.5.1.2.6).
Standard-response sprinklers are acceptable for both wet and dry systems. Quick-response sprinklers are acceptable for wet systems but are not used in the following (2.5.2.1.2.2 for nonstorage, 2.5.4.1.2.2 for storage): spray application of ignitable liquids; hydraulic equipment using ignitable liquids; occupancies with ignitable liquids other than those incidental to a process and kept in safety containers of 19 L (5 gal) or less; vehicle assembly or repair areas with fuel in the tanks; large areas shielded from sprinkler discharge; areas where oil, dust, lint or similar combustible residues can build up on ceilings or structural members; and wherever the occupancy-specific data sheet does not recommend them. On dry-pipe and preaction systems, unless the occupancy data sheet says otherwise, the default is a standard-response sprinkler rated nominally 140 °C (280 °F) (2.2.3.3.2, 2.2.4.3.3). Sprinklers under solid walkways are quick-response and rated 70 °C (160 °F) (2.5.1.5.1), and the flowchart in Figure 2.5.1.4 for solid mezzanines also leads to 70 °C quick-response sprinklers in many cases.
DS 8-9 (March 2010, interim revision July 2025) follows the same line for storage: quick-response ceiling-level sprinklers are used only in wet-pipe and antifreeze systems unless the data sheet recommends otherwise for a specific application (2.3.3.2.4), while standard-response ones may be used in wet-pipe, dry-pipe, antifreeze, preaction and refrigerated-area systems (2.3.3.2.5). In-rack sprinklers are listed as quick-response (2.3.4.3.2). Ceiling sprinklers keep the same K-factor, orientation, RTI and temperature rating throughout the system whenever possible (2.3.3.6.1). The DS 8-9 tables give quick and standard response in separate columns, and the designs often differ. Examples from Table 7 (Class 1, 2 and 3 commodities in open-frame racks) for a maximum ceiling height of 9.0 m (30 ft):
| Sprinkler (Table 7, 9.0 m ceiling) | Quick response | Standard response |
|---|---|---|
| Wet, 70 °C, pendent K14.0 (K200) | 12 sprinklers @ 3.5 bar (50 psi) | 18 sprinklers @ 2.2 bar (32 psi) |
| Wet, 70 °C, upright K16.8 (K240) | 12 sprinklers @ 2.4 bar (35 psi) | 18 sprinklers @ 1.5 bar (22 psi) |
| Wet, 70 °C, pendent K11.2 (K160) | 18 sprinklers @ 3.5 bar (50 psi) | 18 sprinklers @ 3.5 bar (50 psi) |
| Dry, 140 °C, upright K16.8 (K240) | No column | 25 sprinklers @ 1.5 bar (22 psi) |
A rough check on the first row with a nominal K200 (Q = K√P, sprinkler flow only, excluding hydraulic imbalance and hose allowance): the quick-response option comes to about 12 × 374 ≈ 4,490 L/min and the standard-response option to about 18 × 297 ≈ 5,340 L/min. The third row shows that there is not always a difference; which option is better has to be read from the table line by line. Finally, note the FM terminology: DS 2-0 no longer uses the terms CMDA, CMSA and suppression mode to describe sprinklers and uses "Storage", "Nonstorage" and "Special Protection" instead.
How to tell them apart on site
- SIN: under NFPA 13 clause 7.2.1 every sprinkler carries one or two letters identifying the manufacturer, followed by three or four digits. This sprinkler identification number (SIN) identifies the sprinkler by K-factor, deflector characteristic, pressure rating and thermal sensitivity. The reliable way to confirm the response type is to match the SIN against the manufacturer's data sheet and the listing or approval record.
- Colour shows temperature, not response: the bulb liquid and the fusible-link frame colours indicate the temperature classification (NFPA 13 Tables 7.2.4.1(a) and (b); EN 12845 clause 14.4 note and Table 37b; FM DS 2-0 Table 2.5.1.1.1). A red-bulb sprinkler may be quick or standard response. For temperature rating selection, see the sprinkler temperature rating selector.
- A thin bulb is not proof: bulb thickness may give a visual hint, but none of the standards reviewed here defines bulb diameter as a criterion for response rating. The SIN and the data sheet decide.
- Concealed and ornamental sprinklers: colour coding is not required for factory-plated or factory-painted ornamental sprinklers, or for recessed, flush or concealed sprinklers (NFPA 13, 7.2.4.3). On these you may need to remove the cover plate to read the SIN. Only the manufacturer may paint sprinklers (7.2.5.2).
- Spare cabinet: to avoid creating a mixed installation during replacements, compare the SIN of every model in the spare sprinkler cabinet with the heads installed in the field.
Scenario comparison
| Scenario | NFPA 13 (2025) | EN 12845 / 12845-2 | FM Global |
|---|---|---|---|
| Office, school, residential (light hazard) | QR, residential, QR CMSA or ESFR (9.4.3.1); one type per compartment (9.4.3.2) | All three ratings allowed (Table 38); with Annex F applied, quick except in large or tall rooms (F.4) | QR acceptable on wet systems; the occupancy data sheet decides (DS 2-0, 2.5.2.1.2.2) |
| Ignitable liquid spraying / repair of fuelled vehicles | QR prohibited in extra hazard or with substantial flammable liquids (19.2.3.2.2.2) | Table 38 adds no restriction | No QR (DS 2-0, 2.5.2.1.2.2 A and D) |
| Rack storage, CMDA ceiling + in-rack | Ceiling based on standard response, QR only if listed for storage (21.1.2 to 21.1.5); in-rack exempt from 9.4.3.2 (9.4.3.4) | In-rack quick only; ceiling equal or slower (Table 38, 14.5.1) | In-rack QR (DS 8-9, 2.3.4.3.2); ceiling per table column |
| ESFR warehouse | Fast response (3.3.223.4.5); wet unless specially listed (14.2.2); 600 mm curtain + 1.2 m aisle next to standard response (14.2.4) | Wet only (clause 4, 6.5); 600 mm curtain + 1.2 m aisle next to different RTI (6.3) | QR ceiling sprinklers wet or antifreeze only (DS 8-9, 2.3.3.2.4) |
| Cold store, dry system | No general QR ban, but the area reduction applies to wet systems only (19.2.3.2.3.1) | No quick (Table 38); dry CMSA: upright, standard response, 141 °C (6.7.2) | Default standard response 140 °C (DS 2-0, 2.2.3.3.2); DS 8-9 dry columns are standard response only |
| OH2, high ceiling | Standard-response standard-coverage prohibited: ceiling > 9.1 m (10.2.5); 19.2.3.2.5.1(C) gives the same ban at 12.2 m | No response rule specific to this case in the clauses reviewed | For storage, per the DS 8-9 table columns |
| Mezzanine, walkway, below obstructions | Supplemental sprinklers QR, ordinary temperature (9.5.5.3.3.1) | In ESFR/CMSA installations, QR or ESFR below solid mezzanines, with two exceptions (12845-2, 5.2.7) | Below walkways QR 70 °C (DS 2-0, 2.5.1.5.1); mezzanines per Figure 2.5.1.4 |
| Refurbishing an older building, mixed types | Standard response may be added to or replaced in a standard-response light hazard system (30.3.3, 30.3.4); converting to QR means the whole compartment (30.3.5) | Consider the effect of different sensitivity when adding sprinklers (Table 38 note) | No mixed RTI in one area; exceptions with a curtain or elevation difference (DS 2-0, 2.5.1.2.1, 2.5.1.2.5, 2.5.1.2.6) |
| Zone needing a higher temperature rating | Standard response permitted where no listed QR exists in that range (9.4.3.3) | Temperature per clause 14.4; no separate response rule | Same K, orientation and RTI with only a different temperature rating accepted at local hot spots (DS 2-0, 2.5.1.2.7) |
Quick checklist. 1) Which standard, which edition? 2) Hazard class and use (light hazard? ignitable liquids?). 3) System type (wet, or dry/preaction?). 4) Are there in-rack sprinklers? 5) What response type is used next door, and is there a curtain or elevation difference between? 6) Do the selected product's SIN, data sheet and listing or approval record match the response type in the design table?
Frequently Asked Questions
Isn't a quick-response sprinkler always better?
No. It opens earlier, but the standards explicitly exclude it in some uses: NFPA 13 in extra hazard and where there are substantial flammable liquids or combustible dusts (19.2.3.2.2.2), FM Global for ignitable liquid spraying, repair of fuelled vehicles and similar uses (DS 2-0, 2.5.2.1.2.2), and EN 12845 in dry and pre-action Type A systems (Table 38).
Can I tell a quick-response sprinkler by its bulb colour?
No. The colour of the bulb liquid and of the fusible link indicates the temperature classification (NFPA 13 Table 7.2.4.1, EN 12845 Table 37b). The response type is confirmed by matching the SIN on the sprinkler against the manufacturer's data sheet and the listing record.
Can quick-response and standard-response sprinklers share a room?
Under NFPA 13, where quick-response sprinklers are installed every sprinkler in the compartment is quick-response, with exceptions listed in 9.4.3.3, 9.4.3.4 and 9.4.3.5. FM Global DS 2-0 accepts different RTI in one area only with a 0.6 m noncombustible draft curtain or a 0.6 m elevation difference plus a 2.3 m space clear of combustibles. EN 12845-2 also requires a 600 mm curtain and a 1.2 m aisle between areas of different RTI at the same ceiling height.
Can quick-response sprinklers be used on a dry system?
EN 12845 Table 38 does not allow quick response in dry or pre-action Type A systems. FM Global recommends standard-response 140 °C sprinklers on dry-pipe systems unless the occupancy data sheet says otherwise. We found no general ban in the NFPA 13 clauses reviewed, but the design area reduction for quick-response sprinklers applies to wet systems only (19.2.3.2.3.1).
What should I fit when one sprinkler is replaced in an older light hazard system?
NFPA 13 clause 30.3.4 permits individual standard-response sprinklers in an existing light hazard system to be replaced with new standard-response sprinklers. If you decide to convert the system to quick response, every sprinkler in the compartment is changed (30.3.5).
SprinkCalc — Fire Sprinkler Design Across Three Standards
SprinkCalc covers hazard classification, design density and area, K-factor selection, water demand and hydraulic calculations for NFPA 13, FM Global and BS EN 12845 in a single iOS app, and exports a professional PDF report.
Download SprinkCalc on the App StoreNFPA 13, Installation of Sprinkler Systems, 2025 edition · BS EN 12845:2015+A1:2019 · EN 12845-2:2024 (ESFR and CMSA) · FM Global DS 2-0 (October 2021, interim revision April 2025) · FM Global DS 8-9 (March 2010, interim revision July 2025). Clause and table numbers refer to these editions; confirm the edition in force, the authority having jurisdiction and the insurer for your project. The findings here are typical defect patterns, not an account of events at any particular site.