Technical·9 min read
How Is Sprinkler System Water Demand Determined? — Based on BYKHY and TS EN 12845
What truly sizes a sprinkler system is its water demand: pump power, tank volume and pipe diameters all follow from it. BYKHY requires sprinkler design to TS EN 12845. This article explains the three approaches to water demand (hazard class, special design, storage), fully/pre-calculated pipe systems, the TS EN 12845 design density–operation area table, fire hose reel + hydrant additional flow, the +30% area increase for dry systems, an ESFR worked example (12×371 = 4452 L/min + 946 = 5398 L/min), fire pump selection (critical-area pressure + 0.5 bar) and water tank duration (30/60/90 min) with concrete tables.
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The single thing that determines a sprinkler system’s pump power, water tank volume and pipe diameters is its water demand. In Türkiye the framework for this calculation is set by BYKHY (the Regulation on Fire Protection of Buildings): the regulation requires fixed sprinkler systems to be designed to TS EN 12845. In this article we cover how water demand is determined — the three approaches, the design density and operation area tables, the hose additions, and pump and tank sizing — with concrete values and worked examples. (We covered how a sprinkler works in a separate article; here the focus is the water that sizes the system.)
Three approaches to determining water demand
On the BYKHY and TS EN 12845 basis, sprinkler water demand is determined by one of three approaches:
- Hazard-class approach — based on the design density and operation area corresponding to the space’s risk class (low/ordinary/high). This is the most common.
- Special-design approach — special applications such as residential-type sprinklers and water curtains.
- Storage-areas approach — high-rack storage; density/area design or special-application systems such as ESFR.
In each approach the design criteria are set, then the pipe diameters that satisfy those criteria are finalised by hydraulic calculation. The minimum water supply is then found by adding the hose water demand required for fire hose reels and hydrants to the sprinkler water demand.
Fully calculated or pre-calculated pipe system?
How pipe diameters are determined also determines which water-demand rule applies.
- Fully calculated pipe systems: All pipe diameters are determined by hydraulic calculation. The regulation essentially requires this, and water demand is found by the density/area criterion.
- Pre-calculated pipe systems: Some diameters are read from a pipe schedule table, some by hydraulic calculation. Allowed only in limited cases: low and ordinary hazard spaces, new buildings not exceeding 465 m², additions/revisions to existing buildings, and systems where the pump feeds only the sprinklers. Not allowed in combined systems.
In pre-calculated systems the water demand is read from a table (only for K80 sprinklers):
| Hazard Class | Flow (L/min) | Duration (min) |
|---|---|---|
| Low Hazard | 1000 | 45 |
| Ordinary Hazard | 2000 | 60 |
Because the table method requires high pressure and large pipe diameters, the fully calculated system is generally preferred in practice; hence the main focus is the density/area criteria.
Design criteria by hazard class (TS EN 12845)
In fully calculated systems, the core of the water demand is the following design density (water per unit area per minute, mm/min) and operation area (the area assumed to operate simultaneously, m²) table. These values come from TS EN 12845 Table 3 and apply only to standard spray sprinklers:
| Hazard Class | Design Density (mm/min) | Operation Area — Wet/Preaction (m²) | Operation Area — Dry (m²) |
|---|---|---|---|
| Low Hazard | 2.25 | 84 | Not allowed (OT1 used) |
| Ordinary Hazard 1 | 5.0 | 72 | 90 |
| Ordinary Hazard 2 | 5.0 | 144 | 180 |
| Ordinary Hazard 3 | 5.0 | 216 | 270 |
| Ordinary Hazard 4 | 5.0 | 360 | Not allowed (YTİ1 used) |
| High Hazard Process 1 | 7.5 | 260 | 325 |
| High Hazard Process 2 | 10.0 | 260 | 325 |
| High Hazard Process 3 | 12.5 | 260 | 325 |
| High Hazard Process 4 | Deluge systems used | — | — |
The theoretical base of sprinkler water demand = design density × operation area. For example, in an Ordinary Hazard 1 wet system,
5.0 mm/min × 72 m² = 360 L/min is the minimum starting point; the actual system flow comes out higher in the hydraulic calculation because heads near the pump discharge more water.Key rules:
- Dry / preaction system: With density unchanged, the operation area is increased by 30% (this is the reason for the wet→dry column difference in the table). Because water reaches the pipe late, a larger area is accounted for.
- Pitched roof: For a slope exceeding 1 metre rise per 6 metres (>16.7%), the operation area is increased by 30% (except storage).
- Sidewall and quick-response sprinklers: Quick-response sprinklers are not allowed in high hazard; sidewall sprinklers can be used in low hazard, and in ordinary hazard if approved.
- Small space: Even if the space area is smaller than the operation area, the design density may not fall below the table value.
Minimum water supply: sprinkler + hose reel + hydrant
In combined systems (sprinkler + fire hose reel + hydrant), an additional hose water demand is added to the sprinkler water demand. The additional flows and durations are by hazard class (BYKHY Article 92):
| Hazard Class | Duration (min) | Fire Hose Reel Flow (L/min) | Hydrant Flow (L/min) |
|---|---|---|---|
| Low Hazard | 30 | 100 | 400 |
| Ordinary Hazard | 60 | 100 | 1000 |
| High Hazard | 90 | 200 | 1500 |
The addition logic: the first 100 L/min hose reel flow is added to the most remote reel; the second 100 L/min is calculated by increasing it according to the sprinkler’s required pressure at that design point. The hydrant addition is made at whichever of the mains connection point or the site hydrant connection point is closer to the system riser. If the pumps feed only the sprinklers, the hose reel and hydrant demand is not considered when sizing pump capacity.
In open-connected areas with different hazard classes not separated by a physical barrier or a partition that delays heat transfer, the higher-hazard-criterion area is extended 4.6 m toward the lower-hazard area — i.e. the boundary zone is also calculated to the higher criterion.
Storage and ESFR — worked water demand example
High-rack storage is handled separately. Because TS EN 12845 does not give detailed tables for storage configuration, the density/area and especially ESFR criteria are taken from NFPA 13 tables and manufacturer approval data. The ceiling sprinkler is chosen by selecting a K-factor according to the design density: for density ≤ 8.2 L/min·m² use K80; between 8.2–13.9 use K115; above 13.9 use storage-approved sprinklers of K160 and above.
ESFR worked example (from the application manual, wet system):
- Storage arrangement: Racked · Combustibility class: Class IV · Storage height: 6.1 m · Ceiling height: 7.6 m
- Sprinkler: ESFR K = 201 (metric; imperial K14)
- Design criterion (NFPA 13): 12 sprinklers / 3.4 bar minimum pressure
- One sprinkler flow:
Qm = K√P = 201 × √3.4 ≈ 371 L/min - ESFR sprinkler water demand:
12 × 371 = 4452 L/min - Fire hose reel + hydrant addition:
946 L/min - Total water demand: 4452 + 946 = 5398 L/min
In ESFR, the operation area is taken as at least 12 sprinklers (4 on each of three branch lines) in the critical hydraulic area (and at least 89 m²). In storage areas the operation area for dry/preaction systems is increased by 30% but may not exceed 557 m².
Fire pump selection
The output of the hydraulic calculation is the flow + pressure point the source must provide. The pump must exceed this point with a safety margin:
- Pump flow = the flow required for the critical hydraulic design area.
- Pump pressure = the pressure required for the critical area + 0.5 bar.
- The closed-valve (zero-flow) head must be at most 140% of the rated value; the head at 150% flow must not be less than 65% of the rated value; the pump may be used for system demands up to 130% of its rated flow.
- Redundancy: If there is a single pump, a backup of the same capacity; if there are multiple pumps, at least 50% of the total capacity must be backed up.
These sizing details are covered at length in our fire pump and NFPA 20 article. In dry and preaction systems, the valve set type and water delivery delay also affect pump/area selection.
Water tank capacity and duration
If the mains cannot meet the required flow/pressure, a fire pump station and tank are installed (BYKHY Article 91). The tank volume is the product of the system flow and the minimum operating duration:
| Hazard Class | Minimum Operating Duration (min) |
|---|---|
| Low Hazard | 30 |
| Ordinary Hazard | 60 |
| High Hazard Process | 90 |
| High Hazard Storage | 90 |
- If the building has only a fire hose reel system, the tank must be at least 12 m³ (200 L/min × 60 min).
- If it has only a perimeter hydrant system, the water demand must cover at least 1900 L/min for 90 min.
- The volume set aside as fire reserve may not be used for any other purpose; the tank must be arranged to serve only the suppression systems.
In combined water supplies the systems must be fully calculated, the duration taken to the most water-demanding system, and the supply must provide the sum of the simultaneous maximum flows from the systems.
Which standard applies in Türkiye?
BYKHY requires fixed sprinkler systems to be designed to TS EN 12845; the hazard class, design density, operation area, hose additions and durations are taken from that standard. NFPA 13 is used as a reference in storage configurations and ESFR / large-drop / control-mode special-application systems, for which TS EN 12845 does not give detailed tables. The two work together in practice; however, approval and permitting require TS EN 12845 compliance.
Summary
Sprinkler water demand starts from the design density × operation area corresponding to the space’s hazard class; the pipe diameters are finalised by hydraulic calculation; the minimum supply is found by adding the fire hose reel + hydrant flow to the sprinkler demand. Dry/preaction systems and pitched roofs increase the operation area by 30%. Storage and ESFR criteria come from NFPA 13 (total 5398 L/min in the example). The pump must exceed the critical-area pressure with a +0.5 bar margin; the tank volume is found by multiplying by the hazard-based duration of 30/60/90 min. In Türkiye the framework is BYKHY + TS EN 12845; NFPA 13 is complementary for storage/ESFR. At A-Pro we calculate and design the water demand of sprinkler systems on the BYKHY and TS EN 12845 basis using hydraulic software; contact us for a free site survey within Ankara for the right solution for your facility.
This content is for informational purposes. Binding water demand and hydraulic design must be carried out project-specifically by a fire engineer based on the actual hazard class, layout and water supply conditions of the space and the current edition of BYKHY, TS EN 12845 / NFPA 13.
© 2027 A-Pro Mühendislik. This is original technical content produced by A-Pro Engineering; all rights reserved. Copying, reproducing or republishing it in whole or in part without attribution and permission is prohibited.
Frequently asked questions
What determines the water demand of a sprinkler system?+
In Türkiye, BYKHY (the Regulation on Fire Protection of Buildings) requires sprinkler systems to be designed to TS EN 12845. Water demand is determined by one of three approaches: the hazard-class approach, the special-design approach and the storage-areas approach. Fundamentally, each space has a design density (mm/min) and an operation area (m²) corresponding to its hazard class; sprinkler water demand starts from the product of these two, while pipe diameters are finalised by hydraulic calculation. Minimum water supply is then found by adding the fire hose reel and hydrant hose water demand to the sprinkler demand.
What is the difference between a fully calculated and a pre-calculated pipe system?+
Systems where all pipe diameters are determined by hydraulic calculation are 'fully calculated pipe systems', and the regulation essentially requires this. If some are read from a pipe schedule table and some by hydraulic calculation, it is a 'pre-calculated pipe system'; it is only allowed in low and ordinary hazard classes, in new buildings not exceeding 465 m², in additions/revisions to existing buildings, and where the pump feeds only the sprinklers. It is not allowed in combined systems. In pre-calculated systems water demand is read from a table: low hazard 1000 L/min (45 min), ordinary hazard 2000 L/min (60 min).
How does a dry or preaction system change the water demand?+
In dry-pipe and preaction systems, because water reaches the pipe with a delay, TS EN 12845 requires the operation area to be increased by 30% without changing the design density. For example, the operation area that is 72 m² for Ordinary Hazard 1 in a wet system rises to 90 m² in a dry system; this raises the total water demand. The same 30% increase also applies to pitched roofs that exceed 1 metre rise per 6 metres (roof slope > 16.7%). In storage areas the increased operation area for dry/preaction systems may not exceed 557 m².
Why are fire hose reel and hydrant flows added to the sprinkler water demand?+
BYKHY (Article 92) requires that in combined systems the minimum water supply be determined by adding additional hose system demand to the sprinkler demand. The additional flows and durations are by hazard class: low hazard fire hose reel 100 + hydrant 400 L/min (30 min), ordinary hazard 100 + 1000 L/min (60 min), high hazard 200 + 1500 L/min (90 min). In the hose reel addition, the first 100 L/min is added to the most remote reel and the second 100 L/min is calculated by increasing it according to the sprinkler's pressure at that design point. If the pumps feed only the sprinklers, the reel and hydrant demand is not considered when sizing pump capacity.
How is the fire water tank capacity found?+
The tank volume is found from the product of each system's required flow and the minimum operating duration corresponding to the hazard class: low hazard 30 min, ordinary hazard 60 min, high hazard process 90 min, high hazard storage 90 min. If the building has only a fire hose reel system, the tank must be at least 12 m³ (200 L/min × 60 min). If it has only a perimeter hydrant system, the water demand must cover at least 1900 L/min for 90 min. In storage areas the duration is set by combustibility class and storage height. The fire reserve may not be used for any other purpose.
When does NFPA 13 come into play in Türkiye?+
Because BYKHY ties design to TS EN 12845, the hazard class, design density, operation area, hose additions and durations are taken from that standard. However, TS EN 12845 does not give detailed tables for storage configurations and special-application sprinklers (ESFR, large-drop, control-mode); in those cases the NFPA 13 tables and manufacturer approval data are used as reference. So in practice the two standards are used together: layout and hazard class from TS EN 12845, high-rack storage and ESFR criteria from NFPA 13. Approval and permitting require TS EN 12845 compliance.
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