The sprinkler storage tank is governed by Clause 9.3 of TS EN 12845+A2:2026 (TS p.49-54 / EN p.46-51). This article gives the tank types, the definition of effective capacity, Tables 9 and 10 for pre-calculated systems, the volume formula for fully calculated systems, the refill times, the conditions for reduced capacity tanks, the suction pipe clearances (Table 12) and the strainer rules, each with its source.
9.3.1: Types of tank
The standard lists three types (TS p.49 / EN p.46). Storage tanks shall be one or more of the following:
- a pump suction tank;
- a gravity tank;
- a reservoir.
9.3.2.1: Effective capacity and general rules
Clause 9.3.2.1 General (TS p.50 / EN p.47): For each system a minimum water volume is specified. This shall be supplied from one of the following:
- a full capacity tank, with an effective capacity at least equal to the specified water capacity;
- a reduced capacity tank (see 9.3.4), where the required water volume is supplied jointly by the effective capacity of the tank plus the automatic infill.
The definition of effective capacity: the effective capacity of a tank shall be calculated by taking the difference between the normal water level and the lowest effective water level.
Three further rules:
- If the tank is not frost proof, the normal water level shall be increased by at least 1.0 m and ice venting shall be provided.
- In the case of enclosed tanks, easy access shall be provided.
- Except for open reservoirs, tanks shall be provided with an externally readable water level indicator.
Pre-calculated systems: Table 9 and Table 10
Clause 9.3.2.2 (TS p.50 / EN p.47): Table 9 shall be used to determine the minimum effective volume of water required for LH and OH pre-calculated systems. The volumes of water indicated shall be reserved solely for the use of the sprinkler system.
Table 9: Minimum water volume for pre-calculated LH and OH systems (TS p.50 / EN p.47)
| Group | Height h of the highest sprinkler above the lowest sprinkler (m) | Minimum water volume (m³) |
|---|---|---|
| LH (wet or pre-action) | h ≤ 15 | 9 |
| LH (wet or pre-action) | 15 < h ≤ 30 | 10 |
| LH (wet or pre-action) | 30 < h ≤ 45 | 11 |
| OH1 wet or pre-action | h ≤ 15 | 55 |
| OH1 wet or pre-action | 15 < h ≤ 30 | 70 |
| OH1 wet or pre-action | 30 < h ≤ 45 | 80 |
| OH1 dry or alternate / OH2 wet or pre-action | h ≤ 15 | 105 |
| OH1 dry or alternate / OH2 wet or pre-action | 15 < h ≤ 30 | 125 |
| OH1 dry or alternate / OH2 wet or pre-action | 30 < h ≤ 45 | 140 |
| OH2 dry or alternate / OH3 wet or pre-action | h ≤ 15 | 135 |
| OH2 dry or alternate / OH3 wet or pre-action | 15 < h ≤ 30 | 160 |
| OH2 dry or alternate / OH3 wet or pre-action | 30 < h ≤ 45 | 185 |
| OH3 dry or alternate / OH4 wet or pre-action | h ≤ 15 | 160 |
| OH3 dry or alternate / OH4 wet or pre-action | 15 < h ≤ 30 | 185 |
| OH3 dry or alternate / OH4 wet or pre-action | 30 < h ≤ 45 | 200 |
| OH4 dry or alternate | Use HH protection |
NOTE: Excluding sprinklers in the sprinkler valve room.
Table 10: Minimum water volume for pre-calculated HHP and HHS systems (TS p.51 / EN p.48)
| Design density not exceeding (mm/min) | Wet systems (m³) | Dry systems (m³) |
|---|---|---|
| 7.5 | 225 | 280 |
| 10.0 | 275 | 345 |
| 12.5 | 350 | 440 |
| 15.0 | 425 | 530 |
| 17.5 | 450 | 560 |
| 20.0 | 575 | 720 |
| 22.5 | 650 | 815 |
| 25.0 | 725 | 905 |
| 27.5 | 800 | 1 000 |
| 30.0 | 875 | 1 090 |
The dry system column is about 25 % higher than the wet column (for example 225 → 280 at 7.5 mm/min and 875 → 1 090 at 30.0 mm/min). These volumes too are reserved solely for the sprinkler system.
Fully calculated systems
Clause 9.3.2.3 (TS p.51 / EN p.48): The minimum effective volume of water shall be calculated by multiplying the maximum flow demand (Qmax) by the duration specified in 8.1.1.
Clause 8.1.1 Duration (TS p.45 / EN p.42) gives the durations:
| Hazard class | Minimum duration |
|---|---|
| LH | 30 min |
| OH | 60 min |
| HHP | 90 min |
| HHS | 90 min |
NOTE: For town mains, inexhaustible sources and all pre-calculated systems the duration is implicit in the requirements given in this standard.
Worked example (using the 9.3.2.3 formula only): if the hydraulic calculation for an HHS facility gives Qmax = 1 750 l/min, the minimum effective volume required is 1 750 l/min × 90 min = 157 500 l = 157.5 m³. (The Qmax value is an example input; on a real project it comes from the hydraulic calculation.)
Refill rates
Clause 9.3.3 Refill rates for full capacity tanks (TS p.51 / EN p.48):
- The water source shall be capable of refilling the tank in no more than 36 h.
- The outlet of any feed pipe shall be not less than 2.0 m horizontally from the suction pipe inlet.
Worked example: for a 200 m³ full capacity tank the average refill rate required is 200 / 36 ≈ 5.56 m³/h, that is about 93 l/min. (The tank volume is an example input.)
For comparison, the time is much shorter for pressure tanks. Clause 9.5.5 (TS p.58 / EN p.55) requires the pressure tank to be fully charged and pressurized by the air and water supplies in no more than 8 h, and requires the water supply to be able to make up at a rate of at least 6 m³/h at the gauge pressure of the tank.
Reduced capacity tanks
Clause 9.3.4 (TS p.51 / EN p.48) requires five conditions together:
- a) The inflow shall be from a town main and shall be automatic, via at least two mechanical float valves. The inflow shall not adversely influence the pump set suction. The failure of a single float valve shall not impair the required infill rate.
- b) The effective capacity of the tank shall be no less than that shown in Table 11.
- c) The tank capacity plus the inflow shall be sufficient to supply the system at full capacity as specified in 9.3.2.
- d) It shall be possible to check the capacity of the inflow.
- e) The inflow arrangement shall be accessible for inspection.
Table 11: Minimum effective capacity of reduced capacity tanks (TS p.52 / EN p.49)
| Hazard class | Minimum effective capacity (m³) |
|---|---|
| LH (wet or pre-action) | 5 |
| OH1 wet or pre-action | 10 |
| OH1 dry or alternate / OH2 wet or pre-action | 20 |
| OH2 dry or alternate / OH3 wet or pre-action | 30 |
| OH3 dry or alternate / OH4 wet or pre-action | 50 |
| HHP and HHS | 70 (footnote a) |
Footnote a: But in no case less than 10 % of the full capacity.
Reduced capacity tanks are governed by Clause 9.3.4; Clause 9.5.5 concerns charging a pressure tank with air and water and has nothing to do with reduced capacity tanks.
Effective capacity and suction chamber dimensions
Clause 9.3.5 (TS p.53 / EN p.50): The effective capacity of storage tanks shall be calculated as shown in Figure 4, where N is the normal water level, X is the low water level and d is the nominal diameter of the suction pipe. Table 12 specifies minimum dimensions for the following:
- A: from the suction pipe to the low water level (see Figure 4);
- B: from the suction pipe to the bottom of the sump (see Figure 4).
Further provisions (TS p.54 / EN p.51): if a vortex inhibitor is installed with the minimum dimensions specified in Table 12, dimension A might be reduced to 0.10 m. A tank might be provided with a sump in order to maximize the effective capacity.
Table 12: Suction pipe inlet clearances (TS p.54 / EN p.51)
| Nominal diameter of suction pipe d (mm) | B minimum (m) | A minimum without vortex inhibitor (m) | Minimum dimension of vortex inhibitor (m) | A minimum with vortex inhibitor (m) |
|---|---|---|---|---|
| 65 | 0.08 | 0.25 | 0.20 | 0.10 |
| 80 | 0.08 | 0.31 | 0.20 | 0.10 |
| 100 | 0.10 | 0.37 | 0.40 | 0.10 |
| 150 | 0.10 | 0.50 | 0.60 | 0.10 |
| 200 | 0.15 | 0.62 | 0.80 | 0.10 |
| 250 | 0.20 | 0.75 | 1.00 | 0.10 |
| 300 | 0.20 | 0.90 | 1.20 | 0.10 |
| 400 | 0.30 | 1.05 | 1.20 | 0.10 |
| 500 | 0.35 | 1.20 | 1.20 | 0.10 |
Worked example (using table values only): for a DN200 suction pipe, A = 0.62 m without a vortex inhibitor and A = 0.10 m with one. The 0.52 m difference corresponds to 0.52 × 100 = 52 m³ of effective capacity in a tank with a base area of 100 m². (The 100 m² base area is an example input.)
9.3.6: Strainers
Clause 9.3.6 (TS p.54 / EN p.51):
- In the case of pump sets under suction lift conditions, a strainer shall be fitted upstream of the foot valve on the pump suction pipe. It shall be fitted so that it can be cleaned without the tank having to be emptied.
- In the case of open tanks feeding pumps under positive head conditions, a strainer shall be fitted to the suction pipe outside the tank. A stop valve shall be installed between the tank and the strainer.
- Strainers shall have a cross-sectional area of at least 1.5 times the nominal area of the pipe and shall not allow objects greater than 5 mm diameter to pass.
Maintenance periods
Storage tanks are covered in two separate periods:
- Clause 20.3.5.2 (TS p.125 / EN p.122): except for tanks designed so as not to need maintenance within 10 years in accordance with 9.6.2 b), all tanks shall be inspected internally at intervals of no more than 3 years, drained and cleaned if necessary, examined for corrosion and, where necessary, repainted or the corrosion protection renewed.
- Clause 20.3.6 (TS p.125 / EN p.122): at intervals of no more than 10 years, all storage tanks shall be cleaned, inspected internally and their shells dealt with as necessary. NOTE: cleaning tanks generally requires them to be drained, but alternative solutions might be acceptable in order to save water.
In addition, Clause 20.3.4.4 (TS p.125 / EN p.122) requires the float valves in water storage tanks to be checked in the yearly routine to make sure that they operate correctly, and Clause 20.3.4.5 requires the pump suction strainers and the settling chamber and its screens to be inspected and cleaned as necessary at least once a year. In the weekly routine, Clause 20.2.2.2 b) (TS p.122 / EN p.119) requires all water levels in elevated private reservoirs, rivers, canals, lakes and water storage tanks, including pump priming water tanks and pressure tanks, to be checked and recorded.
On the monitoring side, Annex H, H.2.4 (TS p.156 / EN p.153) requires an indication before the water storage level falls by more than 10 % of the nominal fill level.
What the standard does not specify
TS EN 12845+A2:2026 does not prescribe the material of the storage tank (galvanized panel steel, reinforced concrete, GRP and so on). What the standard does say about materials concerns water quality and corrosion protection rather than the tank shell itself: Clause 8.1.2 (TS p.45 / EN p.42) requires the water to be free from fibrous or other matter in suspension that could build up deposits in the system pipework, and requires salt or brackish water not to be retained in the sprinkler installation pipework; Clause 9.6.2 b) (TS p.59 / EN p.56) requires the tank, in order to count as a superior single water supply, to be painted or given other corrosion protection that extends the need for draining for maintenance to at least 10 years.
Field note
The vortex inhibitor column of Table 12 and the sump provision in Clause 9.3.5 serve the same purpose: reducing the difference between the physical volume of the tank and its effective capacity. What enters the calculation is not "how many m³ the tank holds" but "how many m³ lie between the normal level and the lowest effective level"; on projects that difference is lost not at the top of the tank but at the bottom. (This paragraph is engineering commentary.)
Frequently Asked Questions
How is the effective capacity of a tank calculated?
TS EN 12845+A2:2026 Clause 9.3.2.1 (TS p.50 / EN p.47): the effective capacity of a tank shall be calculated by taking the difference between the normal water level and the lowest effective water level. If the tank is not frost proof, the normal water level shall be increased by at least 1.0 m and ice venting shall be provided. Easy access shall be provided in the case of enclosed tanks; except for open reservoirs, tanks shall be provided with an externally readable water level indicator.
What is the minimum water volume for pre-calculated LH and OH systems?
Table 9 (TS p.50 / EN p.47) gives it according to the height of the highest sprinkler above the lowest sprinkler. Examples: LH (wet or pre-action) 9 m³ for h ≤ 15 m, 10 m³ for 15 < h ≤ 30 and 11 m³ for 30 < h ≤ 45. OH1 wet/pre-action: 55, 70, 80 m³. OH2 wet/pre-action (and OH1 dry/alternate): 105, 125, 140 m³. OH3 wet/pre-action (and OH2 dry/alternate): 135, 160, 185 m³. OH4 wet/pre-action (and OH3 dry/alternate): 160, 185, 200 m³. For OH4 dry or alternate, HH protection is used.
How quickly must a full capacity tank refill?
Clause 9.3.3 (TS p.51 / EN p.48): the water source shall be capable of refilling the tank in no more than 36 h. In addition, the outlet of any feed pipe shall be not less than 2.0 m horizontally from the suction pipe inlet.
What conditions apply to a reduced capacity tank?
Clause 9.3.4 (TS p.51 / EN p.48) lists five conditions: a) the inflow shall be from a town main and shall be automatic, via at least two mechanical float valves, shall not adversely influence the pump set suction, and the failure of a single float valve shall not impair the required infill rate; b) the effective capacity of the tank shall be no less than that shown in Table 11; c) the tank capacity plus the inflow shall be sufficient to supply the system at full capacity as specified in 9.3.2; d) it shall be possible to check the capacity of the inflow; e) the inflow arrangement shall be accessible for inspection.
What clearance is required between the suction pipe and the low water level?
Table 12 (TS p.54 / EN p.51) gives it by nominal suction pipe diameter. For DN100, for example: B minimum 0.10 m to the bottom of the sump; A minimum 0.37 m without a vortex inhibitor; A minimum 0.10 m with a vortex inhibitor (minimum dimension of the vortex inhibitor 0.40 m). For DN200 the values are 0.15 m, 0.62 m and 0.10 m respectively (vortex inhibitor 0.80 m). The standard states that where a vortex inhibitor is installed with the minimum dimensions specified in Table 12, dimension A might be reduced to 0.10 m.
References
- TS EN 12845+A2:2026 Clause 9.3.1 General (TS p.49 / EN p.46)
- TS EN 12845+A2:2026 Clauses 9.3.2.1, 9.3.2.2, Table 9 (TS p.50 / EN p.47)
- TS EN 12845+A2:2026 Table 10, Clauses 9.3.2.3, 9.3.3, 9.3.4 (TS p.51 / EN p.48)
- TS EN 12845+A2:2026 Table 11 and its footnote (TS p.52 / EN p.49)
- TS EN 12845+A2:2026 Clause 9.3.5, Figure 4 (TS p.53 / EN p.50)
- TS EN 12845+A2:2026 Table 12, the vortex inhibitor and sump provisions, Clause 9.3.6 Strainers (TS p.54 / EN p.51)
- TS EN 12845+A2:2026 Clause 9.5.5 Charging a pressure tank with air and water (TS p.58 / EN p.55)
- TS EN 12845+A2:2026 Clause 9.6.2 b) (TS p.59 / EN p.56)
- TS EN 12845+A2:2026 Clause 8.1.1 Duration, 8.1.2 Continuity (TS p.45 / EN p.42)
- TS EN 12845+A2:2026 Clause 20.2.2.2 b) (TS p.122 / EN p.119)
- TS EN 12845+A2:2026 Clauses 20.3.4.4, 20.3.4.5 (TS p.125 / EN p.122)
- TS EN 12845+A2:2026 Clauses 20.3.5.2, 20.3.6 (TS p.125 / EN p.122)
- TS EN 12845+A2:2026 Annex H, H.2.4 Liquid levels (TS p.156 / EN p.153)
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Download MEP Calc on the App StoreTS EN 12845+A2:2026 (EN 12845:2015+A2:2026) Fixed firefighting systems — Automatic sprinkler systems — Design, installation and maintenance; TS EN 12845-2:2025 (EN 12845-2:2024) ESFR and CMSA sprinkler systems. Every figure in this article is taken from the published standard text; clause, table and page references are listed under References. General information only, not a substitute for the standard or for a design review.