EN 12845 Reduced Capacity Tanks

Building the sprinkler tank at full capacity is not possible on every site. TS EN 12845+A2:2026 permits the tank to be reduced on installations that have an automatic infill from a town main, but that permission rests on five non-negotiable conditions and a fixed table of minimum volumes. Source: TS EN 12845+A2:2026, Clause 9.3.2.1, Clause 9.3.4 and Table 11.

Full capacity or reduced capacity?

Clause 9.3.2.1 (TS p.50 / EN p.47) defines a minimum water volume for every system and states that it shall be provided in one of two ways:

• Full capacity tank: a tank whose effective capacity is at least equal to the specified water capacity.
• Reduced capacity tank (see Clause 9.3.4): a tank where the required water volume is provided jointly by the effective capacity of the tank and an automatic infill.

The effective capacity is the difference between the normal water level and the lowest effective water level. Where the tank is not protected against frost, the normal water level is raised by at least 1.0 m and ice venting is provided. Easy access is provided on covered tanks; except for open reservoirs, all tanks are fitted with a water level indicator readable from outside.

Find the full capacity first

The reduced capacity is derived from the full capacity, so the target volume has to be known first.

Pre-calculated LH and OH systems, Table 9 (Clause 9.3.2.2, TS p.50 / EN p.47):

Here h is the height of the highest sprinkler relative to the lowest sprinkler; the NOTE to the table excludes sprinklers in the sprinkler valve room.

Pre-calculated HHP and HHS systems, Table 10 (TS p.51 / EN p.48): depending on the design density, from 225 m³ at 7.5 mm/min to 875 m³ at 30.0 mm/min for wet systems, and from 280 m³ to 1 090 m³ for dry systems.

Calculated systems, Clause 9.3.2.3: the minimum effective water volume is the maximum demand flow rate (Qmax) multiplied by the duration in Clause 8.1.1. The Clause 8.1.1 durations (TS p.45 / EN p.42) are: LH 30 min, OH 60 min, HHP 90 min, HHS 90 min.

Clause 9.3.4: the five conditions for reduced capacity

Clause 9.3.4 (TS p.51 / EN p.48) requires all five conditions together:

1. a) The flow shall come from a town main and shall be automatic, through at least two mechanical ball valves. The flow shall not adversely affect the suction of the pump set. The failure of a single ball valve shall not impair the required infill rate.
2. b) The effective capacity of the tank shall not be less than that shown in Table 11.
3. c) The tank capacity plus the infill rate shall be sufficient to supply the system at the full capacity specified in Clause 9.3.2.
4. d) It shall be possible to check the capacity of the infill rate.
5. e) The infill arrangement shall be accessible for inspection.

Table 11: minimum effective capacities

Table 11 (TS p.52 / EN p.49) gives the floor below which a reduced capacity tank may not go:

Footnote a: "but in no case less than 10 per cent of the full capacity." This footnote becomes decisive on high density HHS designs: on a 30.0 mm/min wet system the Table 10 full volume is 875 m³, and since 10 per cent of that is 87.5 m³ the floor rises from 70 m³ to 87.5 m³. If the same system is built dry, the full volume is 1 090 m³ and the floor becomes 109 m³.

How does the infill rate come out?

Read together, Clause 9.3.4 c) and Clause 8.1.1 reduce the calculation to a single line: the missing volume has to be made up within the duration for the class.

Example (arithmetic derived entirely from the standard): OH3 wet, pre-calculated, the highest sprinkler less than 15 m above the lowest sprinkler.

• Table 9, group "OH2 dry/alternate and OH3 wet/pre-action", h ≤ 15 m: full volume 135 m³.
• Table 11, same group: reduced floor 30 m³.
• Difference: 135 - 30 = 105 m³.
• Clause 8.1.1, OH duration: 60 min.
• Required infill: 105 m³ / 1 hour = 105 m³/h = 1 750 l/min.

Under Clause 9.3.4 d) this 1 750 l/min has to be measurable on site; a declaration is not enough. Clause 4.4.4.5 d) (TS p.31 / EN p.28) likewise requires the infill rate to be stated separately in the project documents for reduced capacity tanks.

On the town main side, Clause 4.4.4.3 c) (TS p.31 / EN p.28) requires the pressure/flow characteristic curve of the main to be determined by a test carried out during the period of peak demand, with at least three pressure/flow points taken; the curve is corrected for the friction losses and the static height difference between the test point and the suction tank infill valve.

How it differs from a full capacity tank

Clause 9.3.3 (TS p.51 / EN p.48) imposes a far looser requirement on full capacity tanks: the water source shall be able to fill the tank in a period not longer than 36 hours. The outlet of the feed pipe shall be at least 2.0 m horizontally from the inlet of the suction pipe. With reduced capacity, by contrast, the infill is part of the system during the fire; there is no 36 hour tolerance.

How many, and in which combination?

Clause 9.6.3 (TS p.59 / EN p.56) sets two limits for duplicate water supplies:

• a) More than one pressure tank shall not be used on OH systems.
• b) One tank of the reduced capacity type may be used.

Clause 9.6.2 b) requires the tank to be full capacity for the supply to qualify as a superior single water supply; a reduced capacity tank therefore does not on its own constitute a superior single water supply.

Maintenance and monitoring

• Clause 20.2.2.2 b) (TS p.122 / EN p.119): the water levels in the water storage tanks are checked and recorded in the weekly routine.
• Clause 20.3.4.4 (TS p.125 / EN p.122): the correct operation of the ball valves in the water storage tanks is checked in the annual routine. On a reduced capacity tank the two ball valves are, after all, the lifeline of the system.
• Clause H.2.4 (TS p.156 / EN p.153): an indication shall be given before the water storage tank level falls by more than 10 per cent of the nominal fill level.
• Clause 20.3.5.2 (TS p.125 / EN p.122): except for tanks designed to require no maintenance within 10 years, all tanks are inspected internally at intervals not exceeding 3 years.

Field note: a reduced capacity tank shrinks the volume of the store but not its risk; the system now depends on the town main flowing without interruption during the fire. The requirement in Clause 8.1.2 that the water supply "shall not be liable to be affected by frost, drought, flooding or other conditions which could reduce the flow" weighs far more heavily on this design than on a full capacity tank.

Frequently Asked Questions

What are the minimum effective volumes for a reduced capacity tank?

Table 11 (Clause 9.3.4 b, TS p.52 / EN p.49): LH wet or pre-action 5 m³; OH1 wet or pre-action 10 m³; OH1 dry/alternate and OH2 wet/pre-action 20 m³; OH2 dry/alternate and OH3 wet/pre-action 30 m³; OH3 dry/alternate and OH4 wet/pre-action 50 m³; HHP and HHS 70 m³. The footnote to the HHP/HHS row adds that in no case may it be less than 10 per cent of the full capacity.

Where must the supply to a reduced capacity tank come from?

Clause 9.3.4 a (TS p.51 / EN p.48): the flow shall come from a town main and shall be automatic, through at least two mechanical ball valves. The flow shall not adversely affect the suction of the pump set, and the failure of a single ball valve shall not impair the required infill rate.

How is the required infill rate determined?

Clause 9.3.4 c: the tank capacity plus the infill rate shall be sufficient to supply the system at the full capacity specified in Clause 9.3.2. The duration is taken from Clause 8.1.1: LH 30 min, OH 60 min, HHP 90 min, HHS 90 min. For example, on a pre-calculated OH3 wet system the Table 9 full volume for h ≤ 15 m is 135 m³ and the Table 11 reduced floor is 30 m³; the 105 m³ difference has to be made up within 60 minutes, which means an infill of 105 m³/h, i.e. 1 750 l/min.

How many reduced capacity tanks may be used on one site?

Clause 9.6.3 b (TS p.59 / EN p.56): only one tank of the reduced capacity type may be used in a duplicate water supply. Item a of the same clause also prohibits the use of more than one pressure tank on OH systems.

How is the effective capacity calculated?

Clause 9.3.2.1 (TS p.50 / EN p.47): the effective capacity is calculated as the difference between the normal water level and the lowest effective water level. Where the tank is not protected against frost, the normal water level is raised by at least 1.0 m and ice venting is provided. Except for open reservoirs, tanks are fitted with a water level indicator readable from outside.

References

• TS EN 12845+A2:2026 Clause 4.4.4.3 c), Clause 4.4.4.5 d) (TS p.31 / EN p.28)
• TS EN 12845+A2:2026 Clause 8.1.1, Clause 8.1.2 (TS p.45 / EN p.42)
• TS EN 12845+A2:2026 Clause 9.3.2.1, Clause 9.3.2.2, Table 9 (TS p.50 / EN p.47)
• TS EN 12845+A2:2026 Table 10, Clause 9.3.2.3, Clause 9.3.3, Clause 9.3.4 (TS p.51 / EN p.48)
• TS EN 12845+A2:2026 Table 11 (TS p.52 / EN p.49)
• TS EN 12845+A2:2026 Clause 9.6.2, Clause 9.6.3 (TS p.59 / EN p.56)
• TS EN 12845+A2:2026 Clause 20.2.2.2 (TS p.122 / EN p.119)
• TS EN 12845+A2:2026 Clause 20.3.4.4, Clause 20.3.5.2 (TS p.125 / EN p.122)
• TS EN 12845+A2:2026 Clause H.2.4 (TS p.156 / EN p.153)

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