The pressure tank is regulated in Clause 9.5 of TS EN 12845+A2:2026 and is the water supply with the narrowest scope in the standard: it is accepted as a single water supply only in Light Hazard (LH) and OH1. This article reproduces all subclauses of Clause 9.5, the calculation formula and the control and safety equipment provisions with their sources. Source: TS EN 12845+A2:2026, Clauses 9.5 and 9.6, TS p.57-59 / EN p.54-56.
Where it may be a single supply
Clause 9.6.1 Single water supplies (TS p.58-59 / EN p.55-56) lists the acceptable single water supplies:
- a) a town main;
- b) a town main with one or more booster pumps;
- c) a pressure tank (LH and OH1 only);
- d) a gravity tank;
- e) a storage tank with one or more pumps;
- f) an inexhaustible source with one or more pumps.
There is a limit in the duplicate water supply arrangement as well: Clause 9.6.3 (TS p.59 / EN p.56) states that a duplicate water supply shall consist of two single water supplies independent of each other and lists the restrictions; the first of these is that not more than one pressure tank shall be used in OH systems.
Minimum water capacity
Clause 9.5.3 Minimum capacity (water) (TS p.57 / EN p.54):
| Use | Hazard class | Minimum water volume |
|---|---|---|
| Single water supply | LH | 15 m³ |
| Single water supply | OH1 | 23 m³ |
| Duplicate water supply | LH and OH (all groups) | 15 m³ |
Air volume and pressure limit
Clause 9.5.4.1 General (TS p.57 / EN p.54) sets three requirements:
- The air space shall not be less than one third of the volume of the pressure tank (Va ≥ Vt/3).
- The pressure in the tank shall not exceed 12 bar.
- The air pressures and water flow rates from the tank shall be sufficient to meet the demand requirements of the sprinkler installation up to the point of exhaustion.
The 12 bar limit is consistent with the general maximum water pressure provision of the standard: Clause 8.2.1 (TS p.45 / EN p.42) requires that, other than during tests, the pressure at connections to equipment such as sprinklers, multiple jet controls, water flow detectors, dry pipe and pre-action alarm valves does not exceed 12 bar.
Air pressure calculation
Clause 9.5.4.2 Calculation (TS p.58 / EN p.55) determines the air pressure to be maintained in the tank with the following formula:
P = (P1 + P2 + 0.1 h) × (Vt / Va) − P1
The definitions of the symbols as given in the standard:
| Symbol | Definition | Unit |
|---|---|---|
| p | gauge pressure | bar |
| p1 | atmospheric pressure (p1 is taken as 1) | bar |
| p2 | minimum pressure required at the highest sprinkler at exhaustion of the pressure tank | bar |
| h | height of the highest sprinkler, or of the hydraulically most remote sprinkler, above the base of the pressure tank (negative where the highest sprinkler is below the tank) | m |
| Vt | total volume of the tank | m³ |
| Va | volume of air in the tank | m³ |
The standard also states the source of p2: in pre-calculated systems p2 shall be taken from Table 6 and the friction losses between the design point and the pressure tank shall be added.
Table 6 (Clause 7.3.1, TS p.42 / EN p.39) gives the pressures and flow rates at the control valve set for pre-calculated LH and OH systems; the values are given in the form "bar + ps" and, according to the note below the table, ps is the static head loss (bar) due to the height of the highest sprinkler in the relevant range above the 'C' gauge of the control valve set.
| Hazard class | Flow rate (l/min) | Pressure at the control valve set | Maximum demand flow rate (l/min) | Pressure at the control valve set |
|---|---|---|---|---|
| LH (wet and pre-action) | 225 | 2.2 + ps | - | - |
| OH1 wet and pre-action | 375 | 1.0 + ps | 540 | 0.7 + ps |
| OH1 dry and alternate / OH2 wet and pre-action | 725 | 1.4 + ps | 1 000 | 1.0 + ps |
| OH2 dry and alternate / OH3 wet and pre-action | 1 100 | 1.7 + ps | 1 350 | 1.4 + ps |
| OH3 dry and alternate / OH4 wet and pre-action | 1 800 | 2.0 + ps | 2 100 | 1.5 + ps |
Since a pressure tank may be used as a single supply only in LH and OH1 (9.6.1 c), in practice the first two rows of the table come into play.
Numerical behaviour of the formula
The numbers below are not design data taken from the standard; they are example inputs chosen to show how the formula of Clause 9.5.4.2 works. In a real project p2 comes from Table 6 and the friction losses, and h comes from the project.
Example inputs: p1 = 1 bar (the standard's assumption), p2 = 1.4 bar, Va = Vt/3, that is Vt/Va = 3 (the smallest air volume permitted by 9.5.4.1).
| h (m) | (p1 + p2 + 0.1h) | × 3 | − p1 | p (bar) |
|---|---|---|---|---|
| 6 | 3.0 | 9.0 | −1 | 8.0 |
| 9 | 3.3 | 9.9 | −1 | 8.9 |
| 12 | 3.6 | 10.8 | −1 | 9.8 |
| 15 | 3.9 | 11.7 | −1 | 10.7 |
The table shows two things: for every 3 m increase in height the required gauge pressure rises by 0.9 bar (0.1 × 3 × Vt/Va), and when the air volume is kept at its minimum (Vt/Va = 3) the pressure approaches the 12 bar limit quickly. Increasing the air volume reduces the Vt/Va ratio and therefore reduces the required pressure.
Location, housing and frost protection
Clause 9.5.1 General (TS p.57 / EN p.54):
- The pressure tank shall be dedicated to the sprinkler system and/or the water spray system.
- The tank shall be accessible for external and internal inspection.
- The tank shall be protected against corrosion both externally and internally.
- The discharge pipe shall be at least 0.05 m above the base of the tank.
Clause 9.5.2 Housing (TS p.57 / EN p.54): the pressure tank shall be housed in an easily accessible position in one of the following:
- a) a sprinkler protected building;
- b) a separate sprinkler protected building of Euroclass A1 or A2 construction (or its equivalent in existing national classification systems) used only to house fire protection water supplies and equipment;
- c) an unprotected building located in a 60 minute fire resisting compartment containing no combustible material.
Where the pressure tank is housed in a sprinkler protected building, the area shall be enclosed by construction with a fire resistance of at least 30 minutes. The pressure tank and its housing shall be maintained at a temperature of at least 4 °C.
This 4 °C value belongs to the same family as the other frost protection thresholds of the standard: Clause 8.1.3 Frost protection (TS p.45 / EN p.42) requires the supply pipe and the control valve set to be maintained at a minimum of 4 °C, and Clause 10.3.3 (TS p.61 / EN p.58) requires the pump set compartment to be maintained at 4 °C for electric pumps and 10 °C for diesel pumps.
Charging with air and water
Clause 9.5.5 Charging with air and water (TS p.58 / EN p.55):
- Pressure tanks used as a single supply shall be provided with means of automatically maintaining the air pressure and the water level.
- The air and water supplies shall be capable of fully charging and pressurizing the tank within not more than 8 hours.
- The water supply shall be capable of making up at a flow rate of at least 6 m³/h at the gauge pressure of the tank (p in 9.5.4).
For comparison, the filling time for storage tanks is different: Clause 9.3.3 (TS p.51 / EN p.48) requires a full capacity storage tank to be capable of being refilled by the water supply within not more than 36 hours.
Control and safety equipment
Clause 9.5.6 (TS p.58 / EN p.55) consists of three subclauses.
9.5.6.1 Pressure gauge and safety devices: a pressure gauge shall be fitted to the tank and the correct gauge pressure p shall be marked on the gauge. Suitable safety devices shall be fitted to the tank to ensure that the maximum permitted pressure is not exceeded.
9.5.6.2 Gauge glass: a gauge glass shall be fitted to indicate the water level. A stop valve shall be fitted at each end of the gauge glass and these valves shall be kept normally closed; a drain valve shall also be provided. The gauge glass shall be protected against mechanical damage and the correct water level shall be marked on it.
9.5.6.3 Automatic warning system: an automatic warning system shall be provided to indicate the failure of the devices that restore the correct air pressure or water level. The alarms shall be given visually and audibly at the installation control valve or in a continuously manned location.
A monitoring threshold specific to the pressure tank is in Annex H: H.2.4 Liquid levels (TS p.156 / EN p.153) requires critical liquid levels to be monitored; an indication shall be given before the water storage level falls by more than 10% of the nominal filling level and, in pressure tanks, an indication shall also be given before the level rises to more than 10% above the nominal filling level. Clause H.2.5 Pressures of the same annex (TS p.156 / EN p.153) requires the pressures in the water supplies to be monitored.
Maintenance period
Clause 20.3.5.2 Storage and pressure tanks (TS p.125 / EN p.122): except for tanks designed so as not to require maintenance within 10 years in accordance with 9.6.2 b), all tanks shall be internally inspected at intervals of not more than 3 years and, if necessary, drained and cleaned. The tanks shall be examined for corrosion in accordance with the manufacturer's recommendations and repainted or have their corrosion protection renewed to the extent necessary.
The weekly routine also covers the pressure tank: Clause 20.2.2.2 (TS p.122 / EN p.119) requires all water and air pressure gauge readings on installations, mains and pressure tanks and all water levels including pump priming water tanks and pressure tanks to be checked and recorded.
Field note
The requirement that the pressure tank meets the demand up to the point of exhaustion (9.5.4.1) means that the tank selection has to be verified with a curve rather than a single operating point: as the water empties, the air expands and the pressure falls. The Vt/Va factor in the formula reflects that fall in the initial pressure. (This paragraph is engineering commentary.)
Frequently Asked Questions
In which hazard classes may a pressure tank be a single water supply?
Only in LH and OH1. TS EN 12845+A2:2026 Clause 9.6.1 c) (TS p.59 / EN p.56) lists the acceptable single water supplies and writes the pressure tank as 'a pressure tank (LH and OH1 only)'. Clause 9.5.3 (TS p.57 / EN p.54) gives the corresponding volumes: for a single supply, 15 m³ for LH and 23 m³ for OH1. For a duplicate supply, 15 m³ for LH and all OH groups.
What are the air volume and pressure limits in a pressure tank?
Clause 9.5.4.1 (TS p.57 / EN p.54): the air space shall not be less than one third of the volume of the pressure tank and the pressure in the tank shall not exceed 12 bar. In addition, the air pressures and water flow rates from the tank shall be sufficient to meet the demand requirements of the sprinkler installation up to the point of exhaustion.
How is the air pressure of a pressure tank calculated?
The formula in Clause 9.5.4.2 (TS p.58 / EN p.55) is: P = (P1 + P2 + 0.1h) × (Vt / Va) - P1. Here p is the gauge pressure (bar), p1 the atmospheric pressure (taken as 1), p2 the minimum pressure required at the highest sprinkler at tank exhaustion (bar), h the height of the highest or hydraulically most remote sprinkler above the base of the tank (m; negative where the sprinkler is below the tank), Vt the total volume of the tank and Va the volume of air in the tank (m³). In pre-calculated systems p2 is taken from Table 6 and the friction losses between the design point and the pressure tank are added.
How quickly shall a pressure tank be charged?
Clause 9.5.5 (TS p.58 / EN p.55): pressure tanks used as a single supply shall be provided with means of automatically maintaining the air pressure and the water level. The air and water supplies shall be capable of fully charging and pressurizing the tank within not more than 8 hours. The water supply shall be capable of making up at a flow rate of at least 6 m³/h at the gauge pressure of the tank (p in 9.5.4).
Where is a pressure tank housed?
Clause 9.5.2 (TS p.57 / EN p.54) gives three options: a) a sprinkler protected building; b) a separate sprinkler protected building of Euroclass A1 or A2 construction used only to house fire protection water supplies and equipment; c) an unprotected building located in a 60 minute fire resisting compartment containing no combustible material. Where it is housed in a sprinkler protected building, the area shall be enclosed by construction with a fire resistance of at least 30 minutes. The tank and its housing shall be maintained at a temperature of at least 4 °C.
References
- TS EN 12845+A2:2026 Clause 9.5.1 General, 9.5.2 Housing, 9.5.3 Minimum capacity, 9.5.4.1 General (TS p.57 / EN p.54)
- TS EN 12845+A2:2026 Clause 9.5.4.2 Calculation, 9.5.5 Charging with air and water, 9.5.6.1-9.5.6.3 (TS p.58 / EN p.55)
- TS EN 12845+A2:2026 Clause 9.6.1 Single water supplies (TS p.58-59 / EN p.55-56)
- TS EN 12845+A2:2026 Clause 9.6.3 Duplicate water supplies (TS p.59 / EN p.56)
- TS EN 12845+A2:2026 Clause 7.3.1, Table 6 (TS p.42 / EN p.39)
- TS EN 12845+A2:2026 Clause 8.1.3 Frost protection, 8.2.1 Maximum water pressure (TS p.45 / EN p.42)
- TS EN 12845+A2:2026 Clause 9.3.3 Filling rates for full capacity tanks (TS p.51 / EN p.48)
- TS EN 12845+A2:2026 Clause 10.3.3 Temperature (TS p.61 / EN p.58)
- TS EN 12845+A2:2026 Clause 20.2.2.2 Weekly checks (TS p.122 / EN p.119)
- TS EN 12845+A2:2026 Clause 20.3.5.2 Storage and pressure tanks (TS p.125 / EN p.122)
- TS EN 12845+A2:2026 Annex H, H.2.4 Liquid levels and H.2.5 Pressures (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.