Limited to LH and OH1, an air-to-water ratio of one third to two thirds, a 12 bar ceiling and a heavy inspection burden — why modern projects still prefer a tank and pump set.
On a small manufacturing project the client said "I don't want a pump, I don't want a tank, find me something compact". The first thing that comes to mind is the pressure tank, widely used across Europe in the 1980s. EN 12845 still recognises the solution — but within a scope so narrow that it cannot be applied to more than about 5 % of today's building stock. This article opens the clause and shows why the pressure tank is present in the standard but absent from practice.
Where it sits in the standard
EN 12845 sets out water supply alternatives in sequence: town main, gravity tank, storage tank with pumps, inexhaustible source with pumps, and finally the pressure tank. The supply arrangement clause then places the pressure tank on the single water supply list restricted to LH and OH1 only. For OH2, OH3, OH4, HHP and HHS buildings, a pressure tank alone is not accepted.
How it works: an air cushion carrying the energy
A pressure tank is a pressure vessel filled partly with water and partly with compressed air. When a sprinkler valve opens, the air pushes the water into the network. No pump, no generator, no diesel fuel — just a vessel, a pressure switch and a compressor.
The logic is simple but has a natural limit: as the air expands, pressure falls. So for the tank to deliver steady flow until it empties, it must be large enough and start at a high enough pressure.
Volume and air pressure rules
The standard requires the air space to be at least one third of the total tank volume, so water can occupy no more than two thirds. Internal pressure may not exceed 12 bar.
| Parameter | EN 12845 limit |
|---|---|
| Air volume (Va) | ≥ Vt / 3 |
| Maximum water volume | ≤ 2 Vt / 3 |
| Maximum tank pressure | 12 bar |
| Minimum discharge pipe level | 0.05 m above the tank base |
| Minimum ambient temperature | 4 °C |
| Refill time | ≤ 8 hours |
| Refill rate | ≥ 6 m³/h |
Minimum water volume
| Class | Single supply | Duplicate supply |
|---|---|---|
| LH | 15 m³ | 15 m³ |
| OH1 | 23 m³ | 15 m³ |
| OH2–4 | Not permitted | 15 m³ |
The tank pressure formula
The standard gives the required air pressure as:
P = (P1 + P2 + 0.1 h) · (Vt / Va) − P1
- P — the gauge pressure to be held in the tank, bar
- P1 — atmospheric pressure, bar (taken as 1)
- P2 — the minimum pressure required at the highest sprinkler as the tank empties, bar
- h — the height of the highest (or hydraulically most remote) sprinkler above the tank base, m (negative if the sprinkler is below the tank)
- Vt — total tank volume, m³
- Va — air volume, m³
In pre-calculated systems, P2 comes from the pre-calculated table with friction losses between the design point and the tank added.
Worked example — an OH1 building
An OH1 production building. The highest sprinkler is 9 m above the tank base. Total tank volume Vt = 35 m³, air volume Va = 11.67 m³. P2 from the table plus friction = 1.4 bar.
P = (1 + 1.4 + 0.1 × 9) × (35 / 11.67) − 1
P = 3.3 × 3.0 − 1
P = 9.9 − 1 = 8.9 bar
So 8.9 bar must be held in the tank — already close to the 12 bar limit. At a building height of 12 m, P = 10.4 bar; at 15 m, 11.3 bar. Multi-storey buildings simply cannot be protected this way. That is one of the technical reasons the pressure tank is confined to low, compact buildings.
Location and enclosure conditions
- Inside a sprinklered building, within an enclosure of at least 30 minutes fire resistance;
- or in a separate Euroclass A1/A2 structure dedicated solely to fire protection equipment;
- or in an unsprinklered building within a 60 minute fire-resisting enclosure containing no combustible material.
- The tank and enclosure must be kept above 4 °C.
- The tank must be protected against internal and external corrosion, and accessible for both internal and external inspection.
Controls and safety equipment
- Pressure gauge — with the correct pressure marked on it.
- Safety valve — preventing the tank design pressure being exceeded.
- Glass level gauge — with stop valves at both ends (normally closed) and a drain valve, mechanically protected, with the correct water level marked.
- Automatic warning system — visual and audible alarms for loss of air pressure or water level, transmitted to the installation control valve or a continuously staffed location.
Pressure equipment maintenance obligations
EN 12845 gives only the design minimums. The vessel itself falls under the Pressure Equipment Directive 2014/68/EU, transposed in Turkey as the pressure equipment regulation. In practice:
- The tank must be CE marked with a declaration of conformity; the original calculation report and hydrostatic test certificate must be in the file.
- Periodic inspection — typically internal every four years and hydrostatic every ten — is carried out by an approved inspection body, with records retained.
- Corrosion allowances are tracked; where internal coating degrades, blasting and recoating are required.
- The safety valve is tested annually and gauge calibration verified by daily observation.
Against a tank-and-pump set, the operating cost of those obligations is significantly higher — making the pressure tank not simply a capital cost solution but one with a heavy operational burden.
Common errors
- Showing a pressure tank as the sole supply in an OH2 building. The most common error. The single supply permission is limited to LH and OH1.
- Reducing the air space to 25 %. Compressing the air "to fit more water" breaches the one-third minimum, and pressure will not last until the tank empties.
- Forgetting friction in P2. The tabulated value alone is not enough; all losses between the design point and the tank must be added.
- A refill rate below 6 m³/h. Once emptied, the tank must refill within eight hours; where the town main cannot manage it, a booster pump is required.
- An unprotected glass level gauge. Broken by impact, it becomes a pressurised water jet.
- Forgetting the 4 °C condition. A container-mounted tank outdoors freezes in an Anatolian winter; heating with a thermostat is mandatory.
Comparison with NFPA
NFPA 13 and NFPA 22 still recognise the pressure tank, but treat it as a limited supply solution in modern projects too. NFPA 13 permits 75 % water and 25 % air — three quarters water — against the two thirds of EN 12845. That air-to-water ratio difference is frequently overlooked when comparing the two; the same tank does not size the same way under both standards.
Turkish context
BYKHY refers to "a water tank and pump of adequate capacity" as the sprinkler supply. It does not specifically prohibit pressure tanks, but does require TS EN 12845 compliance. In practice, pressure tank solutions have barely reached approval since 2010, for several reasons:
- Typical residential, assembly and industrial buildings under BYKHY fall into OH2–OH4 or HHS densities, where a pressure tank is already inadequate.
- The high-rise stock (above 12 m) makes the 12 bar limit a bottleneck.
- Periodic pressure equipment inspection deters owners; a tank and pump set is more flexible and cheaper for the same space.
- Insurance assessments place pressure tanks on the non-preferred list.
Where it can still make sense
Niche uses remain:
- Small LH buildings (a 500–1500 m² office or archive, say) where a diesel pump is not wanted, combined with a town main as the second supply.
- Very remote locations without power — although a diesel pump is the standard answer there.
- Retrofit projects where there is no room for a pump room and a compact transitional solution is needed.
Otherwise, the practical reality: choosing a pressure tank on a new project means explaining clearly to the owner the pressure equipment maintenance burden, the insurance surprise, and the inability to extend the system if the building grows.
Frequently asked questions
In which hazard classes can a pressure tank be used?
As a sole water supply, only for LH and OH1. It cannot serve OH2, OH3, OH4, HHP or HHS alone, though it may form one leg of a duplicate supply.
Why one third air and two thirds water?
The air space must be at least one third of total volume. As air expands it pushes water out; with too little air, the minimum discharge pressure cannot be held until the tank empties.
What is the minimum water volume?
For a single supply: 15 m³ for LH and 23 m³ for OH1. In a duplicate supply, 15 m³ is sufficient for LH and all OH groups.
What is the legal framework in Turkey?
The vessel must be CE marked under the pressure equipment regulation, subject to periodic inspection, with the manufacturer's documentation retained on site. BYKHY does not prohibit pressure tanks but requires TS EN 12845 compliance.
Why are pressure tanks avoided in modern projects?
Low volume efficiency (only two thirds is water), a 12 bar ceiling that rules out taller buildings, heavy pressure equipment maintenance obligations, insurer reluctance, and an unfavourable life-cycle cost against a tank and pump set.
SprinkCalc — Fire Sprinkler Design Across Three Standards
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MEP Calc — 86+ Engineering Calculators
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Download MEP Calc on the App StoreBS EN 12845:2015+A1:2019 Fixed firefighting systems — Automatic sprinkler systems. EN 12845-2:2024 (CMSA & ESFR sprinkler systems). NFPA 13 Standard for the Installation of Sprinkler Systems. Turkish Regulation on Fire Protection of Buildings (BYKHY). FM Global Property Loss Prevention Data Sheet 2-0.