EN 12845 Water Supplies: Duration, Continuity and Frost Protection

Working through the case of a 90-minute high-hazard storage tank reduced to 30 minutes on the argument that "we use good pumps".

On an e-commerce warehouse project, the mechanical contractor argued that "a 350 m³ stainless tank is enough, we've fitted the best certified pumps". The hazard class was HHS3, design density 12.5 mm/min, racking 8 m high. The table gave a minimum required volume of 350 m³ — but the question was whether that had been calculated for 90 minutes, or for 60 on the assumption that "modern pumps extinguish faster". Opening the pump room records showed the gap: the tank volume was nominally adequate, but the required flow was 2350 L/min, and at that flow the tank's effective capacity ran out in 79 minutes rather than 95. The reason was simple: refill had not been counted, and the lowest effective water level was not the tank floor but 0.1 m above the vortex inhibitor.

This article covers the three subjects most often got wrong on site — duration, continuity and freeze protection — within the EN 12845 water supply clauses.

Water supply duration

EN 12845 is short and clear: every water supply (apart from the pressure tank exception) must be capable of automatically furnishing the following minimum durations.

Hazard class	Minimum duration	Typical use
LH (light hazard)	30 min	Schools, hospital departments, offices
OH (all ordinary hazard groups)	60 min	Car parks, shops, hotels, OH3 workshops
HHP (high hazard process)	90 min	Chemicals, plastics processing, paint shops
HHS (high hazard storage)	90 min	Logistics warehouses, racking

The key phrase is "automatically furnishing": the supply must hold that duration at the pressure and flow the sprinklers demand, without manual intervention. For a town main, an inexhaustible source or a pre-calculated system, duration is assumed satisfied by the standard's other conditions; but in any system fed from a storage or pressure tank, you must calculate and document it.

For calculated systems the formula is explicit: V_min = Q_max × t. For HHP3 with Q_max of 3250 L/min, the minimum volume is 3250 × 90 = 292 500 L, about 293 m³. In pre-calculated design, the tables give ready volumes — for a 12.5 mm/min wet system, 350 m³, or 440 m³ for a dry system.

A 30-minute tank for a high-hazard warehouse — a common error

I have seen projects shortcut the calculation for years on the reasoning that "60 minutes was enough for the OH3 shop, does raising the racking really make it HHS?" Read alongside the classification annex, palletised rack storage above 4 m falls squarely into high-hazard storage, and the duration jumps from 60 to 90 minutes — so tank volume must grow by at least 50 %. Missing that means certification refusal at third-party audit; worse, in a real fire the system runs dry before the fire service completes its intervention.

Water supply types

EN 12845 recognises four basic types: town main, storage tank (pump suction, gravity or reservoir), inexhaustible source and pressure tank. The configuration most common in Turkey is a suction tank with two pumps — one electric and one diesel, or two electric. A pressure tank is accepted as a sole supply only for LH and OH1, and cannot be used at all for high hazard.

Continuity classes

The standard defines three levels of continuity:

Class	Definition	Typical example
Single	One acceptable supply	A storage tank with a single pump, a town main, a gravity tank
Superior single	One supply with enhanced reliability conditions	A town main fed from both ends; a full-capacity tank with two pumps; an inexhaustible source with two pumps
Duplicate	Two mutually independent supplies	Town main plus storage tank; two separate tank-and-pump sets

Independence is the critical word: to count as duplicate, the two supplies must be entirely independent of each other, and each must meet the pressure-flow requirements on its own. This is frequently confused: drawing from one tank with two pumps gives you superior single, not duplicate. Two separate tanks sharing a pump room are also not duplicate unless the isolation requirements are met.

For high-hazard process foam and deluge systems and large storage warehouses, insurers generally require duplicate supplies. The standard does not always say "shall", but in field practice duplicate has become standard for category one logistics facilities on both capital and operational stability grounds.

Combined supplies

Where sprinklers, hydrants and hose reels all draw from one supply, four conditions apply: the system must be fully calculated; the supply must deliver the sum of the simultaneous maximum calculated flows at the most demanding pressure; the duration must meet the longest required; and duplicate pipe connections are mandatory. Taking two hose lines off the sprinkler tank without calculation breaches all four.

Freeze protection

The requirement is short but decisive: the feed pipe and the control valve set must be kept at least 4 °C. That means a three-layer procedure:

1. Pump room and valve room temperature

Kept above 4 °C by an electric heater, oil-filled radiator or hot water panel, thermostatically controlled, with a low temperature alarm routed to the BMS or fire panel. The standard also prohibits housing pumps, pressure tanks and gravity tank equipment in buildings with hazardous processes or explosion risk.

2. Pipe insulation and trace heating

A feed pipe entering from outside is wrapped in mineral wool or elastomeric insulation, with electrical trace heating over it in cold regions. Self-regulating (PTC) tape is preferred; constant-wattage cables can create local ignition risk. Trace heating faults must be monitored.

3. Tank freeze protection

The standard is explicit: where a tank is not freeze protected, the normal water level is raised by at least 1.0 m and ice venting is provided. That ensures liquid remains beneath a frozen surface, and that an ice layer does not prevent water below it being drawn off. For pressure tanks, the housing must also be kept at least 4 °C.

4. Freeze-prone areas — dry, alternate or antifreeze

Where sprinkler pipework passes through enclosed freeze-prone areas, the answer is not insulation but a change of system type: a dry pipe system, an alternate winter-summer system, or an approved antifreeze solution on small local branches. EN 12845 keeps antifreeze use limited, and in practice no inspector accepts it once design density climbs toward high hazard.

Comparison with NFPA 13

NFPA 13 frames duration differently: 30 to 60 minutes for light and ordinary hazard and 60 to 90 minutes for extra hazard, with storage applications using more complex water demand tables where duration often reaches 90 to 120 minutes. The fixed 90 minutes EN 12845 sets for high hazard is simpler — more conservative in some cases and more relaxed in others. On continuity, NFPA builds a similar hierarchy through single supply, single fire pump plus jockey and two-source arrangements, though "duplicate" is not a formal NFPA term.

Turkish practice

BYKHY refers sprinkler design durations directly to EN 12845 and NFPA 13 without setting its own figures. Even so, most projects we see have sized the tank on a remembered 30 to 60 minutes. Logistics warehouses have been pushed to 90 minutes and duplicate supplies by tighter insurance scrutiny since 2018. If you are designing a new facility, establishing the hazard class correctly from the classification annex at concept stage — and building water supply volume and continuity class on that foundation — is many times cheaper than replacing the tank later.

Common errors — a short check list

• A 30 or 60 minute tank for high-hazard storage. 90 minutes, without exception.
• The "two pumps equals duplicate" fallacy. Two pumps on one tank is superior single, not duplicate.
• Recording gross volume as effective capacity. Dead volume below the vortex inhibitor does not count.
• Not calculating refill rate on a reduced capacity tank. Refill plus tank capacity must together meet the full volume.
• Using a pressure tank as the sole supply for high hazard. Permitted only for LH and OH1.
• Forgetting feed pipe insulation. Below 4 °C is a non-conformity.
• Using a combined supply without calculation. Where sprinklers and hydrants share a tank, the sum of simultaneous flows governs.

Frequently asked questions

Why does high-hazard storage need a 90 minute tank?

Because fire load and growth rate are far higher than in ordinary hazard. With conventional sprinklers rather than ESFR or CMSA, a high-hazard storage fire needs at least 90 minutes of uninterrupted flow until the fire service has completed its intervention. A 30 minute tank in a logistics warehouse is a straight rejection.

What is the difference between single, superior single and duplicate?

Single is one supply. Superior single is a higher-reliability version of one supply — a town main fed from both ends, or a full-capacity tank with two pumps. Duplicate is two wholly independent supplies, each meeting the system demand alone.

What temperature must the tank and feed pipe be kept at?

At least 4 °C for the feed pipe and control valve set, and the same for a pressure tank housing. Where an open tank is not freeze protected, the normal water level is raised by 1.0 m with ice venting provided.

Can antifreeze be used under EN 12845?

Only in a limited way. Freeze protection under the standard is confined to the feed pipe and control valve set. Where local freezing risk exists on sprinkler branches, a dry pipe or alternate system is preferred. Antifreeze is limited to small local branches and is not used in practice for high hazard.

Is a town main alone a valid water supply?

Yes, provided pressure, flow and duration are documented. But for OH3 and above, town main flow is inadequate across much of Turkey, so a storage tank and pumps become necessary in practice. The additional flow for manual firefighting must also be counted.

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