Seasonal wet and dry cycling: the changeover procedure, the 25 % enlarged design area, and the "still wet in winter" traps we meet most often on site.
On a logistics warehouse project in eastern Turkey the client said "half the warehouse is heated, half is not". The classical answer is a full dry installation in the unheated part and a wet installation in the heated part. But the owner wanted one alarm valve, one compressor and one maintenance line. The alternate installation clause of EN 12845 exists for exactly that. In summer it works like a conventional wet system; when winter comes, the same pipework is drained, charged with compressed air and becomes a dry system. Two seasons on one installation — but the costs and the site traps of that changeover must be clearly understood.
What an alternate installation is
The standard defines it as an installation with either an alternate alarm valve or a composite set of a wet alarm valve and a dry alarm valve. In winter the pipework below the alarm valve is charged with compressed air or inert gas and the pipework above with pressurised water; for the rest of the year the installation works as a wet system.
Two points matter here. First, composite set versus single alternate valve: in practice a composite set (a wet and a dry valve side by side) lets you service each separately, while a single-bodied alternate valve is more compact but ties spare parts to the valve maker. Second, "works as a wet system for the rest of the year" creates a design area debate; the standard requires the design area to be increased by 25 % for the dry mode, staying on the safe side in case the fire occurs in winter rather than summer.
Where the 25 % increase comes from
The pre-calculated flow-pressure table and the tank volume table keep wet and dry/alternate rows separate. Taking OH3:
| OH3 scenario | Design flow | Max. demand flow | Min. water volume (h ≤ 15 m) |
|---|---|---|---|
| Wet / pre-action | 1100 L/min at 1.7 + ps bar | 1350 L/min at 1.4 + ps bar | 105 m³ |
| Dry or alternate | 1100 L/min at 1.7 + ps bar | 1350 L/min at 1.4 + ps bar | 160 m³ |
The pump pressures look the same, but the area of operation used is 216 m² for OH3 wet and 270 m² for dry or alternate — the 25 % increase comes from there. Assuming one head per 12 m², the sprinkler count rises from 18 to 23, the hydraulic calculation is run with those extra heads, and the tank grows from 105 to 160 m³. The moment you say "alternate", the tank increases by roughly 50 % — so the storage has to be planned large from day one.
A further note on high-hazard storage: dry and alternate systems should not be used in HHS applications, particularly with more combustible commodities. Where unavoidable, the area of operation is still increased by 25 %. The standard actively discourages alternate systems there.
The 60 second rule
In winter mode an alternate installation behaves like a dry system: when the first sprinkler opens, the air in the pipe must vent before water arrives. That delay adds to the response time, and the maximum water delivery time is fixed:
| Hazard class | Maximum water delivery time |
|---|---|
| LH | 90 seconds |
| OH and HH | 60 seconds |
Measuring it is part of the standard acceptance test: the remote test valve is opened, the alarm valve trips, and the time to the first continuous discharge is recorded with a stopwatch. The standard notes that with an accelerator, pipe volumes above 4 m³ for LH and OH, or 3 m³ for HH, may not achieve the 60/90 s limit. In practice 4 m³ is roughly 250 m of horizontal 100 mm pipe, easily exceeded in a mid-sized warehouse. That is why splitting alternate installations into subsidiary extensions of no more than 100 sprinklers each is the standard approach.
Winter changeover: drain and pressurise
This is where site errors begin. Get the sequence wrong and either water is left in the pipework and freezes, or pressures fail to balance and the alarm valve trips spuriously. The typical procedure:
- Valve room temperature — confirm it is above 4 °C. If not, fix the heating and thermostat before starting.
- Isolate the wet side — close the downstream isolating valve and observe upstream pressure.
- Open every drain in sequence — main drain, manifold drains and the auxiliary drains at every low point. Wait until the water runs out completely; one frozen pocket ruins the whole run.
- Bring the compressor on — charge the pipework slowly with air, to a target typically between 2.5 and 3.5 bar depending on the valve differential.
- Prime the alarm valve — fill the priming chamber with clean water; its level keeps the alarm valve closed.
- Open the downstream isolation slowly — pressurised water below the valve seat and air above it hold the valve closed.
- Test — open the remote test valve and measure water delivery time. It must be within the limit, and the measurement recorded in the logbook.
- Certificate — the maintenance contractor signs a form recording the changeover date, the delivery time and the air pressure.
The spring changeover runs in reverse: air is released, the system is filled slowly from the low point, and every branch is vented to clear air pockets. Air trapped in horizontal pipes causes pressure surges at the first test and spurious alarm valve operation.
The most common field error: still wet in winter
Inspecting an alarm valve at a cold store in mid-December, the valve was still in wet mode — with the outside temperature at −8 °C and the north-facing section of the pipework already frozen. The owner said "we change over every November, this year we forgot". Two problems emerged: a 12 m section of pipe had split, and the water column in the main below the valve had frozen and burst it. Repairs, an additional tank and retesting came to about 18 % of the value of the whole installation.
That is not simply human forgetfulness but a systemic gap. Field practice under EN 12845 has three layers of defence:
- A low-temperature alarm in the valve room — sent to the BMS or the owner's phone below 5 °C. The standard requires the valve room to stay above 4 °C; this alarm is the live monitoring of that requirement.
- A maintenance calendar certificate — the annual maintenance must document that the alternate changeover was carried out at the right time. If the maintenance contractor keeps no record, the standard is breached.
- Pipe temperature monitoring — thermostat-controlled trace heating tape at risk points (roof voids, external wall penetrations), switching on as freezing approaches.
The heart of a sound alternate installation is tying the changeover to a system rather than to a person. Without a thermostat alarm, a BMS notification and a calendar reminder all three, no alternate installation can be called modern.
Subsidiary alternate extensions
It is rare for a whole building to be alternate; usually only a small freeze-prone area — a loading dock, an external store, a cold corridor — is separated from the wet main as a subsidiary alternate extension. The limits:
- No more than 100 sprinklers on one subsidiary extension.
- Where more than two extensions share a control valve set, the total across them must not exceed 250.
Those limits run parallel to the 4 m³ volume threshold, guaranteeing that a subsidiary extension meets the 60 s delivery time. A larger freeze zone must be built as a separate full dry installation.
Galvanising and corrosion
The standard states that galvanised steel should be preferred in dry, alternate and pre-action installations. Alternate pipework holds water through the summer and humid air through the winter, and that wet-dry-wet cycle accelerates internal corrosion. In the field, a ten-year uncoated alternate installation can show up to 20 % flow loss from scale at the sprinkler orifice. Galvanised pipe or a nitrogen generator (pressurising with oxygen-free air) defends against that loss; nitrogen generators are becoming common on EN 12845 sites too, and the standard does not prohibit them.
Comparison with NFPA 13
The nearest NFPA equivalents are pre-action and antifreeze systems, or seasonal use of a dry pipe valve. On the NFPA side the remote area increase for dry systems is 30 %, slightly more than the EN 12845 25 %. NFPA's water delivery limit is 60 s for standard systems and tighter at 50 s for systems above 1000 sprinklers. EN 12845 allows up to 90 s for light hazard, which NFPA does not.
Turkish context
BYKHY refers sprinkler installations directly to EN 12845. It uses the phrase "dry or alternate system" for freeze-prone areas without detailing the changeover procedure; on site, both the equipment and the maintenance flow depend entirely on the alternate installation and maintenance clauses. In central and eastern Anatolia and the higher districts of the Black Sea region, certification of an alternate installation should always require the changeover procedure document and a thermal alarm matrix — without them, an insurance claim is likely to be refused as operator error.
Frequently Asked Questions
What is an alternate installation?
One that runs wet in summer and dry in winter, using either an alternate alarm valve or a composite wet-plus-dry valve set, changed over seasonally.
Why does the tank grow by half?
Because the dry-mode area of operation is 25 % larger. For OH3 that means 270 m² instead of 216 m², taking the minimum tank from 105 to 160 m³.
What is the biggest operational risk?
Forgetting the winter changeover. The defence is systemic: a low-temperature valve room alarm, a documented maintenance calendar and trace heating at risk points.
How large can a subsidiary alternate extension be?
No more than 100 sprinklers, and no more than 250 across several extensions on one control valve set — limits that keep the water delivery time achievable.
SprinkCalc — Fire Sprinkler Design Across Three Standards
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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.