Cold soldering is where the water from the first sprinkler to open cools its neighbour and prevents it operating. The result is counterintuitive: packing sprinklers more closely does not improve protection — it does the opposite.
What happens?
When a sprinkler opens it does not only spray downwards; some of the droplets are thrown sideways and wet the thermal element of the adjacent sprinklers. A wetted glass bulb or solder cannot reach its operating temperature, even while absorbing heat from the hot gas around it, because of evaporative cooling. That sprinkler stays silent while the fire grows.
The design assumes hydraulically that a defined number of sprinklers will operate. If some never open, the fire escapes the design area and the system cannot catch up.
The EN 12845 rule
Under EN 12845 clause 12.3 the minimum distance between two sprinklers is 2.0 metres. It can fall below that in only three cases:
- Where adjacent sprinklers are prevented from wetting each other, for example by a baffle of approximately 200 mm by 150 mm or by intervening construction features,
- For in-rack intermediate level sprinklers,
- On escalators and in stairwells (clause 12.4.11).
NFPA 13 expresses the same physics as a 6 ft (1.8 m) minimum (NFPA 13-2025 10.2.6.4.1) and permits going below it with baffles at least 8 in. (200 mm) long and 6 in. (150 mm) high (10.2.6.4.2), structural members acting as baffles (10.2.6.4.3) and in-rack sprinklers (10.2.6.4.4). The numbers differ, the logic is the same.
What is a baffle? A small metal shield placed between two sprinklers. It physically blocks the sideways droplet from reaching the neighbouring bulb. Where a later addition has brought the spacing below 2 m, a baffle is the most practical fix.
Where it is met on site
- Sprinklers added after refurbishment. A head added when a partition is removed or a duct passes through can end up 1.5 m from an existing head.
- Additional sprinklers under obstructions. A head dropped below a wide HVAC duct can sit very close to the head on the other side of it.
- Narrow corridors. The distance between heads at each end of a corridor can be short.
- Lift and stair shafts. Because of the level difference, the distance can look long on plan while being short in reality.
The vertical version: the water shield
The same problem appears vertically. A sprinkler operating within racking or beneath a perforated platform is wetted by the sprinkler above it. For that reason EN 12845 clause 14.7 requires a metal water shield of at least 0.075 m diameter. A missing water shield on in-rack sprinklers is a frequent finding at inspection.
Checklist
- Is there any sprinkler pair below 2 m on the drawing?
- If so, has a baffle been fitted, and is its size correct?
- Are water shields present on in-rack and under-platform sprinklers?
- Have heads added during refurbishment been checked against the spacing rule?
- Is each upright sprinkler at least 0.15 m from its pipe support (clause 17.2.2)?
Frequently Asked Questions
What is the minimum distance between sprinklers?
2.0 metres under EN 12845. It can fall below only where adjacent sprinklers are prevented from wetting each other (for example an approximately 200 mm by 150 mm baffle or intervening construction), for in-rack intermediate sprinklers, and on escalators and in stairwells (clause 12.3).
Why is cold soldering dangerous?
The design assumes a defined number of sprinklers will operate. If a neighbour is wetted and never opens, the fire escapes the design area and the system cannot catch up.
What size is a baffle?
EN 12845 clause 12.3 gives a baffle of approximately 200 mm by 150 mm as one way of preventing adjacent sprinklers from wetting each other; NFPA 13-2025 10.2.6.4.2 requires baffles at least 200 mm long and 150 mm high. Where a later addition has shortened the spacing, it is the most practical fix.
When is a water shield required?
On sprinklers operating within racking or beneath a perforated platform or floor. Water from the sprinkler above must not wet the thermal element below; a metal shield of at least 0.075 m diameter is mandatory (EN 12845 clause 14.7).
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Download MEP Calc on the App StoreEN 12845:2015+A2:2026 · EN 12259-1 · EN 12259-13 · NFPA 13 (2025) · FM Global DS 8-9. Definitions are for information only.