What is smoke stratification in a large volume?
Smoke stratification is the phenomenon by which hot smoke from a fire stops rising before reaching the ceiling and stabilizes at an intermediate height called the neutral layer. This phenomenon occurs because smoke cools upon contact with ambient air over a large free height, until it reaches a density close to that of the surrounding air: it then stops rising and forms a horizontal layer.
In an atrium, industrial hall or volume with several dozen metres of ceiling height, this layer can stabilize well below the roof or skylight. An incipient fire can thus produce dense, visually detectable smoke without ever reaching a detector installed at the highest point of the building. This is the central challenge of fire detection in large volumes, and the reason why a specific study is necessary from the design stage.
Why are conventional point detectors insufficient in large volumes?
A point detector (optical smoke or heat detector) installed only on the ceiling detects only smoke that physically reaches its installation point. If the smoke layer stratifies several metres lower, as is common in a high-ceiling atrium, the ceiling detector may remain inactive for a significant period while the fire progresses.
There is also a practical constraint: a point detector installed at fifteen or twenty metres height is difficult to access for testing and periodic maintenance required by NBN S21-100-2, which complicates long-term maintenance. This is why standard NBN S21-100-1, which governs the design of detection installations, requires consideration of the building's geometry and selection of appropriate technology when point detection alone cannot guarantee detection within useful timeframes.
How does linear beam detection work at height?
Linear beam detection (or optical linear detector) relies on an infrared emitter-receiver pair that monitors a straight line crossing all or part of the volume to be protected. As soon as smoke intercepts the beam, signal attenuation triggers the alarm, over a range that can cover the entire width or length of an atrium.
The advantage for large volumes is twofold. Firstly, a single device covers a much larger area than a point detector, reducing the number of points to install and maintain at height. Secondly, it is possible to install several superimposed beams at different heights in the same volume, precisely to intercept the smoke layer regardless of the level at which it stratifies. This multi-level approach is the most direct technical response to the stratification phenomenon.
How does aspiration detection work at height?
Aspiration detection (or ASD, aspirating smoke detection) uses a network of pipes with sampling points that continuously draw small quantities of air, conveyed to a remote central optical or laser detector, often at ground level or in an accessible technical room.
This technology offers a specific advantage for atriums: sampling points can be distributed at multiple heights in the same volume, without needing to install a complete electronic device at each level. Only the piping, simpler to route and clean, is located at height, while the detector itself remains accessible for periodic inspection operations. Detection sensitivity is also finer than that of a conventional point detector, enabling earlier detection even when smoke remains diluted in a stratified layer.
What does NBN S21-100-1 provide for large volume design?
NBN S21-100-1 governs the design and installation of fire detection systems in Belgium, including for buildings with atypical volumes such as atriums, industrial halls or high-ceiling reception halls. It requires that the installation study take into account the actual geometry of the building, not a standard scheme designed for standard-height rooms.
Concretely, this means that the choice between point detection, linear beam detection or aspiration detection must result from an analysis of the volume to be protected, ceiling height, possible presence of air currents (skylights, natural smoke extraction, HVAC) that influence smoke behaviour. This study is the responsibility of a qualified installer, ideally BOSEC certified for installed equipment, in consultation with the building's prevention advisor.
What does NBN S21-100-2 provide for maintenance of these installations?
NBN S21-100-2 sets out the procedures for periodic inspection and maintenance of fire detection installations, regardless of the technology chosen, including optical beam or aspiration systems. It applies to both conventional point detectors and specific equipment used in large volumes.
For an atrium, this maintenance requirement directly influences the initial technology choice: an optical beam requires access at height (lift platform, platform) for cleaning optics and emitter-receiver alignment, whereas an aspiration system limits interventions at height to visual inspection of piping, with the central detector remaining accessible from ground level. Anticipating these maintenance constraints from the design stage avoids costly operational interventions and guarantees long-term detection reliability.
How to choose between optical beam and aspiration for an atrium?
The choice between the two technologies depends on volume configuration, accessibility at height and the required sensitivity level. The following table summarizes the main criteria to balance during the study.
| Criterion | Conventional point detection | Linear detection (beam) | Aspiration detection (ASD) |
|---|---|---|---|
| Principle | Isolated optical or heat detector | Emitter-receiver pair on a line | Tube network to a central detector |
| Covers stratification at multiple heights | No, unless multiplying points | Yes, by superimposing beams | Yes, by distributing sampling points |
| Access at height for maintenance | Required at each point | Required for each emitter/receiver | Limited to piping, detector at ground level |
| Sensitivity | Standard | Standard to good | Very fine, early detection |
| Typical atrium use | Low zones or complement | Large open volume, long range | Complex volumes, difficult access |
In practice, many atriums combine both approaches: beams to cover large open areas, aspiration for zones where access at height is particularly constrained or where very early detection is required.