Should all detectors be systematically replaced after a fire?
No. Mass replacement is only justified if contamination or thermal exposure exceeds the thresholds set by the manufacturer for each detector type. The correct decision is made zone by zone, distinguishing devices directly exposed to flames or intense heat from those that only received light smoke deposits from a neighbouring area.
Systematic replacement without justification has a cost with no guarantee of additional reliability if the diagnosis has not been made. Conversely, leaving a contaminated detector in place without technical inspection creates a real risk: a fouled sensor can become either hypersensitive (false alarms) or desensitised (late detection). It is this technical diagnosis, not a blanket rule, that should guide the decision.
How do combustion particles damage a detector?
Combustion particles (soot, tars, acidic residues) deposit inside the measuring chamber of optical smoke detectors and alter how light is scattered within it. This deposit distorts the device's baseline: the sensor may then trigger at a smoke level well below normal, or conversely mask real smoke if the deposit is heavy and opaque.
Thermal detectors are more resistant to this type of deposit, but residue on the sensing element can alter its thermal conduction and delay its response. Aspirating systems (VESDA type) are particularly exposed: filters and pipes become clogged, reducing airflow and lengthening detection time. Finally, combustion gases often contain chlorinated or sulphur compounds that accelerate corrosion of electronic contacts, a fault that may appear several weeks after the incident, with no apparent link to the initial event.
What do manufacturers recommend for checking a detector after an incident?
Manufacturers recommend a visual inspection of the housing (deformation, discolouration, melting marks) that reveals exceedance of the maximum operating temperature declared for the device, followed by a functional test with test aerosol and a sensitivity measurement compared to the model's nominal range. These operations correspond to the inspections provided by the NBN S 21-100-2 standard, which governs maintenance and periodic inspection operations for fire detection installations.
On models with a removable optical chamber, the manufacturer often allows targeted cleaning of this component rather than complete detector replacement. On sealed-chamber models, no internal cleaning is possible: the sensitivity test then becomes the decisive criterion. In all cases, this verification must be performed by a qualified technician, not by simple visual inspection by building personnel.
What objective criteria allow choosing between cleaning and replacement?
The choice between cleaning and replacement is based on verifiable criteria, not visual impression. The following table summarises the most common cases encountered during post-incident diagnosis.
| Observed criterion | Cleaning feasible | Replacement necessary |
|---|---|---|
| Type of deposit | Light smoke, no sticky residue | Greasy soot or embedded tar |
| Thermal exposure | Temperature remained below manufacturer's nominal threshold | Visible housing deformation, discolouration, partial melting |
| Sensitivity test result | Within nominal range after cleaning | Out of range even after cleaning |
| Chamber accessibility | Removable and cleanable optical chamber | Sealed non-removable chamber |
| Device age | Recent detector, far from end of life | Detector near or beyond indicative lifespan (10 years) |
A detector that fails the sensitivity test after cleaning must be replaced, without exception: cleaning is never 100% guaranteed and the manufacturer sets the tolerances, not the installer.
What are the consequences if a contaminated detector is not inspected?
A contaminated detector left in place without inspection exposes the building to two types of failure: masked detection, where the alarm triggers too late because the sensor has lost its actual sensitivity, and repeated false alarms, which ultimately lead occupants to ignore real alarms. This latter effect is particularly dangerous in establishments where staff manage several false alerts per month: vigilance erodes.
Without targeted diagnosis after the incident, this fault will often only be detected at the next periodic inspection provided by NBN S 21-100-2, which may represent several months of uncontrolled exposure for the operator. This period constitutes an identifiable and avoidable security gap, which must be documented in the installation register, particularly for potential inspection by the emergency zone (hulpverleningszone).
Who is authorised to perform this verification after a fire?
This verification must be performed by a qualified technician from the installer, ideally a BOSEC-certified company, which has the sensitivity testing tools and manufacturer references necessary to decide between cleaning and replacement. Simple visual inspection by internal building personnel cannot detect internal fouling of the optical chamber or sensitivity drift.
Following the intervention, the results (detectors cleaned, tested compliant, replaced) must be recorded in the installation register, alongside the periodic inspection reports provided by NBN S 21-100-2. The establishment's prevention adviser must be informed of the result, and the emergency zone (hulpverleningszone) may request to review it during its next visit.