A fire alarm that cries wolf is more than an inconvenience. Repeated false alarms cause occupants to ignore the system, create fatigue among building users, and can result in the fire and rescue service charging for attendance. In some cases, businesses have been fined for repeated unnecessary calls to emergency services. Understanding why false alarms happen — and how to reduce them — is one of the most practical skills a responsible person or fire alarm engineer can develop.
What Is a False Alarm?
A false alarm — properly called an unwanted fire signal (UFS) — is any activation of the fire alarm system that isn't caused by an actual fire. This includes:
- Malicious false alarms: deliberate activation of a call point
- False alarms due to equipment failure: a detector or panel component that activates without an external trigger
- Unwanted alarms from other causes: the most common category — detectors responding to non-fire stimuli like cooking fumes, steam, dust, or insects
BS 5839-1 focuses primarily on reducing unwanted alarms from other causes, as these are the most preventable.
Common Causes of Unwanted Fire Alarms
1. Cooking Fumes and Steam
This is the single most common cause of unwanted alarms in commercial buildings with kitchens or break rooms. Smoke detectors — particularly optical detectors, which are highly sensitive to larger aerosol particles — can be triggered by burnt toast, steam from a kettle, or fumes from a grill. Even light cooking odours in a small, poorly ventilated kitchen can cause repeated activations.
Why it happens: The detector can't distinguish between the aerosols from a kitchen and the aerosols from a fire. Both scatter infrared light in the same way.
2. Dust
Construction or renovation works generate significant quantities of dust. Fine particulate matter entering optical detectors can cause gradual contamination or sudden false alarms during peak disturbance. Vacuuming without dust extraction or removing suspended ceiling tiles nearby are frequent culprits.
Older ionisation detectors, once common in the UK but now less frequently installed, are particularly sensitive to fine dust particles.
3. Insects
Small insects — particularly spiders — entering a detector head can trigger false alarms. Spiders are attracted to the slight warmth of detector electronics and can spin webs inside the sensing chamber. A spider moving through the optical chamber of a smoke detector is a classic cause of a single isolated false alarm with no obvious source.
4. Steam
Steam from showers, dishwashers, and autoclave equipment behaves similarly to smoke in certain detector types. Buildings with sports changing rooms, commercial kitchens, or medical facilities often experience regular activations from steam ingress if detectors are poorly positioned.
5. Aerosol Sprays and Chemical Vapours
Aerosol sprays — deodorants, cleaning products, air fresheners — release fine particles that optical detectors can detect. In small rooms where someone sprays near a detector, activation is common. Some industrial cleaning chemicals and paint fumes also trigger alarms.
6. Rapid Temperature Changes
Heat detectors respond to temperature, not smoke. Fixed-temperature heads activate at a set threshold (typically 58°C or 90°C). Rate-of-rise heads respond to rapid temperature increases. Both can be triggered by: opening an oven door nearby, direct sunlight heating a poorly positioned detector, or heat from kitchen equipment directly below.
7. Detector Age and Contamination
Detectors have a finite service life, set by the manufacturer's recommendation for that device, so check its datasheet rather than assuming a fixed age. Ageing or contaminated detectors can drift, so they activate at lower stimulus levels. Contaminated optical chambers scatter internal light even without external particles, causing random or frequent nuisance alarms.
What BS 5839-1 Says About False Alarm Management
BS 5839-1:2025 deals with the limitation of false alarms in a dedicated section (Section 3, Clauses 29 to 33), covering responsibility, categories and causes, investigation, the design process and the measures used to limit false alarms. As summarised in the Fire Industry Association, Guide to the changes in BS 5839-1:2025:
- The commissioning or handover organisation should advise the user to arrange suitable investigation, and action where appropriate, on every occasion that a false alarm occurs (subclause 29.6).
- The commissioning or handover organisation should also explain the categories of false alarm to the user, so that each incident is assigned the correct category.
- A preliminary investigation is recommended when the false alarm rate over the previous 12 months exceeds four false alarms per 100 detectors per annum. For systems with more than 40 automatic fire detectors, an in-depth investigation is recommended when the average rate exceeds five false alarms per 100 detectors per annum. The FIA notes these trigger points did not change in the 2025 revision.
For the calculation and worked triggers, see BS 5839-1:2025 false alarm rate: how to calculate it and when to investigate.
The standard also recommends maintaining a false alarm log — noting the date, zone, detector or call point, suspected cause, and action taken. This log is essential for identifying patterns and demonstrating due diligence.
Practical Steps to Reduce False Alarms
Alarm Confirmation (Two-Knock)
One of the most effective technical measures is alarm confirmation — requiring two detectors in the same zone to activate before sounding a full evacuation alarm. The length of any confirmation or investigation period is set in the system design and fire emergency plan, not by a standard default, and manual call points normally bypass it. If the second detector activates within the confirmation period, full alarm is raised.
This approach significantly reduces evacuations from single spurious activations without compromising life safety.
Detector Relocation
If a specific detector repeatedly causes false alarms due to its environment, moving it is often the most reliable long-term solution. Heat detectors instead of smoke detectors in kitchens, beam detectors rather than point detectors in high-dust environments, and multi-sensor detectors in catering areas can all reduce false alarm rates.
Multi-Sensor Detectors
Modern multi-sensor detectors combine optical smoke sensing with heat sensing and apply intelligent algorithms to reduce false alarms. They are significantly less likely to respond to steam, cooking fumes, and dust than conventional optical detectors, making them the preferred choice for open-plan offices, reception areas near kitchens, and similar environments.
Detector Sensitivity Adjustment
Addressable systems allow individual detector sensitivity to be adjusted at the panel or via system software. Reducing sensitivity during high-risk periods (morning kitchen rush, cleaning times) and restoring it afterwards can reduce false alarms without removing detector coverage.
Alarm Receiving Centre Protocols
If the system is monitored, agreeing an investigation delay with the ARC before the fire and rescue service is called allows time for the responsible person to investigate. This is a common and legitimate arrangement — but the investigation procedures must be properly documented, and the delay must not compromise life safety.
Staff Training and Signage
Many false alarms are avoidable with simple occupant awareness. Signage near kitchen areas reminding staff to use ventilation before cooking, clear procedures for reporting false alarms, and training for responsible persons on investigating activations all contribute to reduction.
When to Escalate
If you're experiencing more than two unwanted alarms per month, or if a cause cannot be identified after reasonable investigation, escalate to your fire alarm maintenance contractor. Persistent false alarms should trigger a formal review under BS 5839-1 Category B or C.
Persistent unwanted alarms are not just an operational problem — they represent a genuine safety risk, because occupants stop taking the system seriously.
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