Smoke detector vs heat detector — Which do you need?
The differences between smoke and heat detectors, what each responds to, where each should be installed, and how to choose the right type for each environment in your building.
Smoke detectors and heat detectors both trigger fire alarms — but they respond to completely different phenomena, operate reliably in completely different environments, and are appropriate for completely different rooms. Installing the wrong type is one of the most common causes of both false alarms and missed detections. Getting this decision right for every room in the building is a fundamental part of good fire alarm design.
What Smoke Detectors and Heat Detectors Respond To
| Characteristic | Smoke detector | Heat detector |
|---|---|---|
| What it detects | Smoke particles or combustion products in the air — either by light scatter (optical) or ionisation | A rise in air temperature — either to a fixed threshold (fixed temperature) or at a rate faster than normal (rate of rise) |
| Response speed | Fast — detects smoke early in a fire, often before flames develop | Slower — responds only when heat has built up sufficiently; fire is more developed by the time detection occurs |
| Sensitivity | High — responds to very small concentrations of smoke particles | Lower — requires significant temperature rise or high absolute temperature |
| False alarm risk in clean environments | Low — very reliable in clean, undisturbed air | Very low — not affected by smoke, dust, or steam |
| False alarm risk in hostile environments | High — steam, dust, fumes, and cooking smoke all trigger optical and ionisation detectors | Low — only triggered by genuine heat, not by airborne particles |
| Spacing under BS 5839-1 | 7.5m between detectors, 5.3m from walls | 5.3m between detectors, 3.75m from walls |
Types of smoke detector
Optical vs Ionisation Smoke Detectors
There are two main types of smoke detector, which respond differently to different fire types:
Optical (photoelectric) detectors use an LED light source and a photosensor. In clean air, the sensor sees no light. When smoke particles enter, they scatter the light beam onto the sensor, triggering an alarm. Optical detectors are particularly effective at detecting slow smouldering fires — the kind that produce large visible smoke particles before breaking into flame. They are the most common type in modern commercial installations and are recommended by BS 5839-1 for most applications.
Ionisation detectors use a small radioactive source to ionise air between two charged plates, creating a small current. Smoke particles disrupt this current, triggering the alarm. Ionisation detectors are more sensitive to fast flaming fires that produce smaller combustion particles. They are prone to false alarms from dust and are less commonly specified in new commercial installations than in previous decades — optical or multi-sensor detectors are generally preferred.
Types of heat detector
Fixed Temperature vs Rate-of-Rise Heat Detectors
Fixed temperature detectors trigger when the air temperature at the detector reaches a specific threshold — typically 57°C, 83°C, or higher for environments with elevated ambient temperatures. They are simple and reliable but respond only when the fire has already generated significant heat.
Rate-of-rise detectors trigger when the temperature is rising faster than a defined rate — typically 8°C to 12°C per minute — regardless of the absolute temperature. This gives faster response to a developing fire than a fixed temperature device. Most modern heat detectors are combination units that trigger on either a rapid rate of rise or on reaching the maximum fixed temperature threshold — giving both early warning from the rate-of-rise function and guaranteed activation at the fixed threshold even in slow-burning scenarios.
Where each type is used
Choosing the Right Detector for Each Environment
| Room or area | Recommended detector | Reason |
|---|---|---|
| Offices, corridors, meeting rooms | Optical smoke detector | Clean air, low false alarm risk, good sensitivity to smouldering fires |
| Bedrooms and sleeping areas | Optical smoke detector | Early warning essential for sleeping occupants — optical gives earliest detection of smouldering fires common at night |
| Commercial kitchens | Heat detector (fixed temperature or rate-of-rise) | Cooking fumes and steam cause constant false alarms with smoke detectors — heat detectors are immune to cooking by-products |
| Boiler rooms and plant rooms | Heat detector — rated for ambient temperature | High ambient temperatures and fumes make smoke detectors unreliable; choose a detector with an appropriate temperature rating |
| Dusty areas — workshops, warehouses | Heat detector | Dust particles trigger optical detectors; heat detectors are unaffected by dust |
| Bathrooms and shower rooms | Heat detector | Steam from showers triggers smoke detectors; heat detectors give more reliable and less disruptive performance |
| Server rooms and data centres | Aspirating smoke detection (ASD) | Very high sensitivity required for early detection of equipment overheating before visible smoke; ASD draws air through sampling pipes to a central highly sensitive detector |
| Garages and car parks | Heat detector or carbon monoxide detector | Vehicle exhaust fumes trigger smoke detectors; heat or CO detection avoids false alarms while detecting genuine fire risk |
Multi-sensor detectors
Multi-Sensor Detectors — the Best of Both
Multi-sensor detectors combine optical smoke sensing with heat sensing in a single unit. The panel evaluates signals from both sensors together — an alarm requires both to show elevated readings, or for one to show a very high reading alone. This combination significantly reduces false alarms compared to optical-only detectors while maintaining excellent sensitivity to genuine fires.
Multi-sensor detectors are increasingly the standard choice for general commercial areas — offices, corridors, reception areas, and hotel bedrooms — where the performance improvement over single-sensor optical detectors justifies the modest additional cost. They are recommended in the BS 5839-1 guidance for areas where false alarm reduction is important without compromising detection speed.
Common questions
Frequently Asked Questions
Only if the environment is suitable for a smoke detector. If a heat detector was specified for a kitchen, bathroom, or dusty area, replacing it with a smoke detector will result in frequent unwanted alarms from the normal activities in that space. The detector type must match the environment. If you want faster detection in a specific area, discuss the options with a fire alarm engineer — there may be a multi-sensor detector or a different smoke detector type suitable for the environment that offers better performance than the current heat detector without the false alarm risk.
A smoke detector positioned too close to a kitchen or cooking area will react to cooking fumes, steam, and airborne grease particles — all of which scatter light in the same way as smoke from a fire. The solution is to replace the detector in or near the kitchen with a heat detector, or to relocate the smoke detector further from the cooking area. This is one of the most common detector positioning errors in commercial and domestic installations. See our false alarm guide for more detail on causes and solutions.
The Regulatory Reform (Fire Safety) Order 2005 requires appropriate means of detecting and warning of fire — it does not specify detector types. The type required in each location is determined by the fire risk assessment and by the design requirements of BS 5839-1. A competent system designer will specify the correct detector type for each area based on the environment and the level of protection required.