Introduction: Gas and Heat Detectors
Fire alarm systems are built on detectors that respond to different signs of combustion: smoke, heat, flame, and gas. A gas fire detector detects the presence of carbon monoxide (CO), and less commonly methane or propane, long before an open flame appears. A heat fire detector activates when a threshold temperature is reached or when the temperature rises sharply. These two types address fundamentally different scenarios: gas detectors are good for early detection of smouldering fires with incomplete combustion, while heat detectors are suitable for rooms where smoke and gas are inevitable under normal conditions, but overheating is a danger.
Gas detectors are indispensable in living rooms and bedrooms, where a fire often starts from smouldering wiring or a dropped cigarette. Heat detectors are installed in kitchens, boiler rooms, garages, and saunas — places where high temperature is the only reliable criterion. Understanding the difference between these two principles helps the designer avoid false alarms while not missing a real fire. More details on all types of fire detectors and regulations are provided in the overview about fire detectors.
Comparative Characteristics Table
| Parameter | Gas Fire Detector | Heat Fire Detector |
| Controlled indicator | Concentration of carbon monoxide (CO), sometimes methane or propane | Air temperature or its rate of rise |
| Early detection of smouldering fire | Very high; CO is released before smoke and flame appear | Low; heating to threshold 54–78 °C is required |
| False alarms from dust, steam, cooking | Low; gas sensor is selective | Medium; steam and sharp temperature changes may trigger differential models |
| Resistance to dusty environments | High; dust does not affect the gas sensor | High; heat detectors are not sensitive to dust |
| Operating temperature range | −10 to +55 °C | −40 to +70 °C; some models up to +100 °C |
| Typical thresholds | 50–100 ppm carbon monoxide | 54, 64, 72, 78 °C depending on class |
| Response inertia | 1–3 minutes during slow smouldering | From 10 seconds (differential) to several minutes (maximum) |
| Types | Addressable, radio-channel, explosion-proof, combined with smoke | Addressable, analogue-addressable, explosion-proof, differential and maximum |
| Cost | Higher than heat, comparable to smoke | Lower; the most budget-friendly type |
| Typical rooms | Living rooms, bedrooms, corridors, offices | Kitchens, boiler rooms, garages, saunas, warehouses, workshops |
Operating Principle and Controlled Fire Indicator
A gas fire detector is built on an electrochemical or semiconductor sensor sensitive to carbon monoxide. During incomplete combustion of organic materials — wood, fabrics, plastics — CO is released even before visible smoke appears. The detector measures the concentration in ppm and triggers when the threshold is exceeded, typically 50 ppm. This allows detection of a fire at the earliest stage, when a person can still extinguish the source themselves. Some models respond to domestic gas methane or propane and perform a dual function.
A heat fire detector detects either reaching a maximum temperature or the rate of temperature rise. Maximum models activate when the heat-sensitive element heats up to a set threshold — 54, 64, 72, or 78 °C. Differential models respond to a temperature rise rate of 5 to 10 °C per minute, which speeds up detection of fast-developing fires. A heat detector will not notice a smouldering fire without heat generation, but it is indispensable where smoke or steam is constantly present, making smoke and gas sensors useless. A comparison of smoke and heat models is discussed in detail in the article smoke or heat.
Sensitivity and Response Speed
In terms of response speed to smouldering fires, gas detectors outperform heat detectors. Carbon monoxide spreads throughout the room by convection, and its concentration builds up gradually but steadily. The sensor activates within 1–3 minutes after smouldering begins, long before open flames appear. A heat detector may remain silent for tens of minutes until the temperature at the ceiling reaches the threshold. If a fire develops quickly, for example, when igniting flammable liquids, a differential heat detector will activate faster than a gas detector, because the temperature rises rapidly while gas has not yet accumulated to a sufficient concentration.
Gas detectors are sensitive to ventilation: drafts dilute carbon monoxide, and the threshold is reached more slowly. Heat detectors are independent of ventilation, as they respond to air temperature, which rises during a fire regardless of airflow. Therefore, in rooms with powerful supply ventilation, the effectiveness of a gas detector decreases, and it should be supplemented with a heat or smoke detector. Differential heat detectors can also generate false alarms when a door is suddenly opened in winter or a heater is turned on, so their installation requires care.
False Alarms and Noise Immunity
Gas detectors are practically unaffected by dust, steam, or cooking smoke, which distinguishes them favourably from optical smoke detectors. However, they are sensitive to carbon monoxide, which appears in garages from running engines, in kitchens with gas stoves during incomplete combustion, and in boiler rooms when the flame goes out. Therefore, in such rooms, gas detectors may produce false alarms, and preference is given to heat detectors, which respond only to temperature. Heat detectors are not afraid of dust, steam, or gases, but differential models can react to sudden heating from process equipment, such as when opening an oven or starting a heater.
To reduce false alarms, combined detectors that integrate gas and heat channels are used. This solution allows fire confirmation based on two independent indicators: the presence of carbon monoxide and a temperature increase. This improves the reliability of the signal and reduces the likelihood of false evacuation. In analogue-addressable systems, the control panel decides on a fire by analysing the dynamics of both parameters, which is even more reliable. More details on addressable systems are given in the comparison addressable vs. non-addressable.
Types: Addressable, Explosion-Proof, Combined
Gas detectors are available in addressable and non-addressable versions. An addressable gas detector transmits not only the alarm status but also the current gas concentration to the panel, allowing real-time monitoring of the atmosphere. Radio-channel models transmit signals via radio and are used where cable installation is difficult or undesirable. Explosion-proof gas detectors are installed in Zone 1 and 2 areas according to GOST 31610 and are certified under TR CU 012/2011. They feature housings that prevent ignition of explosive mixtures from internal sparks.
Heat detectors are also available in a wide range: maximum, differential, and maximum-differential. Analogue-addressable versions transmit temperature readings to the panel, allowing individual thresholds to be programmed for different rooms. Explosion-proof heat detectors are used in oil and gas facilities where hot gas release is possible. Combined gas-heat devices combine two principles in one housing and are used in rooms with a high risk of smouldering and potential thermal interference.
Installation and Placement
Gas detectors are mounted on the ceiling or wall at a height of 0.3–0.6 m from the ceiling, as carbon monoxide is slightly lighter than air and spreads in the upper zone. For methane, which is lighter than air, the sensor is placed under the ceiling; for propane, near the floor. Heat detectors are fixed to the ceiling at the point of greatest heating during a fire, at least 0.5 m from luminaires and other heat sources. Installation standards, distances between detectors, and room heights are regulated by SP 5.13130.2009. Both types require regular maintenance: sensitivity checks, dust cleaning, and sensor replacement for gas detectors after 5–7 years of operation.
When to Choose a Gas Fire Detector
- Living rooms, bedrooms, children’s rooms — early detection of smouldering fires at night.
- Office spaces with paper documents and office equipment — CO appears before smoke.
- Rooms with high ceilings, where heat detectors take a long time to heat up.
- Facilities where false alarms from dust and steam are unacceptable.
- Early warning systems in explosive industries with carbon monoxide monitoring.
When to Choose a Heat Fire Detector
- Kitchens, canteens, where cooking generates smoke, steam, and carbon monoxide under normal conditions.
- Boiler rooms, garages, workshops where exhaust gases are possible.
- Baths, saunas, drying chambers with high ambient temperatures.
- Warehouses, workshops, production rooms with dust and possible sharp temperature fluctuations.
- Rooms with powerful supply ventilation, where gas detectors are ineffective.
Typical Mistakes in Selection and Installation
- Installing a gas detector in a kitchen with a gas stove. False alarms from combustion products make the system useless.
- Using a heat detector in a bedroom. In a smouldering fire, it may not activate until the fire spreads.
- Mounting a gas detector near the floor for carbon monoxide detection. CO is lighter than air; the detector should be placed higher.
- Placing a differential heat detector next to a heating radiator. Sharp temperature changes cause false alarms.
- Neglecting maintenance: gas sensors lose sensitivity over time, and the detector stops responding to real fires.
- Choosing a standard heat detector for an explosive area without an Ex certificate. This violates TR CU 012/2011 and poses a direct explosion hazard.
Frequently Asked Questions
What is the difference between a gas detector and a heat detector?
A gas detector responds to carbon monoxide produced during smouldering, while a heat detector responds to temperature rise. Gas detectors detect fires earlier but are useless in rooms with background gas. Heat detectors are more reliable in kitchens and boiler rooms.
Can a gas detector be installed in a garage?
Not recommended. Exhaust gases contain carbon monoxide, which will cause constant false alarms. For a garage, a heat detector is preferable.
Which type is better for a residential house?
A gas detector. It detects smouldering fires at an early stage when a person is asleep and does not smell smoke. Heat detectors are ineffective in bedrooms.
What is a combined gas-heat detector?
It is a device that integrates two sensors: gas and heat. It activates when carbon monoxide appears or when the temperature rises, reducing false alarms and improving reliability.
How often should the gas sensor be replaced?
Usually every 5–7 years. The sensor life is limited, and sensitivity decreases after its useful life. The exact period is specified in the product datasheet.
Is an explosion-proof detector different in operating principle?
No, the principle is the same, but the housing and electrical circuits are designed to eliminate sparking and withstand internal explosions. Such models are mandatory in Zone 1 and 2 areas.
Conclusion
Gas and heat fire detectors do not compete but complement each other. Gas detectors are indispensable for early detection of smouldering fires in residential and office premises. Heat detectors are for kitchens, boiler rooms, garages, and industrial facilities where smoke and gas are normal. The choice of a specific type depends on the fire scenario and background conditions. A competent combination of these detectors in one system ensures reliable protection of the facility at any stage of a fire.
