Smoke and Fire Detection | Heat and Smoke Detectors

Smoke and fire detection can be done using two types of devices, “Heat detectors” and “Smoke detectors”.

Heat detectors generally detect the heat that comes as a result of combustion. The devices trigger an alarm when they sense a rapid rise in temperature or when a certain temperature is reached.

A smoke detector is an electronic fire-protection device that automatically senses the presence of smoke, as a key indication of fire, and sounds a warning.

The four types of fire detectors are heat, ionization, photoelectric, and ionization/photoelectric are discussed in this article.

The differences in each of these how they detect fires, heat being from temperature, and the other three being from smoke.

Heat Detectors

Heat detector is a device that can detect heat and an increase in air temperature. Its purpose is to alert homeowners and business owners that there is a fire before the fire becomes irreversible.

Heat detectors have two main classifications of operation,

  1. Rate-of-rise heat detector
  2. Fixed temperature heat detector

They are ideal in situations where smoke detectors may cause false alarms such as steamy, humid, or dusty environments. Occasionally occupied areas like storage facilities, warehouses, or machine rooms also use these types of detectors.

The heat detectors are paired up with control panels which sound the alarm, initiate the water sprinklers or call the fire department.

smoke and fire detection, heat detectors,

Heat detectors : Image source

Rate-of-Rise Heat Detector

Rate-of-rise heat detectors sense the heat in the room. It has two heat-sensitive thermocouples or thermistors. One thermocouple monitors heat transferred by convection or radiation while the other responds to ambient temperature.

The detectors register threshold (say 70 oC) as a baseline; when the heat in the room rises rapidly above threshold, the alarm is activated.

smoke and fire detection, heat detectors,

When a house fire starts, the temperature in the room will rise rapidly. This type of sensor is triggered by the “rate of change” rather than the temperature itself.

Rate-of-Rise (ROR) heat detectors operate on a rapid rise in some element temperature increase per minute, irrespective of the starting temperature. Generally, the alarm is triggered when the ROR is a 15 oC increase in less than a minute.

Rate-of-rise thermal detector responds faster than the fixed temperature detector.

Fixed Temperature Detector

This is the most common type of heat detector. The fixed temperature detector focuses on the actual room temperature, rather than the rate of change of temperature.

When the temperature in the room crosses a predetermined point, the alarm is activated. Typically, the fixed temperature that triggers the alarm is around 58oC.

A fixed temperature detector contains a bimetallic switch element that closes at a specified temperature limit. The switch is normally composed of two different metals having a different temperature coefficient of expansion.

Smoke and Fire Detection | Heat and Smoke Detectors

As this bimetallic element heats the metal with higher coefficient of expansion, it causes the switch to bend. Which results in closing the switch and thus indicating an alarm condition.

Which heat detector is better?

The Rate-of-rise heat detector and Fixed temperature heat detector has their own advantages. It cannot be said that one type of heat detector should always be used instead of another.

If a rate-of-rise heat detector is installed above a closed oven then whenever the door is opened a false alarm could be generated due to sudden rise in heat. In this case the fixed temperature detector would probably be best.

If a fixed temperature heat detector is installed in a room filled with highly inflammable or explosive materials then a fast-flaming fire could exceed the alarm threshold due to thermal lag. In such case the rate-of-rise heat detector should be preferred.

Thus, the selection of heat detector is based on the place where it is to be used.

Advantages of Heat Detectors

  • Heat detector has lower cost than smoke detector units.
  • They also come with a low life cycle cost.
  • They are more reliable than smoke detector units because they are less prone to false alarms.
  • Heat detectors can operate under varied environmental conditions.
  • They are not affected by dusty or dirty environments.
  • Require minimal maintenance.
  • They are not affected by electromagnetic interference.
  • The devices can resist contagions such as IR and UV rays.

Drawbacks of Heat Detectors

  • Although they are great at heat detection, they won’t be able to detect a smouldering fire quickly. They are best for detecting high-heat fires.
  • They take more time to identify a fire than a smoke detector.

Where to Install Heat Detectors?

Heat detectors are suitable for application where other smoke detectors are not useful and cause false alarms.

For example, any environment like warehouses, factories, laundry room, storage facilities or garages which has a high number of airborne particles due to increased steam, dust or moisture will not benefit from smoke detectors.

In such cases, heat detectors become a preferred choice as they focus on temperature to detect fires. This will allow extra time to evacuate the building or to put out the fire, if possible.

Heat detectors are better choices in this case of the kitchen in home. Smoke detectors may not be the ideal choice as kitchen can be filled with smoke during normal cooking.

Smoke Detectors

Heat detectors commonly have a label on them that reads “Not a life safety device”. Smoke detectors are preferred over heat detectors because heat detectors are not meant to replace smoke detectors in the bedrooms or in the hallway outside of the bedrooms.

A smoke detector is an electronic fire-protection device that automatically senses the presence of smoke, as a key indication of fire, and sounds a warning.

There are three types of smoke detectors.

  1. Ionization smoke detector
  2. Photoelectric smoke detector
  3. Ionization/Photoelectric smoke detector

Commercial and industrial smoke and fire detectors issue a signal to a fire alarm.

Household smoke detectors, or smoke alarms, issue an audible and/or visual alarm locally from the detector itself. They can be battery-powered single units or several interlinked hardwired (mains-powered) devices backed up by batteries.

Ionisation smoke detector

Generally, ionization detectors are more responsive to fires resulting from flammable liquids, wood or paper starting on fire. This fire is called flaming fire because this type of fire produces a lot of flames with a limited amount of smoke. Most house fires are having these fires, which is why ionization alarms are popular in homes.

Ionisation smoke alarms are highly sensitive to small smoke particles and respond faster to fast flaming fires than photoelectric smoke alarms, but not to smouldering fires.


As shown, it is constructed by placing small radioactive source or component between reference chamber and sensing chamber.

smoke and fire detection, smoke detectors,

The ionization chamber is an aluminium can containing the ionization source. The can has slots in it to allow air flow. The can itself acts as the negative plate of the ionization chamber. Under the can is a ceramic holder that contains the positive plate of the ionization chamber.


Ionisation alarms carry a small amount of radioactive material between two electrically charged plates, which ionises the air and causes current to flow between the plates.

Americium-241 is used as radionuclide in this type of smoke detector. The radioactive material is placed between two electrically charged plates.

Americium-241 that electrically charges, or ionizes, the air molecules between two metal plates. This produces a small electric current flowing from one plate to the other in the air. Normally, current is stable in sensing chamber.

When smoke enters the chamber, it disrupts the flow of ions, thus reducing the flow of current and activating the alarm.


  • Its cost is less than photoelectric smoke sensors
  • It outperforms photoelectric smoke detector when detecting fast flaming fires with little visible smoke.


  • It is very sensitive, which can lead to false alarms.
  • They are slower than photoelectric sensors in detecting smoke particles from smouldering fires.
  • Use of radioactive material is a concern.

Photoelectric Smoke Detector

Photoelectric smoke detectors respond an average of 15 to 50 minutes faster to fire in its early, smouldering stage, before it breaks into flame. This technology works by detecting light that is reflected off particles from a light beam inside the sensing chamber.


The figure depicts inner design of photoelectric smoke detector of light scattering type. It consists of a light source and a photo detector.

smoke and fire detection, smoke detectors,

In this type of smoke detector, light is transmitted in the area which is normally not seen by the photodetector.


When no particles are present in the sensing chamber, the light beam does not strike the light detector. Hence in normal condition there will be no light received by the detector circuit.

In the smoky condition, photoelectric detector senses sudden scattering of light when smoke enters the detector chamber. When it reaches a certain threshold level the alarm is triggered.


  • Ideal for detecting dense smoke.
  • It does not contain radioactive materials, making it safer for use.


  • It is sensitive to dust particles and insects, meaning that regular maintenance is needed.
  • Maintenance cost is more.
  • Requires more current to operate.

Ionization/Photoelectric Smoke Detector

Both Ionization detectors and Photoelectric detectors can detect either fast-burning “flaming” fires or slow-burning “smouldering” fires, but each technology has its particular strengths.

Ionization detectors detect small black soot particles from flaming fires more quickly because they are produced in greater numbers and reduces current between the plates.

Whereas, Photoelectric detectors are more sensitive to particles that are larger in size and white or light-coloured, and thus more reflective.

Therefore, having both types of smoke detectors installed detect multiple types of fires. A combination these smoke detectors is the best way to protect residence from a fire.

When both forms of smoke detection are together on one device it helps to ensure that regardless of the type of fire, it will be detected as soon as possible.

For MCQs on Smoke and Fire Detectors

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