A heat detector is a device that senses an increase in temperature and activates an alarm. There are two types of heat detectors: fixed temperature and rate-of-rise. A fixed-temperature heat detector is set to activate at a specific temperature, while a rate-of-rise heat detector is activated when the temperature increases at a certain rate.

Some heat detectors use a thermistor, which is a type of resistor that changes resistance with temperature, to sense the increase in temperature. Others use a bimetallic strip, which is a strip of metal that bends when heated, to activate a switch. Heat detectors are often used in fire alarm systems to detect the presence of a fire and alert building occupants to evacuate.


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Heat Detector  – A heat alarm is designed to detect heat instead of smoke, the alarm contains a thermistor that is set to respond to temperatures above 58°C.

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When a fire breaks out hot air from the fire will rise and enter the sensor chamber.

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When the temperature inside the chamber reaches 58°C a signal is sent to the integrated circuit which causes the alarm to sound alerting the occupants to the fire heat Detectors are normally used in environments where a smoke detector might generate false alarms.

Heat detectors can be situated in the kitchen and the garage, because heat alarms don’t react to smoke they are not prone to false alarms from cooking and exhaust fumes.

The rate of Rising heat detectors will alarm if the temperature rises very quickly, or if the temperature reaches a set threshold. This type of detector would be the first choice in an environment where a smoke detector could not be used.

In some environments, such as boiler rooms, fast rates of rise of temperature can be expected normally, meaning that there would be a risk of false alarm when using a rat-of-rise device. In this case, fixed temperature detectors give an alarm once the temperature has reached a preset threshold, most commonly 58°C or 78°C for EN54-5 Class AS or BS respectively.

Heat Detectors Coverage

When mounted on a flat ceiling, heat detection devices have an individual coverage of a 5.3m radius. However, these radii must overlap to ensure there are no “blind spots”. Therefore individual coverage can be represented by a square measuring 7.5 X 7.5m giving an actual coverage area of 56.3m² per device.