Definition
A temperature detector (heat detector) warns of fire by watching for an abnormal rise in ambient temperature. Unlike a smoke detector, it responds more readily to open flame or high-temperature radiation, so it suits dusty rooms, oily kitchens, and other places where a smoke detector should not be fitted. Used with smoke detectors and combustible-gas detection, it is one layer of a multi-signal fire warning system.
How it works
The sensing element is usually a thermistor, a thermocouple, or another temperature sensor. It alarms when ambient temperature passes a set point, such as 60℃ or 70℃, or when the rate of rise passes its limit. A fixed-temperature detector alarms at the set temperature. A rate-of-rise detector alarms when the temperature climbs too fast. A combination detector does both. The output can be a contact (normally open or normally closed) or an analog signal (4-20mA), so it can land on a fire alarm panel or an IoT platform.
Smart fire protection and IoT
A conventional detector is wired to the fire alarm panel. In an IoT layout it can carry WiFi, NB-IoT, or LoRa and send temperature and alarm state to the cloud platform for remote watching, a history curve, and linkage across points. With intelligent video analysis and behavior analysis, the same hazard can be judged from more than one source. SimtoLink can connect fire devices such as heat detectors, collect the data, push the alarm, and integrate them through an API.
Selection and placement
Selection follows the room (kitchen, warehouse, equipment room, underground space), the temperature range and alarm threshold, explosion protection and ingress rating, how it is powered (mains, battery, or the fire bus), and whether it fits the fire system already installed. Mounting height and spacing should meet fire codes such as GB 50116, and the detector should stay off air-conditioning outlets and away from heat sources. Calibration and maintenance keep the sensitivity and hold the false-alarm rate down.
What Triggers a Rate-of-Rise Alarm
A heat detector is the temperature judgment in a fire-alarm chain: it raises a fire alarm when ambient temperature reaches a set point, or when temperature rises too fast over a unit of time. It does not record a process curve, and it is not used to control air conditioning. A fixed-temperature type watches absolute temperature, a rate-of-rise type watches the rate of rise, and a combination type watches both. In kitchens, boiler rooms, and dusty workshops, smoke is often present and smoke detectors false-alarm. A heat detector has a chance of separating open flame or abnormal heat from that background.
It can be used when mounting height, spacing, and the area covered meet the fire-detection requirements of the place, and there is a gap between the alarm threshold and the normal highest temperature. A threshold pressed against summer room temperature, or against the normal temperature of an equipment enclosure, false-alarms every day. A threshold far above the temperature the probe location can reach when combustibles ignite sounds only after the fire has spread. It measures air around the probe, not the center of the flame. With a high ceiling, or when hot air is blown apart, the temperature rise at the probe slows.
Installed at an air-conditioning outlet, beside a steam pipe, or next to a steel beam in the sun, a normal hot airflow is taken as a fire. Paint baking just after construction, or an open furnace door, also causes a short rise. A rate-of-rise detector that operates when a door opens to admit a hot workpiece is not highly sensitive. The place was chosen wrongly. If an alarm is silenced only on the platform and not reset on the fire-alarm control panel, the bus state and the IoT state disagree, and the duty operator sees two conclusions.
The thermal element inside a heat detector is a sensor, but the alarm logic is a limit judgment. It does not hand a temperature value to a process to optimize. Treating the detector's on-off signal as a continuous temperature for a cold-store curve yields either only a step, or no process value at all. A place that needs continuous temperature measurement should choose a temperature element, not treat a fire threshold as a control target.
Fixed temperature and rate of rise answer different questions. Fixed temperature suits a place where the ambient temperature itself can rise to a dangerous value. Rate of rise suits a place that is usually not hot but heats quickly in a fire. Using only rate of rise in a workshop that already has a periodic heat source can alarm every time a door or a furnace opens. Using only rate of rise in a loft where temperature climbs slowly may wait until it is already very hot without having reached the rate limit. A combination type also has to have both thresholds checked. Testing only one of them does not make the whole detector acceptable.
After installation, do a functional test. The method should let the sensing element actually feel temperature, or a specified simulated signal, not merely short a wire on the control panel. A short proves only that the circuit is continuous. It does not prove the thermal element still works. Dust cover or a coat of paint keeps heat outside the housing. The test interval should be able to find that failure. When a detector address does not match the drawing, the alarm point points to the wrong room. If the networked platform keeps the wrong address, the remote location is wrong as well.
Used together with smoke and gas detection, state which acts first and how it is confirmed, so a false alarm at one point does not start interlocking for the whole zone. Once interlocking starts a fan or closes a valve, it should be possible to record which detector triggered it. Recording only "fire alarm" makes it impossible later to judge whether the heat-detector position or another detector should be adjusted. After a heat detector returns to normal, confirm the fire alarm has been reset. It is not enough for the icon on the IoT platform to turn green again.