Heat Detectors


A heat detector is a fire detection device. It is similar to a smoke detector; however it is more resistant to false alarms in areas with a large amount of dust and debris. Since it does not detect smoke, it is therefore not meant as a replacement for a smoke detector. In fact, this type of sensor is used to detect changes in ambient temperature; alarms are triggered when the temperature gets too hot.

The following will discuss:

• Types of Heat Detectors
• Operation
• Configuration
• Applications

Heat Detector- Types and Operation

There are two types of heat detectors on the market today:

• Fixed Temperature
• Rate of Rise

Fixed Temperature

A fixed temperature heat detector makes use of a metal alloy. When the ambient room temperature exceeds the specified temperature for that particular device; the alloy melts and in turn give rise to an alarm. Normally the fixed temperature for these detectors is 115°F to 135°F. It is important to replace this type of heat detector once an alarm has been triggered; if the metal alloy melts, the detector is worthless.

Fixed temperature heat detector

Rate of Rise

This type of heat detector employs a sensing component; used to detect rapid or slow/steady increases in the ambient room temperature. This type is slightly more sensitive than the fixed temperature detector.

There are two scenarios that can cause an alarm:

• A sudden and abrupt change in temperature.
• The detector observes a 12°F to 15°F increase per minute within the room. This “rate of temperature increase” can be programmed on some models.

One type of detector is not better than the other; rather they have different purposes. For example, you would not put a rate of rise type in a kitchen; for the reason that when the oven door is opened a sudden change in room temperature would cause an alarm. A fixed temperature is not well suited in an area with highly combustible materials; in that an earlier warning is usually required.


There are two ways with which heat detectors can be configured:

• Independently
• Two or more in parallel


One heat detector is directly wired to an input on the fire alarm panel. This is perhaps the better configuration in that when an alarm occurs, the area of the fire is indentified.

Two or More in Parallel

Two or more in parallel heat detectorAs explained in the following diagram. Two or more detectors are wired in parallel to one input on the fire alarm panel. This is useful when a large area must be protected (industrial application). If all devices are within the same room, then the area in alarm will be easily identified. If however, the devices are mounted in separate areas this can cause confusion as to which area is in alarm.


The following is a list of possible applications for heat detectors:

• Garage
• Closet
• Kitchen
• Laundry room
• Attic
• Machine Shop
• Welding Shop