How to Deploy Heat Detectors

The need for automatic fire detection in conjunction with manual activation is a matter of record.

The following passage from the Fire Alarm Signaling Systems Handbook, published in 1987 by the National Fire Protection Association (NFPA), is but one example of the importance of automatic detection technology: “Human beings are excellent but unreliable fire detectors. Various mechanical, electrical, and electronic devices in use today mimic the human senses of smell, sight, hearing, taste and touch to detect the environmental changes created by fire.”

Last month’s “Fire Side Chat” discussed the correct placement of NAC devices, manual pulls and spot-type automatic smoke detectors. This month, we’ll explore one of the most inglorious and unappreciated fire detection technologies in use — traditional heat detectors.

When Heat Detectors Are Required
There’s little doubt automatic smoke detectors are the preferred detection technology when the ultimate aim is to protect life and property. But there are situations where smoke detectors cannot be used. There are also applications where heat detection is actually preferred over smoke detection.

For example, heat sensors are often used in confined areas where a sudden rise in heat is expected. In tests involving this type of setting, it was found that heat detectors actually do a better job of detecting a fire than smoke detectors. In this regard, several applications come to mind, such as motor vehicle garages and furnace rooms — both of which are prone to flash fires. Here, heat detectors are apt to detect the fire more quickly than their smoke detection cousins.

Another aspect associated with heat detector use involves environmental conditions where smoke detection devices are unlikely to render the desired result. This would include applications such as manufacturing where dust and other elements could otherwise cause false alarms in smoke detection devices.

Another reason why fire alarm technicians use heat and not smoke detectors involves potentially hostile conditions. One example includes the dust and oil often encountered at the bottom of an elevator shaft. Attics also pose a problem to smoke detectors because of the high temperatures that can exceed the unit’s specified operating temperature.

Heat Sensing Technology Choices
Knowing what kind of heat detector to use in a specific installation is important. Using the incorrect technology will not only cause operational problems, but also can result in property loss due to delayed detection, or no detection at all.

In order to determine what type of heat detector to use in any given application, it’s important for fire technicians to thoroughly understand the various types on the market. They should also understand the limitations of each one and which one the authority having jurisdiction (AHJ) is more apt to pass.

There are two basic types of heat detection technologies in use today: line- and spot-type. Line-type involves detection in a line, such as special heat-sensitive cable like that made by Protectowire Fire Systems of Plymouth, Mass. In this case, the Protectowire Linear Heat detection cable consists of two solid, steel conductors separated by thin, heat-sensitive insulation.

These two conductors are placed on a bind so when enough heat is introduced into the environment the heat-sensitive insulation melts, allowing the two steel conductors to come into contact with one another. The objective is to initiate an alarm condition.

Another variation of this involves the use of fiber-optic cable. Protectowire’s fiber-optic technology is able to determine the temperature of the environment through which this fiber-optic cable runs. Not only that, but the control equipment, which is made especially for this purpose, can determine the exact spot within the facility where an abnormally high or rapid increase in temperature has taken place.

Another linear type of heat detection system uses a coaxial-type cable that relies on a semi-conductor, thermister-type insulator sandwiched between an outer metallic covering and an inner center conductor. A slight current, which is made to flow through the circuit at all times, acts to supervise the sensor.

As the outside temperature rises, so does the current flowing between the center and outer conductors. A current comparator circuit within the internal circuits of the control system is designed to identify thermal increases and sound the alarm.

Spot-Type Uses 3 Basic Methods
Within the spot-type heat sensor category you will find three basic technologies at work: fixed, rate compensation and rate-of-rise.

Bimetallic fixed-temperature heat detectors are comprised of two metal strips as contacts — each having a different temperature coefficient. As the temperature increases in the area surrounding the detector, the two metal strips begin to flex or move. If the temperature continues to rise to the detector’s set point, the two strips will eventually come into contact with one another, causing the alarm to sound.

Within the fixed-temperature category you also will find a nonrestorable detector that uses some kind of fusible link or a special solder that holds a spring-loaded plunger switch in check. When a fire occurs, the solder will then melt, allowing the plunger to move and the internal contacts to short out, causing an alarm to take place.

The rate-of-rise heat detector is probably the most commonly used in real life.

These sensors are designed to detect relatively sudden rises in temperature. Electronic heat sensor technologies have begun to replace the older mechanicals, and rightly so as fire detection moves into the high-tech realm. These sensors rely on a similar semi-conductor technology, as is used within the coaxial, line-type cable mentioned earlier. This type of spot sensor provides a similar supervisory current to internal circuits that monitor it for relatively rapid changes.

Adhering to Placement by Code
Placement of heat detectors begins with the axiom that a heat detector’s rated detection spacing must be reduced in order to cover all portions of a room.

“In laying out detector installations, designers work in terms of rectangles, as building areas are generally rectangular in shape. The pattern of heat spread from a fire source, however, is not rectangular in shape. On a smooth ceiling heat spreads out in all directions in an ever-expanding circle. Thus, the coverage of a detector is not, in fact, a square, but rather a circle whose radius is the linear spacing multiplied by 0.7” (Section A-5-2.4.1, NFPA 72, 1996 Edition).

The specifics concerning placement are a detailed process, one that cannot be left to chance. Those details, however, are far too extensive to be included in a single fire column. In next month’s “Fire Side Chat,” we’ll continue our discussion about automatic heat/smoke detectors with the details of placement.

In the meantime, fire technicians should obtain a current edition of NFPA 72, National Fire Alarm Code where they can review Section 5.6, Heat-Sensing Fire Detectors, in which the details behind heat detectors are revealed. Be sure to read the same section in Annex A.

Additional information on heat detection technology and code issues can also be found on FireNetOnline (FNO) at

For the complete version of this story, see the August issue of Security Sales & Integration magazine.

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