The rather blazing hot summer has gotten me thinking about heat detectors, or as they are referred to within NFPA 72, The National Fire Alarm and Signaling Code, Heat-Sensing Fire Detectors. Heat detectors, in one form or another, have been around since the first fire alarm standard was issued in the late 1800s. While smoke detectors are the primary method of automatic detection now installed as part of an automatic fire alarm system, the heat detector still has its uses and advantages over smokes in a number of applications.
Smoke detectors provide early detection of an incipient fire and are listed as life-safety devices, but they cannot be installed in all locations or environments. This is especially true for dusty or dirty environments or those exposed to weather or moisture. There are several manufacturers that produce smoke detectors that can work within a number of harsh environments, but these may not be practicable for every application.
Heat detectors should be considered when providing detection in environments that may be dirty, dusty, damp or wet, external, exposed to fog or mist. They may also be considered for those times when the installation of smoke detectors has caused unwanted alarms. One item of note is that heat detectors are not listed as life-safety devices, as they only detect a rise in heat, not the products of combustion.
4 Heat Detector Types Explained
There are four primary types of heat detectors on the market today:
Fixed Temperature — Generally constructed using bimetallic components, this detector activates when a preset temperature is reached. Some models may also use eutectic solders. The most common temperature settings are 130° and 190° F. When the temperature within the detector reaches the set parameter, it will go into an alarm state. If the sensor is bimetallic, once the detector fires it will have to be changed as this component cannot be used again. Note that this type of detector reacts not to the temperature outside but the internal temperature environment. The difference between the external and internal temperature is called thermal lag.
Rate-of-Rise — These are also constructed using bimetallic components, but may also be using thermistors. These detectors look for a sudden rise in temperature, generally 10° F during a one-minute time period. For example, the temperature of the environment may be 65° and rises to 80° within one minute. In this case, the rate-of-rise component of the detector would go off, but if the cause of the temperate increase was removed the detector would restore. Thermal lag comes into play with these detectors as well. The rate-of-rise detector also has a fixed-temperature component.
Rate Compensation — These detectors address thermal lag, and will trip when the external temperature reaches the rated limit. This detector will activate quicker than a fixed or rate-of-rise device.
Line Type — The three detectors discussed above are referred to as spot-type detectors, in which the device is located in a fixed position and will respond to temperate increases within the listed spacing. In most cases, this is within a circular pattern, with the detector being in the center. Linear detectors typically follow a cable that is installed throughout the protected property or space. The coverage is based from the position of the cable outward. This type of detector may be installed in difficult-to-reach or -cover spaces within an occupancy. Depending on the type being used, the cable may be zoned so as to provide a location from which the alarm trip originated.
Design and Installation Guidelines
The installation and spacing of heat detectors is found in Section 17.6 of NFPA 72, 2013 edition. There are a number of requirements the system designer and installer need to be aware of when heat detectors are used. The following are key excerpts from Section 17.6 followed by my comments:
188.8.131.52 The heat detection design documentation shall state the required performance objective of the system. The designer of the system must provide a brief statement as to the expected design performance of the heat detectors that are to be installed.
184.108.40.206.2.3 Spot-type heat detectors shall also be marked with their Response Time Index. RTI was added back to the requirements for heat detectors in the 2010 edition. When originally included in NFPA 72, it came from the requirements for sprinklers. The method for calculating RTI for sprinklers, however, would not work for a heat detector. Researchers at FM Global developed a method for the calculation of RTI for heat detectors and it was then placed back in 72. RTI provides a method to measure the speed of a detector’s response.
220.127.116.11 Detectors having fixed-temperature or rate-compensated elements shall be selected in accordance with Table 18.104.22.168 for the maximum expected ambient ceiling temperature. The temperature rating of the detector shall be at least 20° F (11° C) above the maximum expected temperature at the ceiling.
This is a section that is overlooked during the design and installation of heat detectors. If installing detectors within an attic space or other area that may be subject to high heat, the detector cannot be listed for a temperature that is less than 20° F higher than the expected maximum temperature. For example, you would not install a 130° fixed temperature detector in a space that may exceed 110° or 120° during the summer. Nor would there be any cases to install a rate-of-rise in an attic space.
22.214.171.124.3.1 Unless otherwise modified by 126.96.36.199.2, 188.8.131.52.2 or 184.108.40.206, spot-type heat-sensing fire detectors shall be located on the ceiling not less than 4 inches (100mm) from the sidewall or on the sidewalls between 4 inches and 12 inches (100-300mm) from the ceiling.
This rule has not changed in a number of editions. Do not confuse the requirements for heat detectors with smoke detectors. The exclusion of the 4 inches has been removed for smoke detectors, but not heat detectors.
220.127.116.11.3.2 Unless otherwise modified by 18.104.22.168.2, 22.214.171.124.2 or 126.96.36.199, line-type heat detectors shall be located on the ceiling or on the sidewalls not more than 20 inches (510mm) from the ceiling.
Line-type or linear detectors may be installed on a side wall, but they must be at least 20 inches from the ceiling. Spacing between links would be in accordance with the manufacturer’s published instructions and NFPA 72.
Shane Clary, Ph.D., has more than 37 years of security and fi re alarm industry experience. He serves on a number of NFPA technical committees, and is Vice President of Codes and Standards Compliance for Pacheco, Calif.-headquartered Bay Alarm Co.