Heat detectors for fires have been around since the mid-1950s. Since most residential deaths from fire are due to smoke inhalation, not heat, the development of smoke alarms was a great advancement in reducing deaths. Today, heat detectors are primarily employed where it is impractical to use a smoke detector due to environmental conditions, such as in outdoor elevator lobbies. There are now very few code requirements for many types of heat detectors, but we will review them here along with best practices. 

One of the most common applications for a heat detector is in sprinklered elevator shafts. The intent is to use a heat detector that has a lower response time index and lower temperature setting than the sprinkler head to provide a means to disable elevator power prior to activation of the sprinklers. After all, electricity and water are never a good mix.

It takes all kinds

Heat detectors include restorable and nonrestorable fixed-temperature detectors, rate-of-rise heat detectors, rate-compensation detectors, combination fixed-temperature detectors, rate-of-rise detectors and line-type heat detectors. It is important to choose the right type of heat detector for the intended application. Heat detectors have a listed spacing, while smoke detectors have a “nominal” spacing of 30 feet. Therefore, the spacing has to be reduced for ceilings higher than 10 feet, per Table 17.6.3.5.1.

Restorable fixed temperature heat detectors have a sensing element that contains two dissimilar metals that expand at different rates when heated. When the detector reaches its listed temperature rating, the detector will go into alarm. When the sensing element cools, the detector will restore to its normal condition and can be reused. Chapter 14 of NFPA 72 allows these detectors to be tested over a longer period of time. You must test 20% of these detectors per year so all are tested in a five-year period. 

Nonrestorable fixed-temperature detectors obviously cannot be heat-tested. At the initial acceptance test, you can short across the contacts on the back of the detector to ensure that if the detector went into alarm, the control panel would show an alarm. Table 14.4.3.2 of NFPA 72 states, “After 15 years from initial installation, replace all devices or have 2 per every 100 detectors laboratory tested. Replace the 2 detectors with new devices. If a failure occurs on any of the detectors removed, remove and test additional detectors to determine either a general problem involving faulty detectors or a localized problem involving 1 or 2 defective detectors. If the detectors are tested instead of replaced, repeat tests at intervals of 5 years.”

Fixed-temperature heat detectors also use Table 17.6.2.1 for information on the temperature classifications and associated color codes. You should always use a detector that is rated a minimum of 20 degrees above the expected maximum ceiling temperature to avoid unwanted alarms.

Too hot too fast

Rate-of-rise detectors operate on the principle of the air inside the chamber heating up from the fire and expanding. The pressure from the expansion will allow the alarm contacts to touch and initiate the alarm. The typical rate-of-rise temperature is approximately 12–15 degrees per minute. 

Remember that it is the rate of change that matters, not the temperature of the air. For example, don’t install rate-of-rise detectors next to air conditioning vents. When the air conditioner comes on, the air will cool, and when it turns off, the air will heat back up. That is a common source of unwanted alarms when using rate-of-rise detectors. It is common to see combination heat detectors that contain both a fixed-temperature element and a rate-of-rise chamber. Rate-of-rise detectors are usually tested annually.

Rate-compensation detectors are similar to fixed-temperature detectors, but operate faster by compensating for thermal lag. Thermal lag is the difference in time it takes the surrounding ceiling to reach a given temperature versus the time it takes the heat detector heating element to reach the same temperature. For a fixed-­temperature detector, the ceiling temperature may be much higher than the rated temperature of the detector before it activates. These are commonly found in outdoor elevator lobbies. Rate-compensation detectors are usually tested annually.

The last type of heat detector we will discuss today is the line-type heat detector. These specialty detectors are commonly used in cable trays or to protect gas lines. Although they are nonrestorable, they can be cut and the affected portion can be replaced. 

In my next article, I will discuss the various spacing criteria for heat and smoke detectors. There are many choices because of ceiling heights and types. It is somewhat difficult to understand this section of Chapter 17 to determine the correct way to install detectors in these areas. I will offer tips on how to more easily determine the correct method.

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