Frigid Forecast: How does the environment affect detection?

By Wayne D. Moore | Feb 15, 2024
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I am often asked about what detection device is best for a specific environment. The most recent question was whether a certain device could be used in freezing weather. Other questions center around dusty or high ceilings.

I am often asked about what detection device is best for a specific environment. The most recent question was whether a certain device could be used in freezing weather. Other questions center around dusty or high ceilings.

Environment is crucial

Even for normally heated environments, temperature variations can affect detection. Specifically, in Chapter 10, Fundamentals, we are told in Section 10.3.5 of NFPA 72, National Fire Alarm and Signaling Code, that “Equipment shall be designed so that it is capable of performing its intended functions under the following conditions:

  1. At 85 percent and at 110 percent of the nameplate primary (main) and secondary (standby) input voltage(s)
  2. At ambient temperatures of 32°F (0°C) and 120°F (49°C)
  3. At a relative humidity of 85 percent and an ambient temperature of 86°F (30°C).”

Section 10.4.3 states, “Equipment shall be installed in locations where conditions do not exceed the voltage, temperature, and humidity limits specified in the manufacturer’s published instructions.”

So, can a device be used in a freezing environment? No, unless the device is listed for temperatures below 32°F. We could find out by reviewing the manufacturer’s specification sheet for the device. There is a dearth of listed devices that will operate in a freezing environment.

The only options for these environments would seem to be heat detection. However, a device listed for 32°F should never be used in an area that is functionally always at freezing temperatures unless it is listed by a national testing laboratory (e.g., UL or Intertek) for this kind of environment.

There will be a delay in detection time, depending on the device’s operating temperature in a freezing environment.

The space’s ambient temperature is always an issue for concern, regardless of the type of detection device. Given that the actual detection of a fire and avoidance of false alarms are crucial goals, there is guidance in the code for heat detection that is often overlooked. 

When using heat detection, the code requires that the detector’s temperature rating be at least 20°F above the maximum expected temperature at the ceiling of the protected space to avoid false alarms for heat incursions in the space. As an example, don’t use a 135°F heat detector in an attic where temperatures could exceed the detector‘s operational characteristics.

Smoke detection and temperature

Smoke detection is greatly affected by temperature. When the temperature in the space protected by a smoke detector moves beyond the device’s listed temperatures, the device will operate more slowly.

Again, the code guidance for smoke detectors states in Section, “Unless specifically designed and listed for the expected conditions, smoke detectors shall not be installed if any of the following ambient conditions exist:

  1. Temperature below 32°F (0°C)
  2. Temperature above 100°F (38°C)
  3. Relative humidity above 93 percent
  4. Air velocity greater than 300 ft/min (1.5 m/sec).”

In certain parts of the country in non-air-conditioned spaces, it may be necessary to evaluate the detector’s performance in that space.

The Annex A information for smoke detectors advises us that the national laboratory testing the devices for listing purposes include tests for temporary excursions beyond normal temperature limits. In addition to temperature, Annex A says that smoke detectors should operate reliably under common environmental conditions (such as humidity), velocity variations, mechanical vibration, electrical interference and others. Tests for these conditions are also conducted by a national laboratory. 

Where the device’s stated environmental conditions are exceeded, consult the detector manufacturer’s published instructions for additional information and recommendations.

Smoke stratification

Differing temperatures can cause smoke stratification.

Annex A states that “Stratification occurs when air containing smoke particles or gaseous combustion products is heated by smoldering or burning material and, becoming less dense than the surrounding cooler air, rises until it reaches a level at which there is no longer a difference in temperature between it and the surrounding air.”

This means that when designing smoke detection in a space where stratification may occur, expect delayed smoke detection.

There are many considerations when using any type of detection. It’s important to be aware of the limits of detectors and the environment’s effect on detection when designing systems. / kongvector

About The Author

MOORE, a licensed fire protection engineer, was a principal member and chair of NFPA 72, Chapter 24, NFPA 909 and NFPA 914. He is president of the Fire Protection Alliance in Jamestown, R.I. Reach him at [email protected]





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