Article by Matt Karver – Fire Alarm Sales Representative –

Heat detection has many applications and can be accomplished with several types of installation.  The engineer of record, the authority having jurisdiction, or the owner need to consider the surrounding environmental factors and applicable code requirements and listings when choosing the appropriate heat detection for a specific property.   

Ambient temperature

Ambient temperature is a major factor to consider when choosing heat detection.  If a manufacturing plant or boiler room holds equipment that produce a high-heat environment, a detector with a higher heat rating should be considered.  NFPA-72 requires heat detectors to be rated at least 20-degrees above the ambient ceiling temperature.  NFPA-72 also requires a color-code designation to help identify different heat detector temperature ratings. 

The colors and the associated temperature (Fahrenheit) are as follows:

• 100-134 (Low) = Uncolored
• 135-174 (Ordinary) = Uncolored
• 175-249 (Intermediate) – White
• 250-324 (High) – Blue
• 325-399 (Extra High) – Red
• 400-499 (Very Extra High) – Green
• 500-575 (Ultra High) – Orange

Conventional heat detectors

Fixed heat detectors activate only when the detectors operating element exceeds a set temperature.  The downside to a fixed heat detector is that it will not anticipate a growing fire until it has reached its set temperature.  Rate-of-rise heat detectors sense when the temperature rises at a rate exceeding a predetermined value, usually 15 degrees per minute.  Rate-of-rise heat detectors may not be ideal for situations where ambient temperature can fluctuate rapidly, like an attic.  Some detectors have both fixed and rate-of-rise detection capabilities to offer more protection.  This technology may also fail if a fire is heating slower than the detector’s rating.

Rate Compensation Detectors

Another device that combines both fixed and rate-of-rise sensing technology are rate compensation detectors.  The difference being is that these detectors sense the temperature of the air surrounding the device, where a fixed/rate-of-rise detector monitors an internal sensing element.  Because of this, rate compensation detectors are less susceptible to thermal lag.  These are best suited for outdoor environments where the temperature can vary from extreme cold to warm.