Documentation / heat detectors

Heat Detector Technology From Fenwal Controls

Heat Detector Technology From Fenwal Controls


Heat Detector technology offered in Fenwal’s DETECT-A-FIRE® fire detection units have met with wide acceptance because they are designed with RATE COMPENSATION. This provides a unique advantage over both fixed temperature and rate-of-rise heat detector types because only the DETECT-A-FIRE unit accurately senses the surrounding air temperature regardless of the fire growth rate. At precisely the predetermined danger point, the system is activated.

The heat detector employs two independent methods of detection—rate-of-rise and fixed temperature. The rate-of-rise method detects fires that rapidly grow in intensity. This method responds to abnormally fast temperature increases. The fixed-temperature method detects fires that build temperatures to a high level at a slow rate. This method responds to a specific temperature setting.

A fixed head detector must be completely heated to alarm temperature and therefore a disastrous lag in time may occur with a fast rate fire. Rate-of-rise devices, on the other hand, are triggered by the rate of increase in ambient temperature and are subject to false alarms caused by harmless, transient thermal gradients such as the rush of warm air from process ovens.

The secret of the heat detection unit's sensitivity is in the design. The outer shell is made of a rapidly expanding alloy which closely follows changes in surrounding air temperature. The inner struts are made of a lower expanding alloy. Designed to resist thermal energy absorption and sealed inside the shell, the struts follow temperature changes more slowly.

A slow rate fire will heat the shell and struts together. At the "set point", the unit will trigger, actuating the alarm or releasing the extinguishant. A transient rush of warm air up to 40ºF /min. may expand the shell, but not enough to trigger the unit. By ignoring transient warm air excursions, the DETECT-A-FIRE unit virtually eliminates false alarms prevalent with rate-of-rise devices.

If a fast rate fire starts, the shell will expand rapidly. The struts will close, actuating the alarm or releasing the agent. The faster the fire rate of growth, the sooner the DETECT-A-FIRE unit will react.


  • UL, ULC and FM approved
  • Rate compensation principle ignores rate-of-rise and thermal lag to respond precisely at set-point
  • Vibration resistant for industrial applications
  • Hermetically sealed contacts with significant explosion proof and intrinsically safe listings
  • Over ten temperature settings from 140 to 725° F (60 to 385° C)


  • Corrosion resistant stainless steel and flurocarbon coated shell options
  • Extended lead wires
  • Horizontal units with flush or surface mounting
  • Normally open or closed contacts