The FDPJ series rate-of-rise heat detectors operate on the pneumatic principle, i.e. the expansion of heated air in the air chamber, and respond to rapid rise in temperature caused by fire. A slow or gradual temperature rise as would result from a heating system in the building does not activate the detector because any moderate excess air is released into the atmosphere through the air vent. By the use of a common base, the FDPJ series rate-of-rise heat detectors are interchangeable with NOHMI smoke detectors (FDK series). This interchangeability enables optimum detector types to be chosen for protected areas in case of change in the use of the rooms. The detector is equipped with a response lamp by which it is possible to readily identify the alarming detector. This feature facilitates the functional check of the detector.