One of the most evident and obvious indications of fire is heat. Fire tends to make surrounding things hot. Heat detectors are designed to respond to this increase in heat and convey that signal to a fire alarm control panel.
FM Approved heat detectors are classified depending upon their capabilities: do they respond to a specific temperature, or is it the rate that the temperature is increasing (or a combination)? Additionally, FM Approvals evaluates heat detectors to ascertain their response time. These classifications are important because it affects how far apart the detectors can be spaced.