Heat detectors are devices that sense the presence of heat. They can be wired or wireless. Some are combination fixed temperature and rate of rise. The two most common fixed temperature settings are 135 F and 190 F. Rate of rise, usually a change of 10 F in one minute, will cause the device to go into alarm, but they are self resetting so troubleshooting is difficult unless they are addressable. Some cities do not allow rate of rise heat detectors, and others require alarm verification to be used, due to false alarms. Please check with your local Fire Dept for their requirements.