The heat detector operates by using a matched pair of thermistors to sense heat. One thermistor is exposed to the ambient temperature, the other is sealed. In normal conditions the two thermistors register similar temperatures; but, on the development of a fire, the temperature recorded by the exposed thermistor will increase rapidly, resulting in an imbalance that causes the detector to change into the alarm state.
Rate-of-rise detectors are designed to detect a fire as the temperature increases, but they also have a fixed upper limit at which the detector will go into alarm if the rate of temperature increase has been too slow to trigger the detector earlier. A highly visible external red LED flashes when the detector is in operational standby and changes to steady in alarm state.
Requires MSB-65B base.