The Model HD Risk Area Heat Detector accurately senses the surrounding air temperature and actuates an electrical contact output at a pre-determined temperature. The expansion coefficients of the metal alloys used in the construction of the detector enable the hermetically sealed contact to activate at a precise point. The Model HD Detector can be used as an alarm device to sense excessive heat or fire and warn personnel, or it can be used as a signaling device to sense fire and send a signal to an alarm panel for actuation of a fire suppression system.