A neutral white exterior and an unobtrusive, low-profile housing make the TD heat detector blend into any ceiling. The plastic housing fits onto any standard octagon or square electrical box with a trim plate. The unit will protrude no more than 1" below the surface of the finished ceiling. Using a bi-metal disc as a heat-sensing element, an alarm is initiated when the air temperature exceeds the unit's rated level. Only when the air temperature decreases below the rated level will the detector automatically reset.