A neutral white exterior and an unobtrusive, low profile housing make the TD heat detector blend onto 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 finishing 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.