The D341 Duct Smoke Detector Housings mount on the ducts of heating, ventilation, and air conditioning (HVAC) systems to monitor the presence of smoke in the conditioned air. The housing works with all 24 VDC or VAC or 120 VAC, UL Listed four-wire control panels. The efficient housing design samples the air passing through a duct and detects potentially hazardous conditions. When smoke is detected, the detector sends an alarm signal to the control panel or HVAC control equipment that initiates the necessary action to control air handling systems.