What Are the Differences in Duct Smoke Detectors?

Smoke Sensors

• Homes with duct smoke detectors mounted outside ductwork use ionization or photoelectric smoke sensors to sample airflow within sensing chambers. Photoelectric smoke sensors employ optical sensors or diode light to measure smoke levels in the duct’s air supply. Ionization smoke sensors use small amounts of radiation to ionize air in the ducts. The ionization process creates an electrical current in the airflow, which acts as a detector when the air stream’s conductivity — the ability to create an electrical current -- decreases.

  In addition, externally mounted sensors collect air samples using tubes reaching into the main air supply and return ducts. The smoke detector uses ionization or photoelectric sensors to sample the air. If the smoke sensors detect smoke particles from overheated appliances such as an air conditioner, relay mechanisms automatically shut off the motor to prevent smoke from spreading to other parts of the house.

Area Smoke Detector

• Instead of using externally mounted sensors, homeowners may attach area smoke detectors onto an outlet box located inside the duct. The area smoke detector can also be mounted on the duct’s exterior. Also, the detector is positioned so that the sensor extends into the air supply. This allows the area smoke detector to monitor the air particle concentration and alert the system if air samples are abnormal.

Light Beam Detectors

• Light beam detectors are installed inside ducts and include both an air sample projector and receiver. The duct smoke detector shines an optical light beam directly into the air stream, and searches for areas where light is scattered by smoke particles. These duct smoke detectors are best used inside plenums, or boxed areas that supply, and in certain cases return, air to the duct system.

Air Sampling Detectors

• Air sampling detectors work similarly to smoke sensors, but without photoelectric or ionization technology. These duct smoke detectors insert sampling tubes or other detection devices into the duct’s air stream, however. The tubes extract the air and sample it through the detector. If the smoke particles exceed normal concentration levels, the detector activates an alarm.