The Location of a Duct Smoke Detector

General Location Rules

• In general, duct smoke detectors in supply air ducts are installed after the fresh air inlets, filters, climate conditioning equipment and air blowers. In return air ducts, smoke detectors are generally installed after room air inlets and before any final filters and exhaust fans. Smoke detectors should be installed in the upper half of horizontal ducts. They should be installed 6 to 10 duct cross sections away from bends or openings. That means if you have a 6-by-12-inch duct, you install your detectors 6 to 10 feet downstream from a bend or opening.

Location Standards

• The National Fire Protection Association publishes detailed standards governing location of in-duct smoke detectors, while the detectors themselves must meet Underwriters Laboratories standards. Smoke detectors are required in the ductwork of air supply systems having a capacity greater than 2,000 cubic feet per minute. The detectors must be inspected, tested and cleaned at initial installation and annually thereafter. The testing involves introducing smoke into the detector and verifying that the device reacts correctly. Maintenance involves checking air sampling inlets to ensure they are not clogged or dirty, and cleaning the detector with a vacuum or brush.

One Component

• In-duct smoke detectors are one component of a complete fire and smoke detection system. They are not substitute for the building’s regular open area smoke detectors, heat detectors and fire detectors. In-duct smoke detectors typically wait for 15 seconds or so before sending an alarm. This helps minimize the chance of false alarms.

Basic Configuration

• In-duct smoke detectors fall into two general types with two different configurations for each type. Photoelectric detectors react to the presence of smoke that obscures a light beam, triggering an alarm. Ionization detectors employ a radioactive emitter that ionizes smoke particles by imparting a negative electric charge to them. The ionized smoke particles are attracted to a positively charged plate, triggering an alarm. These detectors come as spot detectors similar to the units you see installed in ceilings of motel rooms. The other configuration uses a sampling tube that projects into the air stream to monitor the air for smoke.