Some duct detectors are best installed at the time of construction, but can be retrofitted in older buildings. In fact, most municipal codes that incorporate NFPA codes require duct smoke detectors in all commercial buildings.

The units are to be located within the air ducts a minimum of six duct-widths away from a source of turbulence, such as an inlet, damper, or bend in the duct system. They should also be located in a uniform, nonturbulent airflow of between 300 and 4,000 fpm.

Fans, blowers, and complete systems may be shut down in the event of smoke detection. Universal voltage models are readily available to accommodate any one of four input voltages: 230, 115, and 24 vac, and 24 vdc.

Duct smoke detectors are available with either an ionization or photoelectric detector head. They typically contain two form “C” alarm contacts and one form “C” trouble contact. Some duct smoke detectors can withstand temperatures as high as 150ÞF.

Visual indicators are mounted on the housing front to make it easy to monitor pilot and alarm status, especially when the units are mounted in a high location. A manual test-reset switch is also generally located on the front of the detector, to make frequent testing easy.

Air sampling is accomplished by two tubes that extend into the duct. An exhaust tube of typically 7 in. is provided with each duct smoke detector.

Intake sampling tubes, which must be ordered separately, are available in three standard lengths: 2.5, 5, and 10 ft. Generally, up to 12 detectors can be connected together to provide complete hvac system protection, controlling all connected blowers, fans, and dampers.

There also are special UL 50-listed, NEMA-rated weatherproof enclosures for outdoor or indoor applications requiring protection from the elements.