How to work duct smoke detectors?

Duct smoke detectors are hidden in the HVAC ductwork of commercial buildings as well as residential structures. They play an important role for fire safety. The forced airflow through HVAC systems can lead to rapid spread of smoke and fire throughout buildings. Duct detectors can stop the spread of fire and save lives.

The duct smoke detectors detect smoke and fumes by measuring the air currents inside air handling equipment. The device will detect smoke or fumes and close the dampers, shut off the blowers and fans to remove oxygen from the duct. This will trigger an audible signal at the fire control panels.

A duct detector has a perforated sampling tub attached to a smoke detector. It extends into the duct. As the air flows through the duct, the sampling tube should be positioned so that the perforations face the airflow. To force the air through the smoke detector, the tube is plugged at the other end. The second non-perforated return tube allows the air to flow back into the duct.

The duct detectors are connected to a circuit board. The device sends a signal when smoke is detected to the circuit board. Many circuit boards have multiple relays that can send signals to other systems. These include the fire alarm control panel, which triggers the alarm, and the HVAC control panel, which closes the dampers and shuts down the fans. A relay will be used to signal the supervisory panel.

Two alarms are standard on most duct detectors. One is a fire alarm that is triggered by smoke detection and one is in trouble. This is indicated on the supervisory board. Trouble conditions can be caused by someone removing the cover or if the power supply to the detector goes out. This helps reduce false alarms.

Where Are Duct Smoke Detectors Required?

For HVAC systems that have airflows greater than 2,000 cubic feet per hour (cfm), and where multiple air handling systems are sharing ducts, if the combined design is greater than 2,000cfm, duct smoke detectors will be required.

But, the code that governs where they are placed in your jurisdiction will determine their location. Because the codes that govern their installation are conflicting, this can make it a difficult question.

• National Fire Protection Association (NFPA) 101 Life Safety Code refers to the NFPA 90A: Standard for the Installation of Heating, Ventilation, and Air Conditioning Systems.
• International Building Code (IBC) or International Fire Code (IFC) references the International Mechanical Code (IMC).

The IMC and NFPA90A both require that detectors be installed in accordance to NFPA 72 National Fire Alarm and Signaling Code. They must also be connected to a fire alarm system whenever one is needed.

NFPA 90A mandates that duct detectors be installed on the supply end of units handling airflows exceeding 2,000 cubic feet per hour (cfm). A duct detector must be installed on the return side for units with higher airflows than 15,000 cfm that serve more floors of a building.

The IMC, however, requires smoke detectors be installed in return systems greater than 2,000 cfm. They must also be installed upstream of any filters, exhaust or outdoor air connections.

The NFPA 90A mandates smoke detectors in the supply ducts. However, the IMC insists that they be in the return ducts. If you have duct smoke detectors in your building, or are a designer working on an air handling system for your home, it is important to consult the Authority Having Jurisdiction (AHJ), if you have any questions about the placement of duct smoke alarms.

Important to remember that a duct smoke detector cannot replace an existing area smoke detector. This is because the duct smoke detector will not detect smoke if the air-handling unit does not run. To meet safety regulations, you still require area smoke detectors.

Have Your Duct Smoke Alarms Regularly Inspected?

It doesn't have to be hard to see. Although duct smoke detectors may not be easily seen, they should still be inspected as part of your fire alarm system. NFPA 72 states that duct smoke detectors must be inspected at least twice a year. This inspection can be done by fire safety professionals or staff and should include the following:

The duct smoke detector should be securely mounted to the duct.

• Also, the circuit board should not show any signs of damage.
• Trouble signals for control units should be easily visible
• Remote annunciators must be in good condition and show no signs or damage.
• All in-line filters must be clean
• Pipes and fittings used in sampling system should be airtight and permanently attached
• It is important to clearly identify the sampling pipe and ensure that sample ports are unobstructed
• It is important that there are no penetrations within the detector's proximity that could cause an obstruction to the airflow through duct.

All fire alarm systems must be tested and inspected annually by qualified professionals. Learn more about fire alarm system inspections here. NFPA 72 is required for duct smoke detectors.

• To ensure that smoke cannot physically enter the detector's sensing chamber, a smoke entry test is performed.
• The manufacturer has specified a test to ensure that the pressure differential between the outlet and inlet tubes is within the allowed range.
• To ensure they are properly sampling the airflow through the duct, the sampling tubes must be tested. You must test the detectors according to the manufacturer's recommendations.

Notifying the authorities about the maintenance of the smoke detector system is crucial. The system may temporarily be out-of-service. To prevent any unwanted alarms, you must also disable the zone or system that is undergoing maintenance.

Summery

There are many certified technicians stay on top of evolving sensor technologies. They can help you determine the types of smoke detectors most appropriate for your needs, as well as the brands of equipment that will be compatible with your current fire protection system. They service and inspect and install all brands of fire alarm systems from a wide range of high-quality manufacturers.

If you are getting ready to build, or if your current fire protection system needs updating or maintenance, don't wait – Click more-info for best guideline for the Best Fire Extinguisher.

Feature Image by Erich Westendarp from Pixabay

I'm working as a technician at Germany computer and telecom, a company that imports and sells Cctv Camera, Access Control System, and other security equipment. I like to gather knowledge about new technology and also inform others.