The importance of duct smoke detectors cannot be understated, argues Rochelle McGee, who explains why a fire alarm system alone is not sufficient to prevent fatal accidents.

Consider the following: At a well-known, highly recommended multi-storey hotel, a small kitchen fire breaks out late in the night. As the fire begins to grow and spread, it sends plumes of thick, black smoke into the air for almost twenty minutes. When enough heat has been generated to trigger the sprinkler system, the fire is quickly extinguished. The fire alarm system activated and all alarm functions performed as expected, there is negligible damage to the actual building itself.

The following morning, over a dozen hotel guests on the upper floors are found dead in their beds, having perished in their sleep. Their cause of death is attributed to smoke inhalation.

How did this happen?

The example above is an honest instance of how toxic smoke and gases produced from fire can travel through an air handling system. Here, the smoke simply circulated into the hotel rooms as the air conditioning cycled on, and sleeping occupants fell victim to the cause.

When an HVAC system supplies conditioned air to virtually every area of a building, smoke introduced into the air duct system has the potential to reach the entire building. Residential apartments, businesses, schools, banquet facilities – any such structure provides the opportunity for smoke-related casualties to occupants or property unless the building is properly equipped with duct smoke detectors.

It is critical that building operators understand the primary purpose of duct smoke detection: to prevent injury, panic and property damage by reducing the spread and recirculation of smoke. National and local safety standards and codes acknowledge the ability of air duct systems to transfer smoke, toxic gases and flame from area to area. This threat is diminished by the proper use of duct smoke detectors.

A duct smoke detector is a device (or group of devices) used to detect the presence of smoke in the airstream of ductwork sections of the HVAC air handling systems used in public facilities. Duct smoke detection can be the first line of defense to shut down the system’s blowers and ensure dampers are actuated when there is a fire. Turning the HVAC system off in an emergency situation prevents recirculating contaminated air.

Duct smoke detectors, as the name suggests, are specifically designed to respond to smoke in these areas. They are not intended for use as open area smoke detectors, as the technologies used in duct smoke detectors are slightly different. In simple terms, duct smoke detectors are designed with an intake and exhaust, monitoring for particles of combustion only within the air flowing through them.

Typical duct smoke detectors include a smoke sensor within a housing mounted outside the duct utilising sampling tubes. It may incorporate either an ionisation or photoelectric type smoke sensor. Air stream sampling is accomplished by sampling tubes that penetrate and cross either the supply or return air ducts, or both. The sampling tubes are positioned so air is drawn through the detector, exposed to the smoke sensor, and then returned to the air stream in the duct through an exhaust tube.

The importance of duct smoke detectors cannot be understated. Ducts, of course, are kept out of view, yet can house a great deal of electrical equipment that can potentially spark fires. For instance, should an HVAC fan motor overheat, the resulting smoke is sensed by the duct smoke detector installed in the main supply duct. The duct smoke detector is equipped with an auxiliary relay that immediately cuts power to the fan motor.

Should something go wrong in a duct – and there are no duct smoke detectors – the problem could go unnoticed until it is strong enough to cause significant damage. Air filter fires would be a common example of this. Duct-mounted smoke detectors stop the HVAC system from feeding air to a fire because it turns the system it monitors off if it senses smoke in the duct. This can be helpful and life-saving in the event of a fire.

Duct smoke detectors also suit equipment protection applications. One example might be in the ventilation/exhaust ductwork leading to the areas that house telephone equipment or critical network servers. Duct detectors can be used to assist in protecting the air-conditioning system itself from fire and smoke damage.

Or, for example, if a fire starts on the second floor of a building, where the HVAC system serving the second floor serves all floors one through four, the smoke will also spread to those floors. If open area smoke detectors are not provided, the only means of automatic detection are the duct smoke detectors located in the return air ducts on each floor ahead of the main return plenum. The quantity of smoke in the duct eventually reaches proportions sufficient to alarm the second floor duct smoke detector, which transmits a signal to the fire alarm system. Evacuation signalling and HVAC shutdown functions are then provided by the duct detector’s auxiliary relay contacts. In these and other situations, duct smoke detection devices are proven effective in helping prevent injury and property damage.

Because smoke detectors designed for use in air duct systems are used to sense the presence of smoke in the duct, it is critical that they are properly installed, maintained and tested.

Fire safety can be largely overlooked in general – but especially in the case of duct smoke detectors. “Out of sight, out of mind” has no business in the world of fire safety, and if you are the owner or manager of a small to mid-sized building, it is important to be aware of the fire alarm systems in place. The small expense needed now can save fortunes and lives later.

The author is Rochelle McGee, Marketing Director at Air Products and Controls. She can be contacted at rmcgee@ap-c.com