Now that the summer has arrived, we often hear the comment that “it’s not the heat, but the humidity”. In simple terms, humidity is the amount of water vapour in the air, and can make the warm temperatures outside feel uncomfortable. Humidity also affects indoor building environments, playing havoc with fire alarm systems and some of their connected smoke detection devices. One such device is the duct smoke detector, which is installed on the air handling systems typically found in most large buildings. High levels of humidity in the building air or condensation within the air handling units duct work can cause false alarms. In some cases, the dust particles increase in mass due to the moisture in the air, causing the sensing chamber within the detector to respond when the density is greater than its preset levels.
This device can sense the presence of smoke in the air streams of HVAC (heating, ventilating, and air conditioning) ductwork. As part of the early warning system, the purpose of duct smoke detection is to reduce the spread (re-circulation) of smoke. The duct smoke detector is one of the final measures required to isolate each area of a building (known as a smoke compartment) that would otherwise be filled with combustion particles. Even when a single HVAC fan motor overheats, the resulting smoke can be sensed by a duct smoke detector located on the main supply duct. In some installations, duct smoke detectors are equipped with an auxiliary relay that immediately cuts power to the fan motor before significant amounts of smoke can be distributed to other occupied areas. In most applications, duct smoke detectors are installed onto both the supply and exhaust ducts, and may be equipped with either ionization or photoelectric type sensors.
From an engineering standpoint, specifications commonly call for duct smoke detectors to be installed near the input/output filters, but downstream of fans, filters, chillers, heaters, and humidifiers. Provisions are also made for duct smoke detectors to be placed in the return air streams, normally at every return air opening (or air exit) within the smoke compartment, or within the duct before air enters the return air system common to more than one compartment. They are generally mounted on the outside of the duct, utilizing sampling tubes that penetrate and traverse either the supply or return air ducts. The sampling tubes are positioned so that air may be drawn through the detector, sampled, and then returned to the air stream in the duct. These smoke-sensing devices are typically calibrated to an operating level based on the normal background particle concentration for the building. If the preset threshold is exceeded, an alarm is created, activating the building fire alarm control panel.
Although the fire alarm control panel identifies the zone and air handling unit of the duct smoke detector, locating the actual device is usually more of a challenge. For this reason, it is advantageous to show the locations of the duct smoke detectors on floor layout drawings. In addition, the fire safety plan should document the sequence of operation and maintenance requirements of these devices. It is also beneficial for drawings to indicate roof locations of air handling units which are not equipped with duct smoke detectors, as well as the main power disconnect switch for each unit.
This article was contributed by Firepoint Inc, serving the GTA since 1997, developing fire department approved fire safety plans for newly constructed and existing buildings. Although every effort is made to ensure the accuracy, currency and completeness of the information, the bulletin publishers and authors do not guarantee, warrant, represent or undertake that the information provided is correct, accurate or current and shall not be liable for any loss, claim, or demand arising directly or indirectly from any use or reliance upon the bulletin information. Bulletin reproduction in whole or in part is prohibited without the written consent of Firepoint Inc. Copyright 2021 – All Rights Reserved. See www.firepoint.ca or call 905-874-9400.