What to do with those pesky smoke detectors:
On every job, you are required to wire or install a device that is provided by others; most of the time, the heating, ventilating and air conditioning (HVAC) contractor will be required to supply the duct smoke detectors and install the detectors in the ducts. What never seems to be coordinated is the required compatibility of the duct smoke detector with the fire alarm equipment supplied by the electrical contractor. In many cases, the duct device’s voltage is 120V AC while the fire alarm system is 24V DC.
Coordinating this type of situation is difficult at best because these devices are most often connected at the end of the fire alarm system installation, and it is not until then that the problems of voltage or compatibility are discovered. Electrical contractors have often asked the following questions.
Why are these devices necessary?
Duct smoke detectors are necessary to prevent the spread of smoke throughout the building by the HVAC system. Unlike spot-type smoke detectors, they are not considered early warning devices.
They shut down the fans and close smoke dampers to prevent the recirculation of smoke from the fire area to non-fire areas.
What codes require their use?
NFPA 90A-2005, Standard for the Installation of Air-Conditioning and Ventilating Systems, along with the International Mechanical Codes outline the requirements for duct smoke detectors. These codes and standards specify when and where duct smoke detectors must be used as well as their placement in the duct system.
The standard allows return duct smoke detectors to be eliminated when the entire space served by the air distribution system is protected by a system of area smoke detectors. The entire space means spot-type smoke detectors are installed in all occupiable areas of the space served by the air handling system and does not include closets or bathrooms. Additionally, the codes and standards do not require duct smoke detectors for fan units whose sole function is to remove air from the inside of the building to the outside of the building (exhaust fans).
Are duct smoke detectors required to be connected to the fire alarm system? NFPA 90A and the International Fire Codes require that all smoke detectors installed to prevent the spread of smoke in a building be installed, tested and maintained in accordance with the National Fire Alarm Code.
In addition, the standard and codes require that where an approved fire alarm system is installed in a building, the smoke detectors used to prevent the spread of smoke must be connected to the fire alarm system in accordance with the requirements of NFPA 72, National Fire Alarm Code, which states that “smoke detectors mounted in the air ducts of HVAC systems shall initiate either an alarm signal at the protected premises or a supervisory signal at a constantly attended location or supervising station.” This means all duct smoke detectors connected to fire alarm systems that are, in turn, connected to a supervising station can be monitored for a supervisory condition rather than alarm. Where must they be installed in the ducts?
For years, the National Fire Alarm Code required that duct smoke detectors be installed to ensure a representative sample of the air stream. The code requires the installation be made in accordance with the manufacturer’s instructions and to any of the following methods:
“(1) Rigid mounting within the duct
(2) Rigid mounting to the wall of the duct with the sensing element protruding into the duct
(3) Installation outside the duct with rigidly mounted sampling tubes protruding into the duct
(4) Installation through the duct with projected light beam”
One of the issues that would crop up often was the recommendation to install duct smoke detectors between 6 and 10 duct-equivalent diameters of straight, uninterrupted run. This recommendation was made to ensure the sampling tubes would obtain a representative sample and that stratification and dead air space conditions would be avoided.
The current code, NFPA 72-2007, has replaced those recommendations with “Sampling tubes should be oriented to overcome thermal stratification due to buoyancy of the smoke in the upper half of the duct. This condition occurs where duct velocities are low, buoyancy exceeds flow inertia or the detector is installed close to the fire compartment. A vertical orientation of sampling tubes overcomes the effects of differential buoyancy. Where a detector is installed on a duct serving a single fire compartment, where the buoyancy exceeds the flow inertia of the air in the duct and the sampling tube cannot be oriented vertically, then the effects of thermal stratification can be minimized by locating the detector sampling tube in the upper half of the duct.”
The installation of duct smoke detectors is another area that electrical contractors must coordinate with the mechanical contractor to ensure a smooth acceptance test and sign off by the fire officials. Don’t let a misstep with the installation and connection of these devices cause a delayed certificate of occupancy. ECMOORE, a licensed fire protection engineer, frequent speaker and an expert in the life safety field, is a co-editor of the current National Fire Alarm Code Handbook. Moore is a principal with Hughes Associates Inc. and is located at the Warwick, R.I., office.