What to Know About Smoke Detectors When Renovating Building Space

By David R. George, Director, Corporate Communications, System Sensor, St. Charles, Ill. October 1, 2006

Tenants come and go. One month, an area is filled with a maze of cubicles, and the next, it’s a suite of private offices. As each new company takes over a space and adapts it to their needs, there’s a completely different set of fire-protection guidelines. In other words, the same versatility of space that makes a building desirable for renters can also present challenges when it comes to fire protection.

To guard against hazards associated with changing uses for building space, it is important for a fire-protection system to provide “total coverage.” Defined by the National Fire Protection Assn. in its NFPA 72 guidelines, total coverage is achieved with the proper type of detectors installed in appropriate locations.

Where to install?

The goal of a fire-detection system is to provide an accurate, early warning of a developing fire in any area of a building. Even pockets of unoccupied space require protection, because detectors may not quickly sense a developing fire on the far side of a wall or behind a closed door, allowing damage to multiply needlessly.

The correct placement of detectors is also important for reliable operation. In general, when only one detector is required in a room or space, it is best to install a ceiling-mounted detector as close to the center of the room as possible. If a central ceiling location is not viable, for example, due to wiring constraints, the detector must have sufficient “open space,” with its edge no closer than 4 in. to a wall. Likewise, a wall-mounted detector must clear between 4 in. and 12 in. from the top of the detector to the ceiling (see Figure 1).

Another consideration for a total coverage plan is the proximity of detectors to the air-handling system. NFPA 72 discusses the potential for detector malfunction if installation is in the path of an airflow supply or return duct. A smoke test to monitor particulate travel direction and velocity is helpful in determining detector placement. Smoke tests reveal potential causes of unwanted alarms, such as an airstream directed at the detector, which could result in dust accumulation that alters sensitivity levels.

How to space?

Spacing detectors 30 ft. apart to protect 900 sq. ft. is the NFPA 72 standard for areas with smooth ceilings and no physical obstructions between ceiling and room contents. An example of an obstruction is floor-to-ceiling shelving stacked with materials. Variables for ceiling height may also be calculated into spacing requirements based on the amount and nature of combustibles present.

Figure 1

To determine appropriate detector coverage for the standard 30-ft. spacing, a simple technique is to map the shape and dimensions of an area. Then, draw a circle with a radius of 21 ft. A single detector may protect any square or rectangle that fits within the circumference of that circle (see Fig. 2). The technique shows that in a hallway measuring 10 ft. wide, two detectors can protect up to 82 ft. of the length (see Fig. 3).

What type of detectors?

To answer this question is to understand the use and contents of a particular area. For example, ionization smoke detectors are quicker to detect flaming fires, such as those commonly found in chemical-storage areas, rather than slow, smoldering fires that most typically occur in offices. Ionization sensors almost immediately recognize fires characterized by combustion particles from 0.01 to 0.3 microns. However, ionization sensors offer limited or slower capabilities when installed in high-altitude locations or near high-humidity sources, such as kitchens or shower rooms.

Photoelectric smoke detectors, on the other hand, quickly respond to smoldering fires characterized by combustion particles from 0.3 to 10.0 microns. This type of detector will instantly identify visible white smoke, but will be slower to respond to black smoke produced by plastics or rubber.

A common solution to detect both types of stimuli quickly is to install a multi-criteria detector that monitors particulate detection in tandem with a thermal-sensor input. Together, the two signals are cross-referenced by an onboard microprocessor that uses algorithms to “process out” false alarms while enhancing the response time to real fires. By monitoring the current signal values of each sensor as well as their trends, such as increasing heat or a decreasing photoelectric signal, multi-criteria detectors actually “learn” the environment, which helps to better reject nuisance alarms and maintain heightened sensitivities.

Sensing smoke in ductwork

National and local safety standards and codes address the ability of air duct systems to transfer smoke, toxic gases and flame from area to area. Oftentimes in fire scenarios, smoke can be of such quantity that it poses a serious hazard to the safety of people several floors removed from the actual fire. This threat is confronted by the use of duct-smoke detectors.

It is critical, therefore, that building operators understand the primary purpose of duct-smoke detection is to prevent injury, panic and property damage by reducing the spread and re-circulation of smoke.

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 not only serves to protect a building’s air-conditioning system from fire and smoke damage, it can also be used to assist in equipment protection applications, for example, in the ventilation/exhaust ductwork leading to the areas that house mainframe computers and tape drives.

Duct-smoke detection can also be the first line of defense to shut down the system’s blowers and ensure dampers are actuated when there is a fire. 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.

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 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 signaling 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.

Installation, maintenance, testing

When an HVAC system supplies conditioned air to virtually every area of a building, smoke introduced into this air-duct system has the potential to reach the entire building. 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.

Figure 2

NFPA 90A, “Standard for Air Conditioning and Ventilating Systems,” requires that smoke detectors listed for duct installations be installed at a suitable location in the main supply duct on the downstream side of the filters to automatically stop the supply fans in systems of more than 2,000 cubic feet per minute. For systems more than 15,000 cfm, additional detectors are required in the return system of each floor, at the point of entry into the common return, or a system of smoke detectors is required to provide total area coverage.

Smoke detectors are designed to be as maintenance-free as possible. However, dust, dirt and other foreign matter can accumulate inside a detector and change its sensitivity. This is especially true with duct-type smoke detectors. They can become more sensitive, which may cause unwanted alarms, or less sensitive, which may reduce the level of protection. Both are undesirable. According to Chapter 10 of the 2002 Edition of NFPA 72, duct-smoke detectors should be tested upon acceptance and re-tested annually. Always check your local code requirements to determine if more frequent testing is required.

Under normal conditions, detectors require routine maintenance at least twice a year, or more frequently in dirtier than normal environments. Notify the proper authorities that the smoke-detector system is undergoing maintenance and that the system will be temporarily out of service. It is also imperative that the zone or system undergoing maintenance is disabled to prevent unwanted alarms.

It is also important to test each detector’s sensitivity. If a detector’s sensitivity is within specifications, nothing further needs to be done. If the detector’s sensitivity is outside its listed specifications, either clean or replace the detector according to the manufacturer’s recommended procedures. Upon completion of any testing or maintenance procedures, be sure to restore the system and notify the proper authorities that the system is back in service.

Figure 3

Other duct-smoke detector maintenance issues that should be routinely checked include:

  • Holes or cracks in duct work near the vicinity of the detector

  • Air leaks where the detector housing or sampling tubes are attached to the duct

  • Wiring terminal screw tightness

Understanding the purpose of duct-smoke detectors, as well as the importance of their proper installation and care, is the key to protecting property and saving lives.