The goal of a fire-detection system is to provide an accurate, early warning of a developing fire in any area of a home. 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. NFPA 72 requires one detector be installed outside each sleeping area or bedroom and on every floor, including the basement.
The correct placement of detectors is also important for reliable operation. Check with the local building inspector, fire department or state fire marshal’s office to find out what they recommend. In general, when only one detector is required in a room or space, it is best to install a ceiling-mounted device 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 inches to a wall. Likewise, a wall-mounted detector must clear between 4 inches and 12 inches from the top of the detector to the ceiling (see diagram).
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 the unit 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 air stream directed at the detector, which could result in dust accumulation that alters sensitivity levels.
Determining what type of detector to install requires understanding the use and contents of a particular area.
Ionization smoke detectors are quicker to detect flaming fires, such as those commonly found in kitchens, rather than slow, smoldering fires that most typically occur in bedrooms. Ionization sensors almost immediately recognize fires characterized by combustion particles from .01 to .3 microns. However, ionization sensors offer limited or slower capabilities when installed in high-altitude locations or near high-humidity sources, such as basements.
Photoelectric smoke detectors, on the other hand, quickly respond to smoldering fires characterized by combustion particles from .3 to 10 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 multicriteria 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, multicriteria detectors actually “learn” the environment. This helps them reject nuisance alarms and maintain heightened sensitivities.
Ultimately, fire and life-safety system renovation does not have to be complicated. Simply be aware of the codes, what they entail and how they impact the home. Today’s technology provides fire alarm systems with greater flexibility and the ability to offer improvements in fire and life safety, while reducing installed costs of complete systems.
David George is Communications Director for System Sensor.
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