Smoke Detection 101

In April, I discussed why we typically install a fire alarm system (life safety—early warning) and began a discussion on some of the issues to be aware of when designing and installing spot type smoke detectors. I promised I would get into more detail regarding smoke detection to help those just getting into the business of installing fire alarm systems and those who might need a refresher. I will discuss applications in the next article.

First, we need to look at some important definitions of smoke detectors (all found in NFPA 72 2016, National Fire Alarm and Signaling Code).

A smoke detector is “a device that detects visible or invisible particles of combustion.” You may or may not have heard of the two types of smoke detector, one that operates on the ionization principle (detects invisible particles and some visible particles of smoke) and the other that operates on the photoelectric light scattering principle (detects visible particles only). Initially here we are describing spot-type smoke detectors, so-called because they are installed on a single spot on the ceiling to detect smoke in a space. These are also the most frequently used smoke detection device for a clear majority of fire alarm systems.

We have an array of smoke detection devices that we can choose from if the spot-type does not fit our application, and these include line-type (also called beam-type) and air sampling (also called aspirating type).

The line-type smoke detector is “a device in which detection is continuous along a path.” The air sampling-type smoke detector is “a detector that consists of a piping or tubing distribution network that runs from the detector to the area to be protected. An aspiration fan in the detector housing draws air from the protected area back to the detector through air-sampling ports, piping, or tubing. At the detector, the air is analyzed for fire products.” Some have called the latter type of smoke detector an “active air sampling type” to distinguish it from duct smoke detectors that use sampling pipes in a duct. Generally, duct detectors are passive sampling type detectors in that the smoke must enter the detector based on the smoke velocity in the duct or when the HVAC is operating which can then “push” smoke into the sampling tubes.

There are two additional variations of smoke detectors that need to be discussed. The first would be the combination type, generally a smoke detector combined with a heat detector or a carbon monoxide detector. The second is the multicriteria detector, which is a device that contains multiple sensors that separately respond to physical stimulus such as heat, smoke, or fire gases, or employs more than one sensor to sense the same stimulus. This sensor is capable of generating only one alarm signal from the sensors employed in the design either independently or in combination. The sensor output signal is mathematically evaluated to determine when an alarm signal is warranted. The evaluation can be performed either at the detector or at the control unit. This detector has a single listing that establishes the primary function of the detector. These are also generally spot-type but also are typically special application devices essentially due to their current high cost.

Now that we have a basic knowledge of the types of smoke detectors available for design and installation, what do we do with this information? Well the answer always depends on the fire protection goals of the owner and the application you are designing for. As stated above, the most frequently used smoke detector is the spot type. Most typical building applications can be met with the photoelectric spot-type smoke detector. Both the ionization and photoelectric are UL-Listed to the same standard; however, the photoelectric type is more stable, especially in commercial residential environments, but the type used should be based on your performance objectives. As stated in my previous article, there are limitations to the spot-type detector.

First, make sure that, when you select your smoke detectors, you ensure the detectors' selection and placement considers the detectors' performance characteristics (ionization, photoelectric, or multi-criteria) and the areas where you plan to install them to prevent nuisance and unintentional alarms or improper operation after installation.

Additionally, the smoke detectors cannot be installed whenever the ambient conditions fall below a temperature of 32ºF or above 100ºF, the relative humidity exceeds 93 percent, or the air velocity is greater than 300 feet per minute (at the detector). And, to minimize nuisance alarms, the smoke detector locations should be evaluated for potential ambient sources of alarms that may be present. The code specifically calls out smoke, moisture, dust, or fumes, and electrical or mechanical influences as sources that may cause nuisance alarms.

The code requires spot-type smoke detectors to be located on the ceiling or, if on a sidewall, between the ceiling and 12 inches down from the ceiling to the top of the detector. This is a new allowance. For many years, the industry assumed there was a dead air space at the ceiling-wall interface and required detectors to not be installed closer than 4 inches to the ceiling when wall mounted. Research has shown that fear to be incorrect. See the figure below from NFPA 72 2016:

NFPA 72-2016

In the absence of specific performance-based design criteria, spot-type smoke detectors installed on a smooth ceiling must follow one of the following requirements:

“(1) The distance between smoke detectors shall not exceed a nominal spacing of 30 ft and there shall be detectors within a distance of one-half the nominal spacing, measured at right angles from all walls or partitions extending upward to within the top 15 percent of the ceiling height.

"(2) All points on the ceiling shall have a detector within a distance equal to or less than 0.7 times the nominal 30 ft spacing (0.7S).”

In all cases, the code requires the manufacturer’s published instructions to be followed. The code also allows other spacing to be used depending on factors such as ceiling height, different conditions other than “normal," or when different response requirements need to be met.

With all of the other types of smoke detectors, linear beam, aspirating and duct type, the manufacturer’s listing requirements must be followed.

In my next article, I will look at the applications where each detector type is used.

About the Author

Wayne D. Moore

Fire/Life Safety Columnist
Wayne D. Moore, a licensed fire protection engineer, frequent speaker and an expert in the life safety field, is a principal member and past chair of NFPA 72, Chapter 24. Moore is a vice president with JENSEN HUGHES at the Warwick, R.I., office. He c...

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