Making Passive Fire Protection Active with System Smoke Detectors

Sept. 8, 2016

Most experts agree that owners and property managers have a lot on their plates. Perhaps one of the most important items on that plate is fire protection/life safety (see the adjacent article, Basics of Passive Fire Protection). The responsibility for fire and life safety systems in a building ultimately lies with the building owner and property manager. A basic understanding of system smoke detectors, therefore, will not only facilitate that responsibility, such understanding will enhance the owner’s decisions on the type of protection required for their building, turning passive into active detection and thereby helping to keep life and property safe.

Strobe notification appliances are versatile, easy to install, and greatly simplify the specification, configuration, and installation of a fire protection strategy.

Although smoke detectors are based on simple concepts, today they utilize advanced technologies in many cases which, in turn, affect an owner’s consideration in their application. For example, a detector should do two things really well:

  1. Produce an alarm signal when smoke is detected
  2. Minimize the impact of unwanted signals

How “well” they do them is often the result of WHERE they are doing, what they do, and how they have been applied. In other words, the integrated detection strategy has to be thought through. Fortunately or unfortunately, it’s the building owner who can help that process more than anyone. Working with a fire protection engineer, for example, the owner’s contribution to the detection strategy can’t be underestimated (i.e., the owner knows the building uses intimately – uses which ultimately determine what has to be protected, and how).

Back to Basics

While there are many detectors (covered throughout this article), the two basic types of detectors for owners to be aware of are ionization and photoelectric. Ionization types use ionized air molecules as a form of detection. When a decrease in a chamber current is sensed by electronic circuitry monitoring the molecules in a chamber, an “alarm” is established. Photoelectric smoke detectors, by comparison, sense smoke by utilizing effects of smoke on light. When smoke particles enter the light path, light strikes particles and is reflected onto the photosensitive device causing the detector to respond.

Sounds simple enough, doesn’t it? But like any other type of component, there are issues that can develop and impede the protection. In an ionization detector, for example, dust and dirt can accumulate and cause the detector to become too sensitive. In a photoelectric detector, light from the light source may be reflected off the walls of the sensing chamber then seen by the photosensitive device when no smoke is present. Insects, dirt, drywall dust, and other forms of contamination can accumulate in sensing chambers and reflect light from the light source onto the photosensitive device (which is why maintenance is extremely important!). Electrical transients and some kinds of radiated energy can also affect the circuitry of both ionization and photoelectric smoke detectors. Such issues can result in false alarms. Owners need to know about this not only from a maintenance point of view, but from a detector “placement” point of view.

Conventional detectors provide reliable and accurate sensing technologies and deliver the highest levels of protection available.

The good news for owners is that allowable sensitivity ranges for both types of detectors are established by Underwriters Laboratories, Inc. (UL). Detector performance is verified in fire tests, and all smoke detectors are required to respond to the same test fires regardless of their principle of operation.

Which Detector Is Right For Your Building?

The characteristics of an ionization detector make it more suitable for detection of fast flaming fires (combustion particles in the 0.01 to 0.4 micron size range). These detectors are detecting very small particles – really small.

Photoelectric smoke detectors, by comparison, are better suited to detect slow smoldering fires that are characterized by particulates a little larger than ionization’s -- in the 0.4 to 10.0 micron range. While each type of detector can sense both types of fires, their respective response times vary depending on the type of fire they are sensing because of their technology. Knowing this difference will help you select (or understand working with your fire protection engineer) WHERE you place them. It is important for the building owner to consider both technologies in evaluating a detector strategy for a building.

However, it is also often difficult to predict what size particulate matter will be produced by a developing fire because protected buildings normally contain a variety of combustibles. This is why the owner’s knowledge of tenants is one of the keys to successful detection strategies. The fact that different ignition sources can have different effects on a given combustible further complicates the selection of the right detector. A lit cigarette, for example, will usually produce a slow smoldering fire if it is dropped on a sofa or bed. However, if the cigarette falls on a newspaper that’s on top of that sofa or bed, the resulting fire may be better characterized by flames rather than by smoldering smoke.

Equipment plays a role in your detection strategy as well. A full-fledged data center may require different protection than a data room in a tenant location. A legal library full of volumes may require different detection approaches than a warehouse full of inventory. It is the owner who understands these variables better than the fire protection engineer, but it is the engineer who understands the pros and cons of detectors. Working together helps deal with the innumerable combustion profiles possible with various fire loads and possible ignition sources.

Yet, even though such cooperation offers proven guidance in detector selection, the owner should have a basic understanding of their judgments and strategies because tenants change. Protecting one tenant’s requirements may not be as effective when a new tenant with different requirements moves in. Owners also have access to the same codes and standards professionals use. The NFPA is an important resource. For example, Tables A., A., and A. of NFPA 72-2010 show a variety of common sources of aerosols and particulate matter that can affect a detector, including such things as moisture (humid outside air, showers, etc.), combustion products and fumes (fireplaces, machining, etc.) and more. Owners can freely consult these references on their own to gain better understanding with their fire protection engineers’ recommendations.

Situations for Other Types of Detectors

A data center is different than a retail establishment. A church is different than a stadium. These different types of structures have different fire detection strategies that affect component selection, and in certain circumstances, ionization or photoelectric smoke detectors will not provide the desired protection.

Because there is not a “one size fits all” detector, a number of advanced technology detectors that have been optimized for specific environments evolved and deserve consideration in your detector strategy for your building. Laser technology smoke detectors, for example, are designed for use in areas that require extremely early warning of fire (i.e., a data center). These devices are designed to detect the earliest particles of combustion making them ideal for clean rooms, computer rooms or telecommunication center, or any area with mission critical operations.

Laser-based smoke detectors are ultra-sensitive to smoke, as much as 100 times more sensitive than standard detectors. Care and judgment of the application is needed to prevent unwanted alarms. Aspiration smoke detectors use a pipe system and fan to draw smoke particulates back to the detection chamber. The pipe configuration, hole placement and hole diameter are designed from algorithms, which take into consideration air flow, room size, sensitivity requirements (speed of detection) and other parameters to determine the optimal set up. Aspiration detectors usually have software created for the pipe design of the room.

Aspirating smoke detectors use a pipe system and fan to draw smoke particulates back to the detection chamber.

Multi-criteria detection is another consideration and combine multiple sensors and technologies that separately respond to physical stimulus such as heat, smoke, or fire gases. An alarm signal is determined through advanced algorithms based on input from these sensors. For example, one multi-criteria detector combines photo and heat signals, whereas another may use four signals: photo, thermal, carbon monoxide and infrared. Combination of sensors offers better immunity to nuisance alarms in challenging environments with faster response times to real fires. Combination carbon monoxide and smoke detectors are also available.

Multi-criteria detectors use several sensors to detect different conditions and sense a wider range of fires with greater accuracy. Often they combine fire detection and life safety carbon monoxide (CO) detection in a single device.

Combining two or more functions into one device improves installation time and cost as well as offering a more aesthetically pleasing final product to the facility.

The application of these special types of detectors should be based on an engineering survey of the facility, and then used in accordance with manufacturer instructions. For example, detectors using laser technology are designed for use in areas that require extremely early warning of fire as in a data center.

Where Detectors Are Placed

Owners have to think about detectors as part of their entire building strategy, not just as this kind or that kind of component. Smoke detectors offer the earliest possible warning of fire and have saved thousands of lives, so their purpose has been tested. But, incorrect application of smoke detectors may not provide early warning of a fire developing on another level of a building. This is why detectors should be located on every level of a building – but perhaps not all the same types of detectors. An integrated strategy incorporates holistic thinking (i.e., detectors may not sense a fire developing on the other side of a closed door; areas where doors are usually closed, detectors should be located on both sides of the door).

Because detector placement is critical to early warning functions, smoke detectors should be installed in all areas of a building being protected. Total coverage is defined by NFPA 72 and includes all rooms, halls, storage areas, basements, attics, lofts, and spaces above suspended ceilings including plenum areas utilized as part of the HVAC system. In addition, coverage should include closets, elevator shafts, enclosed stairways, dumbwaiter shafts, chutes, and other subdivisions and accessible spaces. Again, the owner can influence the choice of detectors by knowing tenant requirements.

In fact, such requirements may mean that fire detection systems that meet local codes or ordinances may not be adequate for early warning of fire. Some codes or ordinances have minimum objectives such as capturing elevators or preventing circulation of smoke through the HVAC systems instead of early detection of fire. The location, quantity, and zoning of detectors should be determined by desired objectives and owner/tenant requirements that meet the minimum requirements of all local codes or ordinances.

In general, when only one detector is required in a room or space, the detector should be placed as close to the center of the ceiling as possible. Central location of the detector is best for sensing fires in any part of the room. If a center location is not possible, the detector may be wall mounted within 12 inches from the ceiling if the detector is listed for wall mounting. When air supply and/or air return ducts are present in a room or space, the detector(s) should not be placed in the path of the air flow supply or return (NFPA 72-2010).

If a center location is not possible, the detector may be wall mounted within 12 inches from the ceiling if the detector is listed for wall mounting. 

Spot type detectors in properly engineered systems may also be placed in return air ducts, or in approved duct detector housings designed for this application. Although duct detectors are not a substitute for open area detectors, they can provide an effective method of initiating building control functions to prevent smoke from being transported from the fire area to other parts of a building.

HVAC Duct detectors greatly simplify HVAC system monitoring, with their tool-free, plug-in sampling tubes speed and simplify installation. 

Smoke tests are often helpful in determining proper placement. Special attention can be given to smoke travel directions and velocity, since either can affect detector performance as pointed out earlier. Some of the general rules include: 1) Placement of detectors near air conditioning or incoming air vents can cause excessive accumulation of dust and dirt on the detectors and cause false alarms, and 2) Detectors should not be located closer than 3 feet from an air supply diffuser or an air return vent.

Where NOT To Place Detectors

If the rule is to place detectors most everywhere, sometimes looking at where they should NOT be placed helps the owner create a better detector strategy. One of the ways to do that is to think about where a detector malfunctions or misfires (i.e., moisture or dust environments). Application-specific devices (specialty detectors) have been created to address such environments. Installing these devices according to manufacturer specifications and AHJ guidelines will reduce unwanted alarms. Here are some points where you may not want to place “normal” detectors.

• Excessively Dusty or Dirty Areas. Installing smoke detectors in such environments may require more maintenance than NFPA recommends (detectors have to be properly maintained). In fact, since the owner is responsible for the Inspecting (usually annually, and some components even more often), addressing this in your integrated detection strategy is important. It may be advantageous to install smoke detectors with a remote maintenance signaling module, or to install heat detectors in place of smoke detectors. Consult with your fire protection engineer, or with the manufacturer.

• Outdoors. Avoid using detectors outdoors, in open storage sheds, or other open structures affected by dust, air currents, or excessive ranges of humidity and temperature.

• Wet or Excessively Humid Areas. These include areas next to bathrooms with showers. Refer to NFPA 72-2010 A. (5) for additional placement guidelines or work closely with your engineer.

• Extreme Temperatures. Avoid very cold or very hot environments, or unheated buildings or rooms where temperatures can fall below or exceed the operating temperature range of the detector. At temperatures above or below the operating range of the detector, internal components may not function properly.

• Areas with Combustion Particles. Areas such as kitchens or other places with ovens and burners should be avoided with detectors. Actually anywhere where particles of combustion are present from vehicle exhausts should not be considered for detectors. When a detector must be located in or adjacent to such an area, a heat detector may be more appropriate.

• Manufacturing Areas. Rooms such as battery rooms, or others where substantial quantities of vapors, gases, or fumes may be present should be avoided in you integrated detection strategy. Strong vapors can make detectors overly sensitive or less sensitive than normal. In very large concentrations, gases heavier than air, such as carbon dioxide, may make detectors more sensitive, while gases lighter than air, such as helium, may make them less sensitive. Aerosol particles may collect on detector chamber surfaces and cause nuisance alarms.

• Fluorescent Light Fixtures. Avoid placement near these types of light fixtures. Electrical noise generated by fluorescent light fixtures may cause unwanted alarms. Install detectors at least 1 ft. (0.3 m) away from such light fixtures.

What to Do About Unwanted Alarms

No detection system is impervious to unwanted alarms. Statistically, as the system size and the total number of detectors increase, the total number of nuisance alarms per year tends to increase. Historical experience in a given installation or data on similar sized buildings with similar utilization patterns can provide a basis for a rough indication of how many nuisance alarms are probable during a 12-month span.; However, no two installations are identical.

In small- to moderate-sized detection systems protecting relatively combustion-free environments, like office buildings, more than one or two unwanted alarms per year would be unusual. In more adverse environments, such as laboratory or manufacturing facilities where combustion processes are present, more frequent alarms can be anticipated. In very adverse environments, one alarm per month might not be considered excessive.

After the first few months, which serve as a shakedown period, it should be possible to arrive at some reasonable expectation for probable unwanted alarms from the system. After that, any unexpected change in frequency or distribution indicates a problem that should be investigated. The best way to monitor alarm frequency and distribution is to maintain an alarm log. Reasons for unwanted alarms include:

• Improper environments: detectors will not operate properly because of temperature extremes; excessive dust, dirt, or humidity; excessive air flow rates; or the normal presence of combustion particles in the air streams surrounding the detectors.

• Improper installation: detectors and their wiring are subject to interference from induced currents and noise in adjacent wiring systems, radio-frequency transmissions, and other types of electromagnetic effects.

• Inadequate maintenance: gradual dust and dirt accumulation on the detector’s sensing chambers.

• Seasonal effects: for example, the reactivation of a building heating system after an extended summer shutdown can cause alarms.

• Building maintenance issues: for example, accidental triggering of a detector’s magnetic test switch or the introduction of plaster dust from drywall repairs into a detector’s sensing chamber.

• Induced current effects from lightning storms.

• Infestation from insects small enough to enter the detector’s sensing chamber.

• Vandalism or mischievous acts: for example, dormitory pranks.

If an alarm occurs and a fire does not exist, the alarm should be silenced by an authorized service technician following procedures recommended by the NFPA. The problem unit must be located and the alarm system controls reset, so that the effectiveness of the detection system is restored. Owners or their representatives should be certain to check all detectors in the zone before deciding that it is a false alarm. If a fire does exist, more than one detector may be in the alarm state, although no signs of fire may be evident in the vicinity of the first activated detector. The fire could be overlooked.

Miscellaneous Causes of Unwanted Alarms

Isolated alarm causes such as a maintenance person accidentally triggering an alarm by touching a detector with a magnetic screwdriver can be ignored, except to periodically remind maintenance personnel to be careful when working around detectors. Owners often renovate. In all cases of renovation, steps also should be taken to protect detectors from dust. Even routine maintenance that requires sawing, sanding, drilling, or other dust-producing operations in the vicinity of the detector heads can trigger a false alarm. In new construction applications drywall dust contamination affects all types of smoke detectors. To help overcome this problem, it is strongly recommended that installation of detector heads be delayed until all trades have completed work and job site cleanup has occurred in accordance with NFPA requirements.

If alarms occur whenever the heating system is turned on after an extended shutdown, due to the accumulated dust burning off as the system components heat, the detector system can be turned off for a short period. Not all unwanted alarms are caused by dirt, interference or other effects on the detectors. If the control panel shows an alarm but no detectors in the zone are indicating an alarm condition, the possibility of interference or a failure of a control panel component should be investigated.

Where to Get Help if the Source of Unwanted Alarms Cannot be Found

In the event a series of unexplained unwanted alarms and/or a review of the Alarm Log indicates that a problem situation exists, the owner should conduct the initial investigation to find a solution. If the owner’s personnel are unable to determine the cause for the alarms, the installer or representative of the manufacturer should be contacted to help isolate the problem. Manufacturers can be contacted by phone for additional suggestions. If assistance is needed, a factory engineer may be able to explain the source of the problem with data from your Alarm Log, a complete description of your alarm system including detector model numbers, make and model numbers of the control panel and other components, and a complete summary of all aspects of the problem that have already been checked.

Checklist for Owners and Installers

The owners of smoke detector-equipped fire alarm systems are responsible for maintaining the integrity of the detection system. This can be accomplished by:

• Maintaining an Alarm Log and training appropriate personnel to properly maintain the system.

• Maintaining a Detector Maintenance Log that records inspection, testing and cleaning data for each detector in the system.

• Maintaining a complete file of information on the alarm system in a readily accessible location. This file should include specifications and installation instructions for the detectors, control panel, and auxiliary devices, wiring diagrams, wire location information, and the manufacturer’s recommendations for isolating the detection system wiring from other electrical wiring to prevent interference and unwanted alarms.

• Making certain that maintenance personnel or contractors working on the building’s electrical systems are given copies of the alarm system wiring layout and locations so that potential interference from other wiring systems can be prevented by proper insulation and spacing during installation.

• Keeping accurate records of installation and modifications to all other building electromechanical systems that could cause interference with the alarm system so that problems can be promptly found and eliminated. Consider keeping records of schematic update schedules, wiring layouts, and wiring location information.

• Making a record of all actions taken during the investigation of a series of alarms, indicating a problem exists. If assistance must be sought from the installer or manufacturer, there will be an indication of the tests that have already been done by the owner’s personnel. These services can be provided by qualified outside organizations.


As indicated, owners have a lot on their plates, and proper fire protection for their assets is perhaps one of the most important items. Understanding detectors and their performance helps owners determine the right integrated protection strategy.

If you need more information on detectors, please feel free to contact System Sensor, the leading manufacturer of detectors in the world. System Sensor's Technical Service department is staffed from 7:30 AM - 5:00 PM Central Time, Monday - Friday. After hours, please leave a message. Your call will be returned promptly during regular business hours. Phone: (800) 736-7672, press 2.

In addition, a complete document center is available 24/7 for owners on a complete variety of detectors. You can download this information at: Finally, System Sensor conducts ongoing webinars for owners and other professionals. These webinars, which include many detector topics and their use, can be accessed 24/7 at:

Going Wireless: when owners should tap fire protection without wires

Smoke detectors can be wired or wireless and using wireless opens up an entirely new concept of fire protection to building owners. Wireless addressable devices provide secure, reliable communication to the Fire Alarm Control Panel across a network and are ideal for applications where it is costly, obtrusive, or possibly dangerous to use traditional wired devices.

A wireless system is configured much like wired detectors in terms of installation and integration – but without the wire. The components are the same: a control panel, detectors, command center, power supplies, and other components of a complete system.

For example, in cases where areas of a building are difficult or impossible to wire, visually sensitive, or have restricted access, wireless sensors provide an efficient, reliable solution. The devices communicate via a proprietary wireless mesh protocol to communicate with the fire alarm system by means of a Wireless Gateway.

Wireless devices in a network develop what’s sometimes called "parent-child" communication links with other devices in the mesh, so that a message originating from a remote device "hops" to the closest parent device, and then to successive parent devices until the message reaches the gateway. Alternate paths are also identified and supervised by the protocol providing approved Class A wireless communication. If a device does not have an established communication path with adequate signal strength, an additional device such as a wireless module may be installed in between so that it will act as a repeater.

In one recent installation, for example, the owner was skeptical of all the wiring options when wireless was suggested. According to the installer, the owner selected wireless devices so that the extensive woodwork in that facility would be unaltered. In addition to respecting the wood design of the facility, the wireless system inadvertently also solved an ongoing issue: continuous repair required as pests chewed through the wiring. The wireless devices ensured the integrity of the system by making sure that no creature can chew through the wire.

Whether choosing wired, wireless or a combination of both, owners now have more options for fire protection than ever before.  Building owners and life safety system installers should be aware of and keep all of these options in mind when planning their next fire system installation. 

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