For a fire protection engineer, there are several jobs related to fire alarm systems in existing buildings, and each requires a slightly different approach.

The most common include:

  • Evaluating an existing fire detection and alarm system.
  • Reviewing local codes to decide whether an existing building and occupancy requires a fire alarm system and, if so, what features are required.
  • Determining if a fire detection and alarm system can be used to provide a desired level of fire protection.
  • Determining how best to install a fire alarm system in an existing building.
  • Determining when repairs trigger upgrades.

This article addresses some of the most common considerations in these categories.


Evaluating a System

When evaluating an existing fire detection and alarm system, the first question that should be asked is: Evaluating for what? Most evaluations are done to determine if the existing system meets local code requirements. That simply requires some knowledge of, or research into, the local codes and is addressed below in the section, "Is a System Required." Some evaluations are done to determine if the existing system is still capable of performing its intended function. This is addressed in the section titled, "Is a System Desired."

Another reason to evaluate an existing system is to determine if the system should be replaced or upgraded to help control life-cycle costs. Just like people, as fire detection and alarm systems age, some require a greater amount of maintenance. Smoke detectors may accumulate dust and dirt even when they are regularly cleaned using manufacturer's instructions. Eventually, their sensitivity might drift to the point where replacement is warranted. A higher sensitivity could directly result in a false alarm as the unit approaches its response threshold. Higher sensitivity can also increase the likelihood of nuisance alarms from sources such as cooking odors, steam or tobacco smoke that normally might not be sufficient to alarm the detector. There is no code requirement for the replacement of aging system smoke detectors. This is because a regular inspection and testing program is an effective method of assuring detector operability. 1 The code requirement for smoke alarms installed in one- and two-family dwellings is different in that a 10-year replacement is required. 2 This is predicated on the assumption that homeowners are less likely to periodically test and clean their smoke alarms.


NFPA 72, the National Fire Alarm and Signaling Code, requires owners to keep records of system activations and operation for a period of one year after the next regular test.2 Owners should be encouraged to keep records for longer periods to assist in evaluating aging effects on system performance.

A system that alarms when there is no fire is effectively "crying wolf" and reducing its effectiveness.3 In addition, there are more direct costs, such as lost productivity for the occupants, alarm company service calls and possibly even fines imposed by local authorities. The benefits of replacing an aging system include reduction or elimination of these costs and might include a reduction in the regular cost of ownership.


Depending on the age of the existing system, a replacement system might add features, such as built-in sensitivity testing, sensitivity drift monitoring and management and logging of system events. Printers that automatically maintain the records required by NFPA 72 are another option. These features can directly reduce the costs of periodic testing and maintenance and record keeping.


Existing systems often are evaluated for their ability to provide adequate occupant notification. Many systems installed to meet older codes might lack adequate occupant notification in the form of audible signal levels in sleeping areas. Older codes simply required a system to be audible so that it could be "heard in all areas" or "adequate to perform its intended function." There was no requirement for a specific audible level. As research was done on awakening effectiveness, new requirements were introduced into codes and standards.4,5,6,7,8 Most recently, research has shown that the frequency (pitch) of the alarm signal, in addition to its sound pressure level, is an important factor in waking effectiveness.2,9,10


A common question raised by owners is whether a system has to be upgraded to meet the levels required by current codes. Many jurisdictions and circumstances permit a system to remain unchanged if that system met the code that was in place at the time the system was installed. However, a fire protection engineer should question whether a system actually meets the requirement of that older code to "perform its intended function" if it does not meet the sound pressure level necessary to awaken and alert someone. Similarly, existing systems are often evaluated for their ability to provide adequate visual signaling to meet current accessibility codes.


If an existing system does require or warrant detector replacement or notification appliance upgrades, the engineer must evaluate whether a total replacement is necessary or whether the existing control system and wiring can be used. This might involve issues of component compatibility. All initiating devices must be listed as compatible with the fire alarm equipment unless they interface only by contact (switch) closure. 2 For example, if the control unit uses addressable technology to communicate with the initiating devices, those devices must be listed to work with that particular control unit. However, initiating devices connected to conventional, zoned systems operate differently. They signal an alarm by either creating a direct short circuit or by changing the amount of current flow on the initiating device circuit (IDC). Therefore, conventional (non-addressable) devices that do require power to operate, such as manual pull boxes and heat detectors, do not have to be compatibility listed with the control unit because they operate by simply shorting the IDC.

Conventional smoke detectors operate in one of two ways. One type connects to an IDC using dry (non-energized) contacts (just like a light switch) and requires a separate power circuit. Those detectors would not have to be compatibility listed to connect to a conventional fire alarm control unit. The second type gets its operating power from the IDC and signals an alarm on the same IDC by increasing the flow of current above some threshold established by the control unit manufacturer. Thus, that type of detector must be compatible with the control unit to ensure that operating and alarm currents are properly balanced. Those types of detectors often are called circuit-powered detectors. They also are commonly called "two-wire" detectors, although that is a poor description.


In general, notification appliances do not require any compatibility listing. They must simply be matched for the proper voltage. There are two types of notification appliance that would require compatibility listing to the control unit. The first is addressable notification appliances. They must be compatible to ensure proper communication with the control unit. The second type that must be listed to be compatible with a particular control unit are those that have a "Special Application" listing. In that case, the manufacturer has designed the notification appliance and the notification appliance circuit specifically to work together. In the case of DC voltage notification appliances, the quantity of appliances on a circuit is limited by the amount of current they draw and the rating of the notification appliance circuit. In the case of AC voltage appliances (speakers), the quantity is limited by the power available from the amplifier.


Quantities of appliances might also be limited by the size of existing wires and the resulting voltage drop. Power supply capacity and existing wire sizes might require panel replacements or adding booster panels to accommodate additional notification appliances and circuits. These issues require specific information from the system manufacturer. If the existing system and wiring have adequate capacity and compatibility for new appliances and detectors, battery capacity still must be checked to ensure the ability to supply the new load.


Is a System Required

A fire protection engineer might be asked to determine if an existing building requires a fire alarm system. If the property is being renovated or if there is a change in occupancy, local codes might require installation of a new system or an upgrade of features for an existing system. Changes in fire codes might also require the installation of a system in a building that did not previously require one. The Spring 2007 article in this series provides more discussion of building and fire codes and local requirements.11 If a system is required, the codes usually spell out the need for one or more of the following:

  • Manual activation
  • Automatic fire detection
  • Occupant notification
  • Emergency forces notification
  • Supervision of other protection systems
  • Activation of emergency control functions

The code may not require all of these features. For instance, a code might require a system that supervises a sprinkler system and provides emergency forces notification, without any requirement for occupant notification.


Is a System Desired

Even if a system were not required in an existing building, one might be effectively used to meet the owner's fire protection goals. More often than not, a system is already in place and additional features are evaluated to determine if they will contribute to the overall fire safety goals. By itself, a fire detection and alarm system is not a complete fire protection system. However, it is a vital component for several fire protection strategies. For example, people cannot be expected to exit a building unless a fire is detected and a warning is signaled early enough to provide adequate egress time. This fire protection system requires a sufficient number of adequately sized and constructed means of egress in addition to an adequately designed detection and occupant notification system.


Similarly, manual extinguishing efforts cannot begin unless the fire is discovered and emergency forces are notified. However, fire department notification by itself does not mean that fire protection goals will be met. For example, it is possible in some situations that a fire will be beyond the capability of the local fire department to control even if detection and signaling are nearly simultaneous with ignition. It might be necessary to have a combination of systems to achieve the desired level of fire protection.


NFPA 550 provides engineers with a tool to evaluate systems and combinations of systems for their ability to meet protection goals.12 The decision trees in that guide are also useful for explaining fire protection concepts to owners and in getting owners to establish realistic goals.


How to Install the System

If a system is to be installed or upgraded in an existing building, exactly how that system will be installed can be a challenge and can greatly affect costs and therefore the viability of the project. Obviously, the building construction features play a large role in system installation methods and costs. Unless the engineer has actual field installation experience, it is most often best to work with a qualified contractor to establish circuit routing and installation methods.


However, there are some strategies that engineers should consider and review with the contractors. For example, wall mounting of detectors can make installation of concealed wires fished up from a basement more economical than ceiling mounting, which might require surface wiring using raceways. Where there are suspended ceilings, the use of ceiling-mounted notification appliances usually is less expensive than wall-mounted units. For both detectors and notification appliances, adding some additional units might reduce installation costs by placing them in locations where it is easier to install them. For spot-type detectors, determining locations using the concept of a protection radius rather than square spacing can reduce the number of detectors required to cover a space.2


Fire protection in existing buildings is usually more challenging than in new construction. When the costs of suppression systems, fire barriers and other protection systems are considered, strategies that include properly designed fire detection, alarm and signaling systems are often necessary to meet goals and local code requirements.



When does a repair trigger a requirement to upgrade? And, when does a component upgrade trigger a full system design upgrade? If a detector or notification appliance is physically damaged or fails during a test, a simple one-for-one replacement is permitted. In the case of a damaged control unit, the answer varies from jurisdiction to jurisdiction.


If a panel is damaged to the point where a replacement is needed, that means that all, or a large part of the system, might not be operational. Therefore, a fast one-for-one replacement is warranted. Many authorities, governing laws, codes or standards would permit the panel to be replaced with comparable and compatible equipment. In other cases, authorities simply insist on getting the latest and greatest system upgrades that are available, regardless of whether they have a properly adopted regulation, code or ordinance requiring such upgrades.


Most governing laws, codes and standards do not specifically state when a component replacement or upgrade will trigger a complete system upgrade to current requirements. Some laws, codes or standards have a value limit at which an upgrade to current codes would be required. In making a recommendation to an owner, it is incumbent on a fire protection engineer to consider the many factors discussed in this article regarding evaluating the system and risks, as well as the system needs and desired performance.



  1. "Mission Effectiveness and Failure Rates Drive Inspection, Testing and Maintenance of Fire Detection, Alarm and Signaling Systems," Fire Protection Engineering, Summer 2002.
  2. NFPA 72, National Fire Alarm and Signaling Code, National Fire Protection Association, Quincy, MA, 2009.
  3. "Fire Alarm Testing Strategies Can Improve Occupant Response and Reduce the 'Cry Wolf' Syndrome," Fire Protection Engineering, Fall 2003, Winter 2004.
  4. Nober, E., Harris, H. and Well, A., "Waking Effectiveness of Household Smoke and Fire Detection Devices," Fire Journal 75, no. 4 (July 1981): 86-91, 130.
  5. Berry, C., "Will Your Smoke Detector Wake You?" Fire Journal (July 1978): 105-08.
  6. Pezoldt, V. J., and Van Cott, H., Arousal From Sleep by Emergency Alarms: Implications From the Scientific Literature: U.S. Department of Commerce, June 1978.
  7. Fidell, S., Evaluation of Effectiveness of Residential Fire Detection System Audible Warning Signals, National Fire Protection Association Annual Meeting, May 1979.
  8. Uzzle, T., "Stimulus and Percept," Sound and Video Contractor (October 1991): 38-48.
  9. Bruck, D., Thomas, I. and Ball, M., "Waking effectiveness of alarms (auditory, visual and tactile) for the alcohol impaired," The Fire Protection Research Foundation, Quincy, MA, June 2007.
  10. Bruck, D., and Thomas, I., "Waking effectiveness of alarms (auditory, visual and tactile) for adults who are hard of hearing," The Fire Protection Research Foundation, Quincy, MA, June 2007.
  11. "Codes & Standards & AHJs Oh My," Fire Protection Engineering, Spring 2007.
  12. NFPA 550, Guide to the Fire Safety Concepts Tree, National Fire Protection Association, Quincy, MA, 2007.