Chapter 3(1/2) - Building Construction and Systems for Fire Investigators

Part 1 – Objectives, Building Systems, Design, Loads, and Materials

Knowledge Objectives

After studying this chapter, you should be able to:

• Identify the design, construction, and structural elements of buildings and describe their effect on fire development, spread, and control. (NFPA 1033: 4.2.2; 4.2.3; 4.2.5; 4.2.8, pp. 43–46)

• Identify and describe types of building construction. (NFPA 1033: 4.2.2; 4.2.3, pp. 46–52)

• Assess the structural integrity of construction assemblies during a fire. (NFPA 1033: 4.2.3, pp. 52–54)

• Identify the components of heating, ventilation, and air conditioning (HVAC) systems and describe their potential involvement in fire. (NFPA 1033: 4.2.3; 4.2.8, pp. 54–56)

• Explain the impact of passive fire protection systems on fire investigation. (NFPA 1033: 4.2.3, pp. 56–58)

• Describe the considerations when documenting a passive fire protection system. (NFPA 1033: 4.2.8, p. 58)

• List the types of fire protection systems.

• Identify and describe the components of common fire alarm and detection systems. (NFPA 1033: 4.2.8, pp. 58–66)

• Identify and describe the operational characteristics of common fire alarm and detection systems. (NFPA 1033: 4.2.8, pp. 58–66)

• Identify and describe the operational characteristics of water-based fire suppression systems. (NFPA 1033: 4.2.8, pp. 69–79)

• Identify and describe the components of fire suppression systems. (NFPA 1033: 4.2.8, pp. 69–84)

• Identify and describe the operational characteristics of non-water-based fire suppression systems. (NFPA 1033: 4.2.8, pp. 79–84)

• Describe how to document fire protection systems. (NFPA 1033: 4.2.8, p. 84)

Skill Objectives

• There are no skills in this chapter.

Introduction

A structure’s design, materials, and construction type influence how a fire develops and moves. These features may have a positive effect by containing fire within a compartment or a negative effect by allowing it to spread from the area of origin.

Proper analysis of fire growth and movement requires an understanding of:

• Building construction

• Building systems

• Fire protection systems

• The definitive fire patterns left by a fire

Building Systems Overview

Many modern construction practices developed from analysis of past fires, including catastrophic conflagrations around the beginning of the twentieth century. More than half of modern code requirements relate to fire protection.

A major cause of severe structural fire damage is the failure to contain or confine the fire. Construction principles aim to keep fire within its room, area, or structure of origin. When a fire leaves its room of origin, the route is commonly an unprotected or improperly protected opening or a construction defect rather than a failure of a protection system. Examples include:

• Open doors

• Unprotected penetrations through fire resistance-rated walls or ceilings

• Openings made for building or utility services

Fire beginning in an interstitial space does not receive the same protection as fire developing inside a compartment. These concealed gaps may exist around the building frame, behind interior walls and the exterior façade, or above a ceiling beneath the next floor or roof deck, such as a cockloft (NFPA 921, Section 7.2.2.5). Fire can develop there without detection and move rapidly without effective barriers.

Investigators need to understand the building’s active and passive systems, including:

• Manual and automatic fire detection

• Fire suppression

• Heating, ventilating, and air conditioning (HVAC)

• Utilities

• Building compartmentation

• The use and location of fire resistance-rated assemblies

Knowing where these systems were installed and how they operate helps determine their performance, their effect on the fire, and whether alterations or failure indicators are present. Apparent system failure does not necessarily mean a fault or deliberate interference occurred; the fire may simply have overwhelmed the system.

An operating HVAC system can move smoke and fire through a structure. Determine the system’s condition and operating status at the time of the fire. Mechanical and electrical inspectors or engineers can assist with unfamiliar systems.

Design, Construction, and Structural Elements

A building’s response to fire or explosion depends on factors such as:

• Construction type

• The integrity and fire performance of structural elements under a fire load

• Active and passive fire protection systems

• The number and arrangement of doors, windows, and other openings

A broken window or opened door introduces oxygen that can greatly increase fire size and speed. Exterior winds can rapidly drive fire down unprotected corridors. The investigator must therefore consider environmental and mechanical conditions that existed during the incident, including ventilation.

The building’s interior layout, interior finishes, services, and utilities can also influence fire origin, development, and spread.

Figure 3-1 – Typical single-family dwelling

Figure 3-1 identifies the following structural components:

Building Design

Fire development and spread in a building largely result from radiant and/or convective heat transfer. Within a compartment, important variables include:

• Room size and shape

• Ceiling height

• The placement and area of doors and windows

• Interior finishes and furnishings

• HVAC systems

• Fuel packages and location

Compartmentation is a central fire-safety design strategy: a structure is divided so fire can be limited to a particular area. Investigators must be prepared to encounter varied construction techniques and building materials that may affect fire spread.

Tip

Become familiar with not only the most common building designs and construction, but also less-common styles of buildings or compartments, such as atriums, stadiums, and tunnels, that could have different impacts on fire spread. Special designs may also incorporate special materials.

Building Loads

Loads are forces acting on a structure. Building design accounts for temporary or movable live loads, including people, furniture, furnishings, equipment, machinery, wind, snow, and rainwater. It also accounts for the dead load that remains in place, including structural components, roof coverings, and mechanical equipment.

A structure may become unstable or fail when the applied load exceeds its design parameters. Possible causes include:

• Extreme wind or snow

• Additional contents or stock

• Added mechanical equipment, such as HVAC or elevator equipment

• A large concentration of people in a limited area

• Water introduced during firefighting

Structural Changes

Structural changes may be deliberate, as during renovations, or may result from the fire itself. Fire can damage supporting elements until they can no longer carry their intended loads.

Always conduct an exterior examination to:

• Identify fire-related structural compromise and potential collapse hazards

• Preserve existing evidence

• Interpret exterior fire damage

• Locate all possible entry and egress routes so an area being processed has at least two ways out

• Identify the building’s utilities

These considerations also apply during the initial interior survey before scene processing begins.

Room Size

Room geometry—including height, width, and especially the distance between walls—affects fire development. With comparable fuel and ventilation, a smaller room reaches flashover sooner than a larger room. Because flashover can spread fire beyond the room of origin, room size is important when reconstructing fire development. In a very large room, heat transfer may never become sufficient to produce flashover.

Compartmentation

This subdivision of a structure into separate sections or units can limit fire spread. Fire commonly spreads horizontally beyond its compartment of origin through doors, windows, unprotected openings, and wall penetrations. Stairways, utility chases, and shafts provide possible vertical routes.

Interstitial and Concealed Spaces

Concealed and interstitial spaces occur in most buildings and can carry fire beyond the original compartment. Interstitial spaces commonly lack fire stops, allowing movement behind wall finishes, within exterior-wall cavities, or above ceilings. Missing fire stops may permit vertical spread. Where fire stops are present, examine whether they remained intact and stopped or limited spread.

Failure to recognize a concealed space can produce an incorrect interpretation of fire patterns. Depending on the building, these spaces may contain:

• Fire sprinklers

• Early-detection devices

• Fire stops

Codes may permit some concealed spaces when they are built from noncombustible material. Using them to store combustible items can promote fire spread and is often prohibited.

Planned Design Versus “As-Built” Conditions

When possible, obtain the building’s floor plan from the owner, fire department, or building department having jurisdiction. Building-department records can show whether permits were issued for interior or exterior alterations. However, walls, wiring, and mechanical systems are sometimes changed without permits.

Original plans frequently differ from what was actually constructed. Determine whether available plans accurately represent the “as-built” structure. Undocumented changes may be identified through:

• Examination of the fire scene

• Comparison with similar houses constructed by the same contractor in the same project

• Witness interviews

• Occupant photographs

Building Materials

The chemical composition, thermal conductivity, and density of building materials influence ignition, fire growth, and fire speed. Plastics and other synthetic materials used in modern construction can significantly increase heat-release rates.

Thermoplastics may change from solids to liquids and then to ignitable gases. While burning, they can form flaming drips and pools that create drop-down damage and patterns. Thermoset plastics instead pyrolyze directly into ignitable gases and generally do not drip or flow. At the area of origin, identify the plastic involved so a melted thermoplastic pool is not mistaken for suspected ignitible liquid.

Table 3-1 — Building Materials

Orientation affects burning behavior. Carpet normally lies horizontally on a floor; placing it vertically on a wall greatly increases flame spread associated with the carpet.

Orientation, Position, and Placement of Loads

Fuel load includes all combustible contents within a building, space, or fire area—including interior finishes and trims—and may be expressed in heat units or as an equivalent weight of wood (NFPA 921).

Do not overlook the orientation, position, or placement of fuel packages. Their location and installation influence how they burn and how quickly fire progresses. For example, a trash can 3 feet (0.9 m) from a burning desk presents a different spread potential from one immediately beside the desk. Interior finishes and furnishings likewise influence fire speed and intensity.

A 2-inch (5-cm) by 4-inch (10-cm) piece of oak is compared with a lower-density 2-inch (5-cm) by 4-inch (1-cm) piece of balsa wood. The oak burns much slower and creates less char.

Steiner Tunnel Test

The Steiner Tunnel Test, ASTM E84, establishes flame-spread indexes used in fire and building codes. Many materials evaluated in the tunnel are not designed or intended for use as wall or ceiling coverings. A tested product’s classification may not represent its actual behavior if it is installed in an orientation or application for which it was not designed. Such an installation would not be an acceptable interior finish under the applicable codes.

Interior-finish requirements depend on occupancy type and size. If an assembly is unrated, there is no standard fire-test information showing how that assembly will react. Code violations involving interior finishes can produce devastating outcomes and have contributed to incidents with multiple fire deaths. Recognizing improper finishes may help explain rapid fire development or loss of life.

Occupancy

Determine whether the building’s use or occupancy classification changed at any time. Occupancy describes the intended use of a structure, such as residential or commercial. A change can introduce fuel loads the original fire-protection systems were not designed to handle. A change of occupancy is often accompanied by changes to the structure and systems, putting the “as-built” structure at even greater variance from the original plans. Egress and active or passive fire-protection safeguards may consequently be inadequate.

For example, a home-improvement store may have additional rack-storage sprinkler protection in its paint section. If the paint is moved elsewhere without extending that protection, the standard sprinkler protection in the new location may be overwhelmed by the fuel load presented by aerosols, paints, thinners, and strippers.

Chapter 15, Analyzing the Incident, provides additional discussion of failure analysis and how it can affect fire spread in a building.

Tip

It is important to determine whether a building’s occupancy classification has changed since it was built or renovated. Different occupancies require different fire protection systems depending on their load, contents, and storage configuration.

Part 2 – Types of Building Construction

Construction methods vary throughout the world; this chapter focuses on the types most common in the United States. Determine and document construction type from the building’s structural elements. Also record:

• Structural components

• Breaches

• Structural changes

• Other conditions that could affect structural integrity or fire spread

Tip

A wood wall stud is commonly called a 2 by 4, reflecting the nominal lumber measurement. The actual dimension of the 2 by 4 [2 × 4] is 1¾ (44.5 mm) by 3½ inches (88.9 mm), which is the dimensional lumber measurement.

Ordinary Construction

In ordinary construction, NFPA 921 describes exterior walls as masonry or other noncombustible materials. The floor, roof, and partition framing are wood assemblies that use braced- or platform-frame methods. NFPA 220, Standard on Types of Building Construction, classifies ordinary construction as Type III.

The term ordinary reflects this construction type’s use in many different buildings. Fire spread can be affected by:

• Open vertical shafts

• Combustible materials

• Multiple ceilings

Figure 3-2: Ordinary construction is used in a wide variety of buildings.

Wood-Frame Construction

NFPA 220 classifies wood-frame construction as Type V. Exterior walls and load-bearing components use wood or other combustible materials, most commonly in residential and commercial buildings of limited size.

Floor joists and vertical supports are generally spaced 16 inches (0.4 m) on center. Nominal vertical members may measure:

• 2 inches (5 cm) by 4 inches (10 cm)

• 2 inches (5 cm) by 6 inches (15 cm)

These members alone offer little fire resistance. Flames and hot gases can penetrate gaps between joists or studs and carry fire outside the area of origin.

Sheathing can provide additional resistance. Examples include:

• Gypsum board

• Lath and plaster

• Mineral tiles

Even non-fire-rated sheathing may provide some resistance. A fire-rated wall, however, may still conduct heat to the underlying members or combustible materials on its opposite side.

A masonry veneer does not change the construction classification when the supporting frame is wood. Fire-resistive membranes applied according to ASTM testing standards can provide wood-frame construction with up to a 2-hour fire resistance.

Table 3-2 — Ordinary Construction Versus Frame Construction

Platform-Frame Construction

Most modern wood-frame buildings use platform framing. The walls for each level rest on a platform or floor, so each platform acts as a fire stop against floor-to-floor vertical spread. Because the barriers are combustible, fire may eventually overcome them.

Important areas of concern include:

• Concealed spaces in soffits, which are the horizontal undersides of eaves or cornices

• Vertical openings made for utilities

Figure 3-3 — Platform-frame construction.

Tip

Checking the floor or roof to see whether it is spongy does not ensure safety with modern lightweight construction.

Balloon-Frame Construction

Unlike platform framing, balloon framing uses exterior wall studs that continue uninterrupted from the foundation wall to the roofline. Codes have long required fire stops in the resulting vertical channels. Fire stops may consist of:

• Wood boards

• Noncombustible material, such as insulation, filling the void

Proper fire stops can give the building fire performance similar to platform-frame construction. Missing fire stops can permit uninhibited vertical spread or ignition caused by burning material falling from the attic. Fire stops may also be removed when wiring, HVAC equipment, or other services are installed.

Open vertical channels can produce rapid upward extension. Open connections between floor joists and the vertical channels also make horizontal extension likely. These channels may allow burning debris to fall and combustion gases to convect downward.

Consequently:

• More extensive burning may occur at the upper level than where the fire originated.

• Fire may emerge remotely from its origin.

• Fire may bypass one floor and reach another.

• Investigators must identify every possible route of fire travel.

Figure 3-4: Balloon-frame construction.

Tip

Fires that have burned in balloon-frame walls destroy the structural integrity of the building. Collapse is a serious threat.

Plank-and-Beam Construction

This method uses large beams supported by posts. Examples of beam dimensions include:

• 4 inches (10 cm) by 10 inches (25 cm)

• 5 inches (12.5 cm) by 12 inches (30 cm)

The beams are spaced more widely than conventional framing—typically 4 or 6 feet (1.2 or 1.8 m) on center. Floor decking has a minimal thickness of 2 inches (5 cm) and is generally tongue-and-groove, which helps limit fire spread.

Other characteristics include:

• Few concealed spaces

• Exterior finish that has no structural value

• Large spans of unsupported finish material that may result in failure of structural sections, with large frame members left standing

• Large interior areas with exposed combustibles that can support flame spread

Figure 3-5 — Plank-and-beam construction has a limited number of concealed spaces.

Post-and-Frame Construction

A visible skeleton of large fitted timbers distinguishes this method, which is similar to plank-and-beam construction. In typical barn construction, the posts provide most of the support and the frame provides a surface for attaching the exterior finish.

Figure 3-6: Post-and-frame construction.

Heavy Timber Construction

Structural members are unprotected wood with a smallest dimension of 6 or 8 inches (15 or 20 cm). Floor assemblies use 2-inch (5-cm) thick, tongue-and-groove end-matched lumber, and no concealed spaces are permitted.

Building codes require bearing walls in heavy timber construction to have a 2-hour rating. Fire spread may still be rapid because of:

• Wood-frame components

• Open spaces

• Large areas of interior combustibles

Figure 3-7: Heavy timber construction has exterior walls that consist of masonry construction and interior walls, columns, beams, floor assemblies, and roof structures that are made of wood.

Contemporary log homes use specially milled logs for their exterior walls and many structural elements. The interior structure is generally 2-by-4 wood framing, although the finishes are commonly made to resemble the milled-log exterior.

This mismatch between the visible finish and the underlying construction may confuse an investigation involving rapid fire spread or structural failure. Large open interior spaces are another hazard because they facilitate fire spread.

Insurance companies helped develop mill construction as an early heavy timber method after recognizing the need to reduce major factory-fire losses.

Typical features include:

• Masonry exterior walls

• Heavy timber columns and beams

• Walls generally assigned at least a 2-hour fire rating

• Rated doors in interior walls between compartments

• No concealed spaces, improving the building’s fire-resistive performance

• Protection of vertical openings to limit fire passage

• Fire sprinkler systems

• Wall scuppers that drain water and reduce water damage

Alternative Residential Construction

Residential construction also includes manufactured housing and steel-framed systems in addition to site-built wood framing.

Manufactured housing is constructed in one or more sections. While being transported, a section measures at least:

• 8 feet (2.4 m) wide

• 40 feet (12.2 m) long

The sections are transported to the site and assembled there, where they can be 320 ft² (29.7 m²) or larger. Another form is built on a wheeled steel frame for transportation and is often called a mobile home.

Manufactured homes are the least expensive structures to build. Some incorporate house-like features such as:

• Steeper-pitch roofs

• Residential-size windows

• Covered decks

Figure 3-8: Manufactured home.

Manufactured homes contain four principal components or subassemblies:

• Chassis

• Floor system

• Wall system

• Roof system

The chassis generally has two longitudinal steel beams braced by steel cross members. These members receive vertical loads from the walls, roof, and floor. Once onsite, the loads transfer to stability devices, which may consist of piers or footings, or to a foundation. Steel outriggers cantilevered from the outside of the main beams bring the chassis to approximately the full width of the structure.

Since the mid-1970s, mobile homes built to the U.S. Department of Housing and Urban Development (HUD) Standard have used more substantial construction than earlier units.

HUD-approved construction generally includes:

• Wood studs

• Interior finishes, most often gypsum wallboard

• Metal, vinyl, or wood exterior siding

• Stronger, more solidly built roof systems, often with gable- or hip-style rooflines

• An asphalt paper/shingle roof covering over oriented strand board (OSB) or plywood attached to rafters or wood trusses

• A wall system bonded into a complete unit between the roof and floor systems by steel ties

Older pre-HUD units commonly use:

• 2-inch (5-cm) by 2-inch (5-cm) exterior wood studs

• Asphalt paper serving as a vapor barrier

• Aluminum siding attached directly to the studs

• Interior wood paneling

• No insulation in the wall voids

• A minimal flat wood-frame roof with a steel exterior skin

Minimal construction and the absence of fire-resistant interior finishes allow faster, more intense fire spread. Newer manufactured homes use gypsum wallboard on walls and ceilings and include code-required smoke alarms. Their fire dynamics are more similar to those found in site-built homes, and fire spread is not as rapid as in pre-HUD homes.

Early detection devices wired to house power reduce the chance of a large fire and lower the risk of death or injury.

A modular home is also called a system-built or prefabricated home. Multiple sections are produced in a controlled offsite facility using technology similar to newer site-built homes. At the selected site, the sections are joined into one residence using a crane or trucks.

Unlike manufactured homes, modular homes have neither axles nor a transport frame. They are placed on a foundation supplied by the contractor. Designs may be single-floor or multilevel and can be customized. Fires in modular homes have burn tendencies similar to those of site-built wood-frame homes.

Steel-framed residential construction is becoming more common in:

• Site-built systems

• Panelized systems

• Pre-engineered systems

It has many characteristics of wood framing but does not add combustible framing material. Extreme heat can nevertheless cause the steel frame to lose structural capacity.

Figure 3-9: Steel will begin to lose its strength at temperatures of 1000°F (537°C).

Manufactured Wood and Laminated Beams

Laminated beams are made by gluing or laminating many wood planks into a single structural beam with performance similar to solid wood. These members are generally intended only for interior use and are commonly called glulam beams.

They behave like heavy timber until failure. Weather exposure reduces their load-bearing ability. During an investigation, document:

• The size of each individual member

• The overall beam dimensions

Wood I-beams have smaller dimensions than floor joists and can therefore burn through and fail sooner than dimensional lumber. Openings cut through the vertical center portion, or web, for utilities may reduce the structural integrity of the web. Gypsum-board protection is required to help delay collapse during a fire.

Lightweight wood trusses are similar in design to other trusses. Individual members may be joined with:

• Nails

• Staples

• Glue

• Metal gusset plates, also called gang nail plates

• Wooden gusset plates

Truss failure can occur when gusset plates fail before the wood members are burned through. When one member fails, the remaining members assume additional loads and may become stressed, which may cause failure of the entire truss.

Rating of Wood-Frame Assemblies

A wall receives a fire rating by being covered with a noncombustible finish, commonly gypsum board. Even then, heat may conduct through the assembly to the combustible members beneath it.

Actual field installation can differ from the construction used during fire-integrity testing. Poor installation may therefore cause a rated wall to fail sooner than its tested rating suggests.

Noncombustible Construction

This construction is used primarily for commercial, industrial, and high-rise buildings. Its materials do not contribute to the fuel load. Examples include:

• Brick

• Stone

• Metal

• Non-reinforced concrete

Metal Construction

Exposed metal is common in unfinished spaces and may fail in as little as 3 minutes during flashover. Fire-rating tests may not predict actual scene performance because real fires are unpredictable and can differ substantially from laboratory conditions.

Investigators should examine:

• Building conditions

• Interior finishes

• Ventilation effects

• Whether exposed metal contributed to earlier-than-expected failure

Ductile metals such as steel deform before failure. This deformation may occur in two ranges:

• Elastic range: The material changes shape under load and then returns to its original shape without losing strength when the load is removed.

• Plastic range: Stress beyond the elastic limit produces permanent deformation, although the material may continue carrying the load.

Elongation or deformation in either range can produce building collapse or damage.

Table 3-3 — Steel Versus Masonry/Concrete

Concrete and Masonry Construction

Mass, high density, and low thermal conductivity give concrete and masonry inherent fire resistance. These materials are strong under compression but weak under tension, and structural failure often begins at connection points.

Failure may result from:

• Heat transfer through the concrete or masonry

• Surface spacing, which exposes the reinforcement to fire temperatures

• Failure of steel connections between components at temperatures well within those found in structure fires

Figure 3-10: Concrete is noncombustible and provides thermal protection around steel reinforcing rods.

Part 3 – Construction Assemblies, Walls, Doors, and Structural Protection

Construction Assemblies

Walls, floors, ceilings, and similar completed units are assembled from manufactured components. These units may or may not have a fire-resistance rating. A door is one component within a larger door assembly.

Even unrated assemblies usually provide some resistance to fire or smoke. Because assemblies interact, failure of one can contribute to failure of another and affect fire growth and spread.

An assembly is designed to function as a complete unit. Its integrity during a fire depends on it being manufactured, installed, and maintained as intended. Conditions that can compromise performance include:

• Fire doors blocked open between compartmentalized fire areas

• Unprotected utility openings through fire-rated walls

• Other holes left in walls

• Missing ceiling tiles

• Ceiling tiles that were not clipped down

• Ductwork passing through an area without the required fire and/or smoke dampers

These defects can allow for the passage of fire.

Fire-resistance ratings come from specific tests conducted under defined conditions. An actual structure fire may be more severe than the test and may cause an assembly to fail before its assigned hourly rating. After a fire, examine the complete rated assembly for flaws in any of its components.

Floor, Ceiling, and Roof Assemblies

These assemblies are a major concern because they are among the first building elements to fail after structural members are exposed to fire. Failure can involve:

• Collapse

• Deflection

• Distortion

• Heat transmission

• Fire penetration

Performance is influenced by:

• Type of structural element

• Protection from the elements

• Span

• Load

• Beam spacing

Water introduced during firefighting adds weight and can contribute to failure. Utility penetrations are also common and are required to be sealed when they pass through a fire-rated assembly, although this is often not done.

Floor assemblies are tested for fire spread from below, not from above. Roof stability can affect both firefighting operations and fire dynamics, making collapse potential a vital scene concern.

Structural elements are not intended to retain their strength under fire conditions. Protective materials such as gypsum wallboard delay their direct involvement. Once that protection fails and the structural elements are exposed, their load-bearing capacity decreases.

Walls

Walls may limit both fire and smoke spread, but they are constructed to many standards and may or may not be fire-resistance rated or load bearing.

Figure 3-11: A. A load-bearing wall provides structural support. B. A non-load-bearing wall supports only its own weight.

Utility penetrations through fire-rated wall assemblies must be sealed. Different wall systems serve different protective functions. Fire walls separate buildings or compartmentalize the interiors of large buildings to restrict fire spread while creating a fire-resistance-rated, structurally stable division. Fire barriers resist the passage of fire and smoke; a gypsum-board fire barrier uses Type X gypsum wallboard. Smoke barriers are continuous vertical or horizontal membranes—such as wall, floor, or ceiling assemblies—constructed to restrict smoke movement (NFPA 921).

Fire walls and fire barriers normally do not have to meet the same requirements as smoke barriers unless they are also intended to perform that function.

An unrated assembly may still provide some fire resistance. Its lack of a rating means that no test was done for that type of wall, ceiling, or floor component, so no established failure time exists.

Assemblies incorporating smoke damper systems are intended to restrict smoke passage. Their openings may or may not also be protected against fire passage.

During an investigation, examine interior walls to determine:

• Which walls are load bearing

• Whether supporting elements were removed, creating structural stress and possible collapse

• Whether walls were intended to resist fire, smoke, or both

• Whether fire passed through an unsealed penetration

Fire walls and smoke barriers are constructed of specific materials that have been tested and proven to prevent the passage of smoke or fire.

Contractors may install lines or other components through compartment separations without closing the openings as required by fire or building codes.

Doors

Doors can determine whether fire remains confined or spreads through a structure. They may be fire rated or non-fire rated and may be made from several materials.

Every opening in a fire-rated wall or partition requires a rated door and its complete associated assembly. Rated components include:

• Frames

• Hinges

• Closures

• Latching devices

• Glazing, when provided and permitted

Fire doors may be constructed from:

• Solid wood

• Steel

• Steel with an insulated core of wood or mineral material

Their insulating value supports egress, especially in multistory buildings, and helps protect combustibles near the unexposed side of the opening from autoignition. The door must be closed to serve as an effective barrier against smoke, heat, and fire.

A door placed in a fire resistance-rated wall assembly should be installed as a complete fire-protection-rated door assembly. Its hourly rating depends on the fire wall’s rating and is generally lower than the rating of the wall system.

Figure 3-12 — A fire door has a label indicating its classification and rating.

The label identifies the door’s classification and rating. Visible markings on the example include:

• CLASSIFIED UL

• Swinging Type Fire Door Rated: 1½ Hours

• No. N-985479

• Minimum Latch Throw - ½" (12.7 mm)

• SECURALL SAFETY STORAGE EQUIPMENT

For the assembly to retain its rating, its hinges, closures, latching devices, and glazing must all operate properly. NFPA 80, Standard for Fire Doors and Other Opening Protectives, provides further information.

During a fire investigation:

• Determine whether a fire door was propped open with doorstops or other objects before the fire.

• If an approved magnetic hold-open device was used, determine whether activation of the fire alarm released the door and whether the door then closed and latched securely.

• If a door was propped open, determine whether that was normal practice or an unusual condition.

• Consider whether firefighters propped the door open during suppression operations.

• Inspect for holes, penetrations, or modifications that prevented the door from conforming to its intended design.

Concealed Spaces

Penetrations commonly provide concealed access for:

• HVAC systems

• Plumbing

• Electrical systems

• Computer lines

• Telephone lines

• Other building functions

Each penetration must be sealed to maintain the rating of the wall or floor it passes through. Openings added after construction are not always properly sealed and may allow fire or smoke to move from one protected area into another.

Construction Materials

Unprotected structural steel loses strength at high temperatures and should be protected from exposure to the heat produced by building fires. Protection methods include:

• Encasement in poured concrete

• Calcium-silicate board systems

• Gypsum board systems

• Spray-on fireproofing

• Intumescent coatings

A less-common method fills hollow structural members so the fill acts as a heat sink and slows the steel’s temperature rise.

Reinforced concrete can be used as a protective coating. Its fire resistance is affected by:

• The concrete’s density

• Aggregates

• Moisture content

Wood encasement may or may not maintain reasonable structural integrity during fire exposure. Important variables include:

• Wood-member size

• Wood moisture content

• The presence of fire retardant intended to delay ignition and reduce combustion

Part 4 – Heating, Ventilation, Air Conditioning, and Air Handling Systems

Heating, Ventilation, and Air Conditioning Overview

Residential and commercial heating, ventilation, and air-conditioning systems may provide heat, cooling, or both. Depending on the settings and the ambient-air temperature, the same system may deliver either heated or cooled air.

When examining a fire scene, investigators often need to analyze:

• The types of systems present

• Their components

• How they perform

• Whether they played any role in the fire

Heating Systems

Heating systems produce heat in several ways. Investigators often need to analyze whether a heating system:

• Served as an ignition source

• Contributed to the fire in another way

• Was not involved

Components of Heating Systems

Fuels

Heating equipment may use electricity or a combustible fuel. Natural gas is typically piped into a building. Propane is normally delivered to a tank that can be found onsite. Other possible fuels include:

• Fuel oil

• Wood

• Coal

• Other unique types of fuels

All-electric heating systems, including electric baseboard systems, do not rely on a combustible fuel supply. Building gas systems are discussed further later in the chapter.

Devices

Furnaces may be powered by electricity, gas, oil, or solid fuel. Most fuel-burning furnaces other than electric units use a vent or chimney to discharge combustion products. Furnaces may be:

• Central air furnaces

• Furnaces mounted in the structure’s floor

• Furnaces mounted on the wall

Boilers are normally made of cast iron or steel. Boilers run on either hot water or steam and may be powered by propane, natural gas, fuel oil, or solid fuel. Safety devices monitor conditions such as pressure, low water level, and temperature to help prevent a fire or explosion.

Radiant or convective heaters may use flat panels installed in walls, floors, or ceilings. Heat may come from electric elements or from water circulated through pipes.

Stoves may burn wood, wood pellets, or coal. Their fire chamber is contained within a metal or soapstone enclosure, and combustion products are discharged through a vent or chimney.

Fireplaces are commonly constructed of masonry and may burn solid fuel or gas. They normally provide no control over the amount of incoming air. Factory-built fireplaces are metal units that use liners, refractories, and insulation to protect the surrounding wooden enclosure. They may also include a gas igniter, an air-circulation mechanism, or both.

Electric heating units include:

• Baseboard heaters

• Permanent wall- or floor-mounted room heaters

• Central forced-air systems

Chimneys and Vents

Chimneys and vents carry exhaust from heat-producing devices to the outside. Masonry chimneys and unlisted metal smokestacks are not tested prior to installation.

Factory-built chimneys, by contrast, are complete assemblies tested to established safety standards. Factory-built vents have specific types and designations corresponding to the furnaces or other appliances for which they are intended.

Controls and Safety Devices

Controls and safety devices are required in heating systems to enhance safety by preventing improper or unexpected operation. They include:

• Pressure switches, which may monitor draft or air flow, fuel pressure on oil and gas systems, or water or steam pressure

• The high-temperature limit, a heat sensing switch that remains closed during normal operation and opens when a preset temperature is sensed

• Door switches, which confirm that required doors are in place

• Flame sensors, which verify that a flame is present when fuel is being supplied and help prevent an accumulation of unburned fuel gas

• Controls that start or stop a burner in response to changes in demand

• Flame rollout detectors, which shut down a gas-fired heater if flame rolls out of the combustion chamber, potentially because of improper draft or soot buildup

• Control thermostats, which regulate operating temperature; for example, a thermostat may activate a circulating fan after a heat exchanger reaches a set temperature

Installation, Use, and Maintenance

Numerous codes and standards address the installation of heating devices and their fuel supplies. Manufacturer instructions may also be necessary when evaluating a specific installation.

Important installation considerations include:

• Appliance placement

• Venting and associated controls

• Fuel supply

• Air required for combustion and cooling

Depending on the circumstances, the investigator may need to determine the installation history and details of a heat-producing appliance. Its use and maintenance must also be evaluated against the manufacturer’s instructions for safe operation.

Heating Systems as Potential Causes of Fire

The presence of a heating device in the area of origin does not by itself establish that the device ignited the fire. A proposed heating-device cause must be tested against other applicable hypotheses as required by NFPA 921.

Testing may show that a device:

• Could not produce enough heat to ignite nearby combustibles

• Was not functioning at the relevant time

• Can be eliminated from consideration for another reason

A qualified specialist may be needed to examine a heating device and identify the particular failure mode that could have contributed to a fire.

When a heating device is located at the origin, consider whether:

1. The appliance was operating at a normal temperature but was against or near fuel within a distance that allowed sufficient heat to cause the device to ignite. If so, determine whether the appliance could produce enough heat during normal operation to cause ignition.

2. A fault or failure caused the device to exceed its normal temperature, heating or igniting nearby material.

3. The device was installed or used outside the manufacturer’s intended use or applicable code requirements. Examples include a chimney installed without proper clearances or a stove or furnace supplied with improper fuel.

Under normal circumstances, the investigator should not attempt to disassemble a heating appliance to determine whether an internal failure occurred.

Related guidance appears in:

• Chapter 5, Legal Considerations for Fire Investigators, regarding spoliation

• Chapter 9, Identification, Collection, and Preservation of Physical Evidence, regarding collection of heating devices for examination

• Chapter 14, Cause Determination, regarding potential fire causes involving heating devices

Air Conditioning and Air Handling

Components

When both systems are present, air-conditioning equipment commonly shares components with the heating system. A condenser coil, often outside the building, removes heat from the refrigerant. The refrigerant is then pumped to the circulation system.

In a system combined with a furnace, an evaporator coil uses the cooled refrigerant to cool air leaving the furnace before the air enters the building.

In both furnaces and air conditioners, an air handling unit includes:

• A blower

• An evaporator coil, when present

• A filter or filters

• Other system components

The unit sends air through supply ducts to occupied areas. Return vents draw air from conditioned spaces back to the furnace through a return plenum. The return pathway may instead use a plenum located between a drop ceiling and the upper floor or beneath a raised floor area.

Commercial air-movement systems may be considerably more elaborate. Air handlers may be installed:

• In an equipment room

• On a roof

• On the ground

Ductwork may contain smoke detectors connected to the building fire alarm system and automatic dampers intended to limit the spread of fire and smoke. Codes may require larger systems to shut down when the fire alarm activates.

Some systems use smoke exhaust fans. Others pressurize selected building areas when an alarm activates to keep combustion products from entering refuge and egress areas.

Investigation

HVAC systems can contribute to the spread of fire and smoke. Heat, smoke, and toxic combustion products may enter the system for several reasons and circulate to other parts of the building, including plenum spaces, whether or not the blower was operating when the fire occurred. Ducts can also serve to supply fresh air—that is, an oxygen supply—to a fire, thereby supporting continuing combustion.

When analyzing fire- and smoke-spread scenarios, consider the presence and layout of the HVAC system and ductwork. Determine whether supply or return ductwork:

• Delivered oxygen to the fire

• Spread fire or smoke into other compartments

• Affected occupants

• Influenced the fire patterns found at the scene

Also determine whether components such as dampers were maintained and functional.

In larger buildings, assess whether air pressure against egress doors slowed or prevented occupant escape and whether smoke handling systems adequately protected occupants during evacuation.

Part 5 – Passive Fire Protection Systems

Part of a fire investigator’s work is determining whether a building was provided with passive fire protection and how that protection affected the incident.

Fire resistance-rated wall and floor assemblies create compartmentalized areas intended to control fire spread. These assemblies may take the form of:

• Occupancy separations

• Fire partitions

• Fire walls

They are intended to keep fire, high temperatures, and flue gases within the compartment of origin while assisting firefighting and evacuation.

Determine whether any passive fire protection system failed and, if it did, how and why. Pay particular attention to damage that may have contributed to fire growth. Possible reasons for failure include:

• Improper design

• Inadequate installation

• A change in occupancy and its associated hazards

• Breaches in compartment walls or damage to applied coatings after the system was initially installed

Design and Installation Parameters of the System

When passive fire protection is present, evaluate whether each system and/or component influenced the fire’s evolution. Consider:

• Which codes, guides, standards, and manufacturer instructions were in effect when the building was constructed

• Whether an update may have been required to bring the system up to the most current code

Analyze the protection provided at door openings, including:

• The door’s fire rating

• Door frame

• Door hardware

• Construction surrounding the door

Evaluate windows in protected openings, including:

• Type of glass

• Glass thickness

• Number of panes

• Opening mechanisms and hardware

• Frame

• Construction surrounding the window

Examine ductwork penetrations through fire-rated walls, partitions, floors, and ceilings to determine their effect on the fire’s evolution. If smoke and/or fire dampers were required, determine whether they operated as required.

Examine every penetration for compliance with applicable:

• Codes

• Standards

• Manufacturer instructions

Examination at Fire Scene

During the origin and cause determination phase, examine whether each passive fire protection system:

• Was assembled properly

• Impeded fire and smoke as expected

• Failed prematurely

Table 3-4 — Considerations for Passive Fire Protection Systems

Documentation and Data Collection

Documentation is important both for recording a passive fire protection system and for understanding any role it may have played in the fire. The investigator may review:

• Design plans

• Design specifications

• As-built drawings

• Equivalencies or alternative levels of protection approved by the authority having jurisdiction

• Building and fire permits

• Invoices for work performed on the system

• Measurements

• Diagrams and/or photographs

• Maintenance records

• Inspection records

• Testing records

Document any occupancy changes that occurred after the original installation and determine whether the passive fire protection remained suitable for the current occupancy and fuel load.

The actual installation may differ significantly from the permit plans if an occupant made changes that are not reflected in the permit materials. Consult local code officials when modifications or installations found in the structure do not appear to comply with the code or permitted work.

Code Analysis

Fire investigators need to be familiar with applicable codes and know where to find installation requirements for passive fire protection systems. Relevant sources may include:

• Building codes

• Fire codes

• Property maintenance codes

• NFPA 101, Life Safety Code, if adopted by the authority having jurisdiction

• Localized code amendments

In addition, it is a good idea to consult with the local building official and fire prevention staff.

Permit applications and/or building permits can provide dates indicating which codes may have been in effect when the passive fire protection system was installed. Both the name of the code and its specific edition are important when determining whether the installation met the requirements in force at that time.

Building codes address matters including:

• Type of construction

• Fire resistance-rated construction

• Requirements for glass and glazing, gypsum board and plaster, and other materials

• References to ASTM and Underwriters Laboratories

Design Analysis

Approved construction documents may contain:

• Drawings

• Specifications

• Design calculations

• Floor plans

• Details

• Cross-section elevations

• Schedules for the work

If computer modeling or calculations are used to assess the passive fire protection system’s performance, the evaluation will likely require data concerning the thermal properties of the walls, ceilings, and floors.

Testing and Maintenance

Determine whether periodic testing or examination was required for the passive fire protection system, including fire prevention inspections. The building owner may be required to maintain the system by ensuring that it is not damaged or breached.

Part 6 – Active Fire Protection Systems and Fire Alarm Systems

Active Fire Protection Systems

Active fire protection systems fall into three general categories:

• Fire alarm systems, including detection and notification appliances

• Water-based systems

• Fire suppression systems

These systems can provide analytical information that helps an investigator determine:

• When and where a fire started

• How the fire progressed

• How system activation and operation affected the fire

System performance may contribute to the formation and testing of hypotheses concerning:

• Origin

• Cause

• Fire spread

It is important to understand, document, and preserve fire protection systems. Technical specialists are often needed during documentation and analysis to prevent the destruction of critical data.

Fire Alarm Systems

Survival in a structure fire depends on the time between:

1. The fire starting

2. Occupants realizing that there is a fire

3. Occupants leaving the building

A properly designed, installed, tested, and maintained fire alarm system improves occupants’ chances of survival by detecting fire early and alerting them to take action.

A fire alarm system can provide direct notification to supervising stations, which then contact public safety answering points (PSAPs) and initiate the appropriate response without waiting for an individual to report the emergency. Some alarms are monitored directly by a PSAP. This alert will expedite the emergency response and, in many instances, may help limit property damage and potentially save a life.

System Components

A fire alarm system connects components and devices that produce audible and visual signals after activation, alerting occupants to a fire emergency and prompting a response.

Its three basic component groups are:

• Control unit

• Initiating devices

• Notification appliances

Primary and secondary power supplies, initiating device circuits, notification device circuits, and alarm reporting services are associated with the control unit.

Initiating devices such as smoke and heat detectors may respond automatically to a fire condition and, through the control unit, initiate an alarm. A manual fire alarm box requires a person to operate it.

Notification appliances provide warnings that prompt occupants to react. Examples include:

• Bells

• Horns

• Strobe lights

Alarm reporting services send signals by telephone or radio to supervising stations.

General System Operation

System operation begins when:

• An initiating device automatically detects or reacts to a fire condition, or

• A building occupant discovers the fire and manually initiates the alarm

The initiating device sends a signal to the fire alarm control unit, which activates the notification appliances. Many systems also transmit the alarm information to an onsite or offsite monitoring location, where operators receive the signal and take the appropriate action.

Modern commercial fire alarm systems often interface with building control, building management, and fire protection systems. These interfaces may initiate additional life-safety operations, including:

• Starting fans to pressurize stairways, elevator shafts, and vestibules

• Closing smoke dampers and doors to compartmentalize the building

• Capturing elevators and moving them to predetermined floors

• Unlocking certain doors to permit safe ingress and egress

Some fire alarm technology dates back 50 years or more. Even the oldest and most basic systems can provide information that is useful during an investigation. When an alarm activates, the FACU can provide information as simple as a marked light with a location identifier, such as the floor number, along with the type of device activated.

Modern FACUs can provide even more comprehensive information. Graphic and touch screen units can display the building footprint and/or alphanumeric readouts showing:

• Device type

• Type of alarm signal

• Floor

• Exact location of the device on that floor

Understanding basic system operation will greatly assist a fire investigator in interpreting the information available after a fire.

Key Components of Systems

Fire Alarm Control Unit

Two types of fire alarm control units (FACUs) remain in use:

• Conventional technology

• Addressable technology

Conventional technology has existed for more than 50 years and provides only basic information, performance, and monitoring capabilities. It remains useful in small buildings with few alarm-system devices, where locating the event does not require extensive travel through the building.

Addressable technology uses state-of-the-art computer and circuit technology to provide extensive event information to first responders. It can also:

• Monitor system integrity for conditions that could inhibit the system or its components from operating properly

• Interface with other building fire protection and life-safety systems

Fire alarm control units generate three types of output signals:

1. Alarm signal

   • Alerts occupants to a fire emergency with the expectation that they will leave the building

   • Notifies the supervising station

   • Initiates any other predetermined action necessary for life safety

2. Trouble alarm signal

   • Notifies a responsible party—usually the building engineer or maintenance personnel—that the fire alarm system has an integrity problem

   • Possible problems include:

     • Ground faults

     • Breaks in system wiring

     • Power or component failure

     • Communications problems

     • Device removal

3. Supervisory alarm signal

   • Notifies a responsible party—usually the building engineer or maintenance personnel—that the system’s normal ready status has changed

   • Possible status changes include:

     • Closure of electronically supervised fire suppression system control valves

     • High or low air pressure in dry-pipe sprinkler systems

     • Fire pump failures or abnormalities

     • Air or water temperatures outside the appropriate range

     • Low water level in a water tank

     • Water pressure that is too low or too high

     • Operation of associated fire protection equipment during nonfire conditions, such as starting a fire pump to exercise the pump and its associated components

Trouble and supervisory signals may be sent to a supervising station, which then notifies the building’s responsible party. If a predetermined period passes without communication with that person, the supervising station may notify the fire department.

To clear any of the three signals—and, in most cases, reset the FACU—the condition responsible for the signal must be investigated and resolved.

Power Supplies

The power supply is integral to the fire alarm unit and provides electrical power to the unit, system components, and system devices. Two power sources are required:

• Primary power: Usually supplied by the local commercial power utility

• Secondary or backup power: Usually supplied by rechargeable batteries

In some factory or industrial installations, natural gas, diesel fuel, or steam engine-driven generators serve as primary and secondary power sources.

The design capacity of both sources accounts for:

• Current draw from all system circuits, components, and devices

• Additional capacity as a safety margin

Additional capacity is especially important for the secondary source because code requirements establish the minimum time the system must remain operational on secondary power without failure.

The electrical circuit supplying power to the fire alarm control unit is not permitted to power other appliances.

Initiating Devices

Initiating devices interact with the fire alarm control unit through automatic or manual operation to activate the system.

Automatic operation. Initiating devices that respond to changes in the monitored environment include:

• Fire detectors

• Water flow detectors

• Component monitoring devices

Fire detection devices may provide spot, line, or video coverage. Spot-type units place one sensing element—such as a smoke or heat detector—at a fixed location, usually on a ceiling, to protect a certain square footage area as determined by the manufacturer; some are installed in HVAC equipment or ductwork. Line-type devices follow a continuous route through the hazard by using electrically or thermally sensitive cable or a light beam. Video systems instead use high-definition cameras to detect pixel changes caused by smoke and flame generation.

Water flow and monitoring initiating devices are addressed later in the chapter.

Manual operation. A person can signal the fire alarm system by operating either of the following:

• Manual fire alarm boxes

• Electronic valve supervisory devices

Manual fire alarm boxes are also called manual pull stations. Although their basic operation is similar, differences include the type of signal generated and the actions required to operate the box. Coded and noncoded signaling are discussed later in the chapter.

• A single action box requires one action.

• A double action box requires two actions.

Operating an electronic valve supervisory device—also called a tamper switch—requires turning a wheel, handle, or other lever. Once it moves a specified distance, the device initiates a supervisory alarm.

Part 7 – Smoke, Heat, and Specialized Detection Devices

Smoke Detection

Smoke detectors initiate an alarm by detecting smoke particles produced by a fire rather than by detecting heat. Sufficient smoke must develop to trigger the detector before other detection systems operate, which makes smoke detectors life-safety devices.

Smoke detection uses two operating principles:

• A radioactive element

• Light that is obscured or scattered

Both types are reliable, although one may respond faster than the other depending on the fire and surrounding conditions. Detector selection should account for:

• Environment

• Building construction and design

• Building contents and their burning characteristics

• Room shape

• Ceiling height

• Airflow and ventilation

These factors determine which operating principle and device are best suited to the location.

Smoke Detectors Versus Smoke Alarms

People often regard smoke detectors and smoke alarms as the same device, but they are used differently. A smoke detector is installed mainly in commercial occupancies as a component of a fire alarm system. It relies on the fire alarm control unit for power and initiates an alarm through that unit. A smoke alarm is used mainly in residential occupancies and combines the sensing and sounding functions. It normally receives power from a house circuit, has battery backup, and is interconnected with the other installed smoke alarms.

With multi-station smoke alarms, activation of one device causes all connected alarms to sound. A single-station smoke alarm is a stand-alone unit powered only by a battery.

A smoke alarm may also be incorporated into:

• A combination system in which security, medical, gas-detection, and fire signals report to a monitoring panel that notifies a supervising station

• A complete fire alarm system that operates similarly to a commercial system

Tip

Smoke detectors and alarms remain some of the best early-warning life-safety devices, but they do have limitations. For example, spot-type smoke detectors cover only a certain area reliably and, over time, may become overly sensitive or undersensitive compared to their original design. This could be problematic because the level of sensitivity must be appropriate for the environment in which detectors are installed. If the devices are too sensitive, false alarms will happen; if they are not sensitive enough, detectors may not react rapidly enough to provide an appropriate warning.

Ionization Smoke Detectors

Ionization detection uses a small quantity of radioactive material and two electrically charged plates.

1. The radioactive material charges air particles inside the sensing chamber, making the air conductive.

2. Ionized particles attach to the oppositely charged plate, producing a small but measurable current.

3. When smoke enters, its particles attach to the charged air particles and reduce current flow between the plates.

4. The detector activates when current falls to a predetermined level.

Ionization detectors are normally spot-type devices. They are best suited to fires that quickly produce more flame than smoke, such as a burning piece of paper, and are the most common smoke alarms in residential occupancies.

Figure 3-13 — Principle of operation for an ionization smoke detector.

The diagram compares normal operation with smoke in the chamber. Under normal conditions, the galvanometer measures normal current flow between the charged plates. Smoke reduces that current, which the galvanometer shows as reduced current flow. The figure identifies the radioactive material that ionizes the air.

Photoelectric Detectors

A photoelectric detector uses a light source and a photoelectric-cell receiver positioned at an angle to one another inside the sensing chamber.

• With no smoke present, the light beam does not strike the receiver.

• When smoke or other particulate matter enters, the light source refracts off the particles into the receiver, initiating an alarm.

Photoelectric smoke detectors are usually spot-type devices. They respond more quickly to smoldering fires because those fires tend to produce larger smoke particles. Rapidly developing flaming fires produce smaller particles that do not refract as much light, which can delay alarm initiation. Photoelectric detectors are commonly used as fire alarm system components.

Figure 3-14 — Principle of operation for a light-scattering photoelectric smoke detector.

The no-smoke diagram shows the light beam passing through the chamber without reaching the photocell. In the smoke-filled diagram, smoke entering through the chamber openings scatters and reflects part of the beam into the photocell.

Air Sampling (Aspirating) Detectors

Air sampling smoke detection is one of the more sophisticated types of smoke detection in use. Also called aspirating detection, it uses a network of pipes with sampling holes to pull air continuously from the protected area into an analyzer.

By continuously sampling the air, the system can identify otherwise undetectable by-products produced as material breaks down during the precombustion stage—well before an occupant would notice them. The detection chamber may be inside or outside the protected room or area. Because it can receive air from several tubes, one system can cover more area than spot-type detectors.

Air entering the chamber is commonly analyzed by highly sensitive light refractions. Other methods, including chemical gas analysis, may also be used.

This approach is generally known as very early warning smoke detection because it can detect products of combustion well before typical spot-type smoke detectors. The system can provide progressively serious alarm levels:

1. A first-level alarm indicates an abnormal air characteristic.

2. A second-level alarm indicates that a fire may be present.

3. A final-level alarm indicates a fire in progress.

Air sampling systems may operate independently or as part of a broader fire protection strategy. They are commonly installed in high-value operations where concealed detection is important, maintenance access is limited, or critical processes cannot be disrupted. Examples include:

• Historic buildings

• Telecommunications facilities

• Computer/server operations centers

• Production facilities

• Electrical substations

• Semiconductor manufacturing facility clean rooms

• Warehouses

• Aircraft hangars

• Textile mills

• Atriums

Figure 3-15 — Air sampling smoke detection system.

• A: The system control panel/analyzer.

• B: The ends of sampling tubes, which contain small holes that capture air samples for analysis by the control panel/analyzer.

Projected Beam Detectors

These detectors send light across a protected area to a receiver. Smoke or another obscuration of the light source reduces the intensity reaching the receiver and initiates an alarm.

The beam can extend more than 300 feet (91 m) between the transmitter and receiver while covering a width of approximately 60 feet (18 m).

Projected beam detection is suited to places where spot detection would be impractical or ineffective, particularly areas with very high ceilings, including:

• Atriums

• Concert halls

• Warehouses

• Gymnasiums

• Factories

Figure 3-16 — Principle of operation for a light-obscuration photoelectric smoke detector.

The figure shows an unobstructed beam traveling directly from a light source to a light sensor. When smoke enters the beam path, it scatters and obscures the light, reducing the amount that reaches the sensor.

Duct Smoke Detectors

Duct smoke detectors are installed on many commercial air distribution systems to sample air moving through building ductwork. Depending on the code authority having jurisdiction, activation may produce either:

• An alarm signal

• A supervisory signal

When particulate matter is detected in the airflow, the detector shuts down the associated air distribution unit. Without that unit moving air, smoke and toxic gases cannot travel as easily from one part of the building or one floor to another. This approach prevents or inhibits occupant exposure to potentially life-threatening products of combustion.

These detectors are not required on every air distribution system. Installation depends on airflow capacity, measured by the cubic feet per minute delivered by the unit. A detector must operate throughout the full range of air velocities, temperatures, and humidity expected at the installation.

Duct smoke detectors may be:

• Mounted inside the ductwork

• Mounted outside the unit and connected to sampling tubes inside the ductwork

• Ganged together in front of an air duct intake

Many duct detectors are mounted above ceilings or too high to see, making it difficult to investigate their activation; inspect, test, maintain, and service the detectors; and, in some instances, reset the detector.

NFPA 72, National Fire Alarm and Signaling Code, requires a remote indicator when a detector is:

• More than 10 feet (3 m) above the finished floor, or

• In a location that is not readily visible

The indicator must identify the associated HVAC unit. Remote lamps or annunciation devices should be installed on a wall, ceiling, or as close to the detector as possible so first responders or maintenance personnel can locate it quickly.

Heat Detection

Heat detectors are among the oldest and most reliable fire-detection devices. They are useful in challenging environments where smoke detection may not be suitable, including:

• Dusty locations with fluctuating temperatures

• Locations where a fire will produce high heat output

• Locations where detection speed is not the primary concern

They respond to either:

• A predetermined fixed temperature

• A specified rate of temperature change

Three operating principles—the fuel, hazard, and associated rate of fire development—determine whether fixed-temperature, rate-of-rise, or rate-compensation detection should be installed.

• Fixed-temperature detectors operate when the sensing element reaches its rated temperature.

• Rate-of-rise detectors measure temperature change over a fixed period.

• Rate-compensation detectors react to predetermined air temperatures.

Most heat detectors are spot-type devices, although line-type detectors may be more appropriate for some hazards and environments.

Because they rely on fire-generated heat rather than smoke, heat detectors usually respond more slowly than other fire detectors, particularly smoke detectors. They are not life-safety devices and should not replace smoke detectors unless the reviewing authority having jurisdiction approves the substitution.

When heat detection replaces smoke detection, the reason is normally an ambient condition or environment that could cause a smoke detector to malfunction or activate inappropriately.

Radiant Energy–Sensing Fire Detection

Radiant energy–sensing detectors can operate without waiting for smoke or heat plumes. They monitor selected portions of the visible or invisible light spectrum produced by:

• Flames

• Sparks

• Embers

They are well suited to manufacturing and industrial environments where non-fire-related sources of radiant energy are more common. These detectors commonly protect high-risk, high-value facilities where rapid and sensitive detection and remote detection of small fires are critical.

Detector selection requires careful evaluation of:

• The environment

• Fuel type

• Nonfire radiant energy sources that could cause false alarms

• Conditions that could prevent an alarm

Examples of protected locations include:

• Petroleum processing, storage, and loading facilities

• Print shops

• Paint application facilities

• Airplane hangars

• Woodworking facilities

• Textile mills

• Museums

• Ordnance facilities

• Computer data rooms

Flame Detectors

These detectors respond to a selected part of the light spectrum. Operating ranges include:

• Ultraviolet (UV)

• Infrared (IR)

• Ultraviolet/infrared (UV/IR)

• Multiple-wavelength infrared

Dual UV/IR and multiple-wavelength IR detectors use two sensing elements. Both elements must detect the required light spectrum band before an alarm is triggered. Dual sensing improves recognition of an actual fire and reduces activation from nonfire radiant energy sources.

Radiant energy detectors require an unobstructed view of the protected area and must be close enough to receive sufficient energy to initiate an alarm. When the view or distance creates a problem, additional detectors are required to ensure complete coverage of the hazard area. Flame detectors may be designed and adjusted for different flames, flame intensities, or both.

Spark/Ember Detectors

These detectors are not fuel specific. They look for sparks or embers produced during manufacturing and are normally installed on, in, or near conveyor belts or duct lines where solid materials move through a process. In these locations, the possibility of a spark or ember starting a fire is high.

These detectors:

• Operate in the infrared range

• Are normally installed in closed, dark environments

• Activate some type of suppression after detecting a spark or ember

Suppression commonly consists of a water spray nozzle connected to an ultra-high-speed water spray system.

Other Types of Detectors

Gas-sensing fire detection may be designed to detect a specific gas, any type of toxic gas, or various gases and vapors associated with processes utilizing hydrocarbons.

Available sensing technologies include:

• Infrared

• Semiconductor

• Electrochemical

• Catalytic beads

These systems protect commercial, industrial, and residential occupancies, including:

• High-hazard environments

• Off-shore oil and gas rigs

• Oil, gas, and petrochemical facilities

• Gas turbines

• HVAC air intakes

• Oil and gas wells

• Enclosed air handling ductwork

A detector may be manufactured exclusively for gas sensing or as a combined gas/smoke detector. Inspection, testing, maintenance, and calibration are necessary to ensure proper operation.