Temporary Fire Alarm Heat Detection

Renovation and construction activities have numerous potential fire ignition sources. Temporary heat detection is frequently utilized to cover an area under renovation, especially in partially occupied buildings when fire sprinklers are disabled. The building code in IBC Chapter 33 and the referenced NFPA 241 and NFPA 72-2022 Section 17.7.2 provide life safety requirements during these operations.

All fire alarm heat detection has UL listed coverage to determine spacing between detectors and distances from walls. When a ceiling surface has solid beams, the prescriptive spacing is reduced based on the beam depth and distance between beams. It is also reduced for high ceilings.

Per NFPA 72 section 17.6.3 the location and spacing of heat detectors is determined the ceiling height, beam depth, and distance between beams.? ??Higher ceilings require a longer time and more heat output to activate the detector.? Deep beams create barriers to the horizontal flow of the ceiling jet and form pocket a pocket which must be filled before spilling into the next bay. This fill-and-spill progression of the ceiling jet is slower than the velocity attained on a smooth, flat ceiling.? Reduction of the heat detector’s UL listed coverage area is required based on the following factors.

Ceiling Height

For ceilings over 10ft and upto 30ft, height detector spacing for spot-type heat detectors detector spacing must be reduced ?per table 6.3.5.1 (NFPA 72 ?17.6.3.5.1).? ?Linear heat detection and pneumatic rate-of-rise tubing heat detectors are exempt from this requirement and do not require a spacing derating factor (NFPA 72? 17.6.3.5.2).??

Solid Beam or Obstruction Depth

A ceiling shall be treated as a smooth ceiling if the beams project no more than 4 in. below the ceiling. (NFPA 72 ?17.6.3.3.1.1). ??This shallow beam depth will have an insufficient effect on the overall flow of the ceiling jet to affect system response.

• ?Where the beams project more than 4 in. below the ceiling, the spacing of spot-type heat detectors at right angles to the direction of beam travel shall be not more than two-thirds of the listed spacing. (NFPA 72 ?17.6.3.3.1.2).
• Where the beams project more than 18 in. below the ceiling and are more than 8 ft. in the center, each bay formed by the beams shall be treated as a separate area. (NFPA 72 ?17.6.3.3.1.3).

The location of the heat detector is based on beam depth:

• For beams less than 12 in. in depth and less than 8 ft on center, detectors are installed on the bottom of beams. (NFPA 72? 17.6.3.3.1.2).
• For beams more than 12 in. deep, they will likely project downward far enough to sufficiently interrupt and divert the ceiling jet.? Also beams spaced more than 8 ft apart, the volume enclosed by the bay formed by the beams will be of a sufficient size that a large quantity of ceiling jet gases must accumulate in that bay before they will spill into the adjacent bay.? Detectors must be placed on the ceiling surface/ hard deck between the beams (NFPA 72? 17.6.3.3.1.2).
• Sloping ceilings (peaked and shed) require a detector within 3 ft of the peak.? For ceiling slopes of 30 degrees or more, the acceleration of the ceiling jet due to buoyancy becomes more of a factor and the spacing must be adjusted (NFPA 72? 17.6.3.4).

Circuit Installation

Temporary fire alarm cable must be installed under NEC (NFPA 70) article 725 and article 800. While fire alarm armored cable (Type MC), power-limited circuits installed in electrical metallic tubing (EMT) or rigid metal conduit (RMC) are all acceptable, these are uncommon for temporary heat detection.? Most installations utilize red-jacketed (free air) cabling fasten directly to the building structure no more than every 3 ft.?? Here are the four types, in increasing fire resistance rating:

• Type FPL power-limited fire alarm cable for general purpose fire alarm use. This listing excludes installation in risers, ducts, plenums and other space used for environmental air unless the cable is installed in conduit.
• Type FPLR power-limited fire alarm riser cable for use in a vertical run in a shaft or from floor to floor. All FPLR cables are listed as having fire-resistant characteristics capable of preventing fire from traveling floor to floor.
• Type FPLP power-limited fire alarm plenum cable in ducts, plenums and other space used for environmental air.
• Type NYC-FPLP power-limited fire alarm plenum cable in ducts, plenums and other space used for environmental air.?? Meets New York City’s Local Law 39 for higher operating temperature.

Most temporary heat detection in installed using the NFPA 72 Class B method. ?A good practice is to provide periodic loops of cable or slack to permit easier field adjustment during construction.? Below are two heat detection solutions which meet code and are common for temporary protection due to renovations.? They are both mechanical fixed-temperature/ non-restorable conventional heat detection which report to an addressable module as a zone.? They are supervised with an end-of-line resistor and can b field modified without programming.? ??Addressable heat detectors have limited environmental temperature ranges.? Conventional heat detection has a wider range, better suited for unconditioned construction sites.

Solution #1 – Spot-type Heat Detection

Spot-type heat detectors (SHD) are installed on octagon electrical boxes located either on the bottom of the beam or in the beam pocket, depending on the ceiling construction.? Fire alarm red-jacketed cable is used, Siemens supplies the addressable monitoring modules and spot-type heat detectors.

Solution #2 – Linear Heat Detection

Linear Heat Detection (LHD) is a line-type form of fixed temperature heat detection cable which can detect a fire anywhere along its entire length and is available in multiple temperatures. It is also weather/ water-proof.? Fire alarm red-jacketed cable is used for the Leader wire, Siemens supplies the addressable monitoring modules, linear heat detectors, mounting hardware, and termination boxes.

NFPA 72 Acceptance and Annual Testing

If the temporary heat detection is operational for more than a year or if it left in place post construction for enhanced above ceiling fire detection, it will require a NFPA 72 annual inspection.? Since either the spot or line solution uses mechanical fixed-temperature/ non-restorable conventional technology, there are special inspection exceptions which may be applied.? Physical testing by applying heat is not required.? Initial acceptance and annual NFPA 72 testing requires shorting the circuit, and recording the loop resistance.? After 15 years from initial installation, replace all devices or have 2 detectors per 100 laboratory tested.? If a failure occurs on any of the detectors removed, remove and test additional detectors to determine either a general problem involving faulty detectors or a localized problem involving 1 or 2 defective detectors.?

When detection is voluntarily elected to remain, it is recommended the end of the circuit be routed back to accessible level and terminate with a local key switch. ?A key-operated switch which creates a ‘dead-short’ across the circuit to simulate a heat activation is acceptable for this type of application.? Like sprinkler heads, these types of detectors are very reliable and require very little service.