Typically, designers place heat detection in spaces where the environment is not acceptable for smoke detection. Additionally, heat detectors are used in fire-rated enclosures where the enclosure will contain the fire for a sufficient period to allow for emergency responders to arrive while the fire is still relatively small.
For additional coverage in a system, know that heat detectors are relatively slow and react only to established fires. The National Fire Alarm Code advises that detectors having fixed-temperature or rate-compensated elements must be selected so the temperature rating is at least 20°F above the maximum expected ambient temperature at the ceiling. The code recommends that detectors be selected to minimize this temperature difference to reduce response time. However, a heat detector with a temperature rating above the highest normally expected ambient should be used, avoiding the detector’s premature operation to nonfire conditions. Typically, heat detector temperature classification is marked using either a color code or printed labels.
When designing such, the performance objective statement should include the purpose of detector placement and the intended response of the fire alarm control unit to detector activation. It can include a narrative of the required response time, a description or matrix of the operations sequence, a tabular list or some other programming requirements.
Heat detectors are generally placed according to assigned spacing—the maximum allowable distance between heat detectors—from Underwriters Laboratories. The linear space rating also is a measure of the heat detector response time (sensitivity) to a standard test fire when tested at the same distance, i.e., the higher the detector’s listed spacing, the faster the response time.
NFPA 72-2007 provides requirements for proper spacing for different ceiling applications. Some considerations that affect spot-type heat detector spacing include the following:
- Avoid “dead air” spaces. Detectors should be mounted 4 inches from the sidewall or on the sidewalls between 4 inches and 12 inches from the ceiling.
- In the case of solid joist construction, detectors must be mounted at the bottom of the joists.
- In the case of beam construction where beams are less than 12 inches in depth and less than 8 feet on center, detectors are permitted to be installed on the bottom of beams.
The code advises that, when laying out such installations, contractors should work in terms of rectangles, as building areas are generally rectangular. However, the heat pattern spreads in all directions. Thus, the detector coverage is really a circle whose radius is the linear listed spacing multiplied by 0.7.
NFPA 72-2007 section 188.8.131.52.1 provides the following requirements for heat detector spacing:
“(1) The distance between detectors shall not exceed their listed spacing, and there shall be detectors within a distance of one-half the listed spacing, measured at right angles from all walls or partitions extending upward to within the top 15 percent of the ceiling height.
“(2) All points on the ceiling shall have a detector within a distance equal to 0.7 times the listed spacing (0.7S).”
Spacing must be reduced for irregularly shaped areas and for ceilings that have solid joists or beams or are sloped. The reduced spacing required when installed on high ceilings is usually an overlooked requirement. Section 184.108.40.206.1 of NFPA 72-2007 states, “On ceilings 3 m to 9.1 m (10 ft to 30 ft) high, heat detector linear spacing shall be reduced in accordance with Table 220.127.116.11.1 prior to any additional reductions for beams, joists, or slope, where applicable.”
Both section 18.104.22.168.1 and Table 22.214.171.124.1 are constructed to ensure detectors perform on higher ceilings (up to 30 feet high) essentially equivalently to the performance of those on a 10-foot ceiling.
A contractor who understands the code and heat detector application and limitations will be better able to meet their customer’s fire protection needs.
MOORE, a licensed fire protection engineer, frequent speaker and an expert in the life safety field, is a co-editor of the current National Fire Alarm Code Handbook. Moore is a principal with Hughes Associates Inc. at the Warwick, R.I., office.