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Heat Detector Spacing for high Ceiling According to NFPA 72


The below lecture base on smooth ceiling.

To determine the spacing of heat detectors, full-scale tests are performed where the heat detectors are arranged in a square pattern installed on (3.1m x 3.1m) centers, this arrangement places the centerline of the fire (2.2m) from the test sprinklers. The basis for this approach takes into account that it is not known where a fire will begin, so the test considers a fire in the center of a square grid. It should be noted that the ceiling height used during these tests is (4.8 meters), with no airflow, and the test sprinklers having an activation temperature of (71.1°C). The (1,200 kW) ethanol/methanol fire is ignited and the greatest heat detector spacing which activates before a sprinkler activates is the listed spacing of the heat detector, for example a heat detector installed on (15.2m x 15.2m) array receives a 15.2m listed spacing if responds to the test fire before the test sprinkler head operates

This standard test is used to assign a heat detector with a relative indication of its thermal response, with a number of variables being considered during this test

·         Fire size.

·         Fire growth rate.

·         Ambient temperature.

·         Ceiling height.

The test typically provides activation within ~2 minutes 10 seconds.

The blue circles represent sprinklers
whereas the orange diamonds represent the heat detectors and red Square is the ignited fire .

What is the reason that detector spacing must be reduced for ceiling higher than 3m?

The Speed of response of a heat detector depends on both the temperature of the ceiling jet gas and the speed of the ceiling jet flow. The higher the gas temperature and flow velocity are, the more rapid the heat detector response. As the fire plume rises, it cools due to fresh air entrainment and volumetric expansion. As the plume cools, its buoyancy is reduced and its upward velocity decrease. These cooling and velocity loss phenomena continue after the plume turns and forms a ceiling jet. The ceiling jet entrains cool air as it moves across the ceiling. As the ceiling jet moves further from the plume centerline, its velocity decrease, as does its temperature.

In case of a room with a high ceiling, the plume is cooler and slower, yielding a ceiling jet that is both cooler and moving at a slower initial velocity. Consequently, if all other variables are held constant, heat detectors installed on high ceiling experience lower temperature at lower velocities than when they are installed on lower ceiling, one way to compensate for the cooler and slower flow of the ceiling jet at high ceiling elevations is to move the detector closer together and accordingly, closer to the fire plume centerline, where velocity and temperature will be higher. To attain a roughly equivalent response to one obtained at normal ceiling height 3m, the detector spacing must be reduced for high ceiling with reference to NFPA 72.  

Here below reduction spacing based on Ceiling height with reference to NFPA 72. 

However, there is a table that allows for reduction of spacing for heat detection, NFPA 72, National Fire Alarm and Signaling Code, does not address spacing consideration for smoke detection based on ceiling heights.

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