can heat detectors be installed on the bottom of open web metal beams? They are approx. 24" deep, the cieling material is some sort of corragated metal. The solid portions of the beams (top and bottom) approx. 4" or less.
The spacing of the beams is 5'.
I'm a journeyman electrician, now working for a company that installs low-voltage security/fire systems.
Thanks
Based on your information, the short answer is "NO".
Mounting on the bottom of the beam is for
solid beam construction and the beam can't be deeper than 12" with spacing 8' on center or less (NFPA 72-2002 5.6.3.1.2). Your 24" depth is too great, even if you had a solid beam.
You can treat the ceiling as "smooth" if the solid portion of the top plate is 4" or less in depth. In this case, you must mount at the ceiling. The reason for this is that the hot air needs to accumulate around the detector for it to operate as listed. If the air rises past the detector and has to build "down" to the 24" elevation you may not get timely activation of the fire alarm system.
While we're at it, heat detector spacing has to be de-rated for mounting heights above 10' (NFPA 72-2002 Table 5.6.5.5.1). It sounds like you may be in a warehouse or other space with a high ceiling.
Also, don't assume that the detector has a 30' listing. Many are listed for 50'. They can also be dual listed, say 25' for FM and 50' for UL. This information will be in the package on the detector's installation sheet.
