I was recently at a hospital where I noticed the smoke detector installation (at left) inside a mechanical room. It made me stop and exam the installation as my first thought was the smoke detector was too close to the air diffuser. As the pictures indicate, the detector was only about 4 inches from the actual air diffuser, and NFPA 72 (1999 edition) A-2-3.5.1 requires detectors to be spaced a minimum of 36 inches from air supply or return diffusers. Now, to be sure, this reference is found in the Annex section which is not part of the enforceable standard, but many (if not all) AHJs use the Annex section to help them determine compliance with the standard. The 36 inch rule is enforced by many of the AHJs which inspect hospitals.
Let’s review what the pictures indicate: This is new construction for this situation and the wall to the left of the air diffuser is a 2-hour fire rated vertical shaft which also doubles as the smoke compartment barrier in this area. In the pictures you will notice a combination fire/smoke damper in the short duct which is connected to the return air system. It is understandable that a fire damper would be required in a 2-hour vertical shaft (or for that matter, any vertical shaft where the duct penetrates the vertical barrier), but a smoke damper? Well, yes, it is required, as 220.127.116.11, exception #2 in the 2000 edition of the LSC says smoke dampers are not required in smoke barriers with fully ducted HVAC systems. The pictures indicate that the return air is not fully ducted, but a case could be made that is is “fully ducted” since there is no suspended ceiling in this mechanical room, and the return air diffuser location is no different than if it was ducted to a ceiling mounted diffuser. Besides, in this situation, the IBC building code is used and they do not have an exception for smoke dampers in smoke barriers.
But back to my first thought: Is the spot-type smoke detector too close to the return air diffuser? According to NFPA 72 (1999 edition) A-2-3.5.1 it is. But one needs to take into consideration section 2-10.4.2.2. of the same standard. This section says if smoke detectors are required in the return air system by other NFPA standards then a smoke detector listed for the air velocity present should be located where the air leaves each smoke compartment or in the duct system before the air enters the return air system. Well, smoke detectors are required to activate the smoke damper, according to NFPA 105 which says smoke detectors that activate smoke dampers need to be installed according to NFPA 72.
Section 2-10.4.2.2 of NFPA 72 (1999 edition) does state that detectors in the return air system are not required if the entire smoke compartment is protected with smoke detectors. In this situation, that is not the case, so it is not an option. Going back to the Annex section A-2-10.4.2.2 of NFPA 72 (1999 edition) we find that the smoke detector needs to be listed for the air velocity present and installed up to 12 inches in front of or behind the opening of the return air system.
So, in summary, I see two issues in this installation: 1). Is the smoke detector listed for the air velocity present? and 2). Is the smoke detector mounted up to 12 inches in front of or behind the opening for the return air system?
For #1, I cannot answer this question, but I did inform the organization that they need to confirm the smoke detector is listed for the air velocity in which it is used. I was later informed that the detector was a Siemens ILI-1 which is rated for 0 – 300 ft./min. The airflow around this detector would have to be measured to confirm it is within the listed operating range of the detector.
For #2, the detector is mounted within 12 inches of the air diffuser, so it is well within the limitations of the specifications.
Based on the above, the spot type smoke detector in the picture appears to be within the requirements of NFPA 72.[Thanks to Greg Waldman of Stanford Healthcare for assistance with this issue.]