A reader recently asked me where in NFPA 72 does it say that heat detectors have to be a minimum of 36 inches away from an air diffuser. He could not find the standard reference and asked if I could point him in the right direction.
Well…. I think he had a point, since there is no requirement in NFPA 72 that says heat detectors have to be 36 inches away from air diffusers. Section 2-3.5.1 and section A-2-3.5.1 of NFPA 72-1999 says spacing for detectors from air diffusers (supply and return) must be 3 feet. This section (NFPA 72 2-3) is referring specifically to smoke detectors, and section NFPA 72 2-2 refers specifically to heat detectors and there is no similar language in section 2-2 concerning minimum distance from air diffusers for heat detectors.
Therefore, one can conclude that heat detectors do not have to comply with the 3 foot spacing from air diffusers like smoke detectors. But that is not how all of the AHJs interpret this. The actual spacing (3 feet) for smoke detectors is found in the Annex section of the standard which is explanatory information and not part of the enforceable standard. But AHJs are free to use this information in the Annex section in determining compliance with the standard. Therefore, AHJs can interpret this section how they want, and many of the AHJs interpret that you need to maintain 3 feet from air diffusers for both smoke and heat detectors.
If you get cited, you can fight the finding by pointing out the reference in the Annex section is only for smoke detectors, but ultimately the AHJs get to interpret this the way they want, and you may end up losing anyway.