In general, duct hangers are limited to a 50 foot(15m) requirement from the fans as the vibration becomes minimal as the air flow smooths out. Deflections are limited to 0.75” (19mm) as these are light capacity supports, and we are isolating sheet metal resonance rather than the fan’s primary disturbing frequency. We continue to be concerned if the duct work was designed for very high velocity in which case the hangers are continued to the point where this velocity drops off.
All air ducts with a cross section of 2ft2(0.19m2) or larger shall be isolated from the building structure by specification H hangers or B floor supports with a minimum deflection of 0.75”(19mm). Isolators shall continue for 50’(15m) from the equipment. If air velocity exceeds 1000 fpm(5.3mps), hangers or supports shall continue for an additional 50’(15m) or as shown on the drawings.