I encountered the installation in Photo A when performing my first punch list as a junior engineer. The mechanical contractor on the project opted to substitute the specified stacked air-handling units (AHUs) with those of a different manufacturer. The substituted AHUs were substantially taller than the specified units, and my team and I had indicated in the submittal-review process that the contractor would be responsible for any conflicts caused by larger-than-specified units.
As shown in Photo A, the contractor transitioned the mixed-air ductwork around existing electrical conduit, rather than relocate the conduit. The duct cross-sectional area was reduced by approximately 30 percent, raising the cooling-coil velocity well over the manufacturer's recommended 500 fpm.
It took some time to convince the contractor that the negative impact to the AHUs' performance was caused by the restricted duct size. However, my team and I stood our ground, and eventually the installation was corrected. Any money saved by the contractor's substitution surely was exhausted when an electrical contractor was hired to reroute the two existing 2-in. conduits and associated conductors.
Mike Trzcinski, PE, LEED AP
Hesnor Engineering Co.
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