Search

"Industry Standard" An Example Why Requirements Matter


As a commissioning company, GMC Cx is focused on quality, defined as meeting the owner’s expectations. Before designing a space we focus on capturing and recording the Owner's Requirements. Without these requirements and expectations, quality is left undefined. This unknown puts commissioning agents in a tough spot because an undefined quality is very hard to measure, verify and enforce.

Recently, we were called back out to a project site after occupancy to investigate a noise issue in a conference room. We were asked to determine the cause and propose a solution.

The conference room is served by a single zone, split system heat pump fan coil supplying 1800 CFM of conditioned air through 5 supply diffusers at 360 CFM each and 3 return diffusers at 500 - 585 CFM each. The fan coil is located on the second floor behind a full height wall and the air is ducted down through 14” ducts to the room diffusers. Supply diffusers are steel lay-in 4 way throw with a perforated face.

There were no acoustical design issues with the airflow, ducting or unit location. However, for HVAC noise sensitive spaces, such as conference rooms, it is recommended to use low NC (low noise criteria) plaque style diffusers with no perforated face. The steel perforated neck-mounted curved blade deflector ceiling diffusers are designed for longer throws, but not low noise. Curved blade deflectors and perforated faces often generate airflow noise.

We measured significant airflow noise (55 dBA) being generated by the diffusers and possibly the ductwork. High sound pressures in the 500 Hz to 2 kHz range indicate airflow noise. There was no indication of fan, compressor or vibration related noise. After reviewing the building construction drawings, TAB report and visiting the site we had identified a few small impact options to reduce HVAC sound and noise. This included replacing the diffusers to low NC type and verifying straight run and properly stretched and supported ductwork a minimum of 3 duct diameters perpendicular to the diffuser.

Original 4-way throw diffuser with perforated face (Left) compared to the new plaque style diffuser (Right)

After applying the suggested fix the sounds levels were reduced by 13 dBA to a more acceptable 42 dBA while the fan coil was running.

Unfortunately I can not call this a success. Since we were not able to capture the Owner's Requirements before the project began, we were not able to enforce any sound level requirements during design, construction, testing or turnover.

Had we known that the owner expected a quieter conference room than "industry standard" we could have indicated the correct style diffuser during design, saving time, money and resources.

Requirements matter because if you do not define them, you can not enforce them. Without requirements there is no basis for quality. Quality then just becomes someone else's definition of "industry standard".

Many owners would not know what diffuser type to specify. It is a very detailed question. However, with the right guidance, a simple NC or dBA requirement specified as 35 or 40 dBA in sound sensitive spaces such as conference rooms could have been recorded. This specification gives the designers and engineers clear direction; clearing the path to a well defined, quality product.


28 views
  • Facebook - White Circle
  • LinkedIn - White Circle

© 2020 by GMC Commissioning, Inc.