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TAB is Often the Key to HVAC Diagnostics

All HVAC equipment and systems are designed to move heat at a specific flow and static pressure. When designers select fans and coils, they must take into account exact airflow in CFM, FPM, room air changes, temperature, humidity, water flow, and load within spaces. Because flow and pressure are at the heart of every system, problems can be diagnosed using TAB (testing, adjusting and balancing).

A commissioning agent for a large production facility called MSC in on several systems experiencing erratic flow, which was causing static pressure swings and instability within the space. Through hood readings and multiple duct traverses, we were able to determine that many of the systems hot-wire anemometers were in sporadic, turbulent airflow, causing CFM fluctuations, thus driving static pressure swings. As a result, the lab pressure cascade failed commissioning.


Ideally, the solution would to be to relocate the hot-wire flow probe, but due to the stainless-steel ductwork configuration and air valve locations, there were limited options for relocation. MSC’s HVAC technician performed additional TAB traversing to find consistent, repeatable flow by moving the probes several inches downstream and slightly off center of the air valve. We then recalibrated the airflow throughout the curve at 10% increments with a NIST-traceable flow hood. Through these steps, costly ductwork reconfiguration was avoided. MSC assisted the balancer and BAS contractor with completing the lab airflow and pressure cascade checkout, to the commissioning agent’s satisfaction.