Case Study: Fan Stall Diagnostics Resolve Commissioning Delays at New R&D Facility

Initial Call

MSC received a call from a client on a new R&D facility construction project, where fan stall issues across several process labs were preventing proper air balancing and holding up building commissioning. The affected labs were dealing with unstable airflow and erratic static pressure that were disrupting the pressure cascade required for laboratory operations. The AHUs were suspected as the root cause, but nothing had been tested or confirmed. MSC dispatched a diagnostic team to assess the system; after gathering information, walking the site, and running initial tests, the team confirmed fan stall as the root cause.

What Is Fan Stall and Why Does It Happen?

Fan stall is one of those problems that can quietly wreak havoc on an HVAC system. It occurs when the CFM running through a fan is too low for the fan's size, most commonly because an oversized fan was selected for the system. This happens more than you'd think. Designers and contractors sometimes spec a larger fan than necessary as a buffer against calculation errors, not realizing the operational problems it can create down the line.

In a laboratory setting, where precise airflow and pressure control are critical, fan stall can be especially disruptive. It doesn't just cause performance issues; it can hold up commissioning, damage equipment, and create safety concerns if pressure cascades can't be properly maintained.

Recognizing the Symptoms

The symptoms the team found at this facility were hard to miss. Unsteady airflow and fluctuating static pressure were the obvious signs, but there was also excessive vibration, loud hammering, and AHU flex connections visibly pulsing in and out. On top of that, several fans showed signs of light mechanical damage, worn flex components, and bearing vibration issues, all consistent with a fan that had been fighting against stall conditions for some time.

A Unique System Design

This project used an unusual control configuration for the supply fans. Rather than using traditional variable frequency drives, the system relied on variable inlet controls, similar in concept to vortex inlet vanes. The fans ran at full speed continuously, with the inlet controls modulating how much air entered the fan to meet static pressure setpoints. The BAS sent an analog signal to adjust the inlet position as conditions changed.

It's a legitimate control strategy, but as the team would confirm, it wasn't the right fit for these particular fans.

Running the Diagnosis

Before drawing any conclusions, the team did their due diligence. They verified that the fans

were the correct approved units, checked that everything was installed properly, confirmed clean 480V power, and looked for any thermal or vibration red flags. The inlet controls were also inspected to make sure they were correctly calibrated and functioning as designed.

Then they ran each fan through its full operating range on the fan curve, capturing everything on video.

Under normal conditions, a fan blade moving through an airstream deflects air as it spins, generating pressure as the blade's attack angle increases. But when the attack angle gets too steep relative to the CFM actually entering the fan, the airflow separates from the blade surface, and pressure drops sharply. That's fan stall. In this case, the fans were simply too large for the airflow the system required, and the inlet controls couldn't compensate enough to keep them out of the stall zone.

The Solution: Variable Frequency Drives

With the diagnosis confirmed, MSC identified the solution. The most reliable and energy-efficient solution for an oversized fan stall situation is installing variable frequency drives. VFDs give you precise control over fan speed, allowing the system to dial back RPMs to match actual airflow demand rather than throttling air entry at the inlet.

To validate the approach before recommending it, MSC tested the fans using a portable VFD on-site. The inlet control modules were taken out of the equation, and the fans were run under VFD control through the full fan curve. The stall condition disappeared.

MSC recommended VFD installation on all laboratory supply fans. While they were at it, the team also performed full preventive maintenance across all units: thermal and vibration testing, bearing replacements where needed, bearing greasing, and installation of Aegis grounding rings to protect the bearings from stray electrical current that can occur with VFD operation.

Back on Track

VFDs were ordered and installed to replace the inlet control modules, and MSC returned to perform startup and verify everything was operating correctly. With the fan stall issue resolved, proper air balancing could finally be completed, and the building commissioning effort that had been delayed was back on track.

The fans weren't broken. They were simply the wrong size for the job, and once that was addressed, everything else fell into place.