Stopping the Cycle of Fan Array Failures: An MSC Case Study
A previous client called upon Mechanical Service Corporation to solve a perplexing and increasingly costly HVAC problem that had persisted despite multiple attempts at resolution. The case involved repeated fan failures in a fan array serving a 160-ton rooftop split system over a short period of time, with no apparent cause. MSC had provided refrigerant piping design assistance two years prior when the system was installed, and now, the client was facing an entirely different challenge.
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Upon our service technician’s arrival, the client disclosed that four out of the nine supply fans in the two-year-old fan array had been replaced, and a fifth fan had recently failed. This was particularly noteworthy as these fans would typically operate for many years without failure.
An examination of the system revealed an excessive amount of accumulated water in the variable frequency drive (VFD) control cabinet serving the fan array. Our technician identified this water as condensation resulting from cold, pressurized air from the system infiltrating the control cabinet through an unsealed 2-inch conduit nipple and mixing with the warm air circulating through the cabinet. The problem was exacerbated at night when the unit shut off, allowing warm, moist outside air to infiltrate the unit and condense on the cooler interior surfaces.
MSC resolved the condensation issue by sealing the conduit opening. However, while problematic, it was not the root cause of the fan failures, so we moved on to examine a short circuit fault code in the VFD. When the technician performed a Megger test, however, no short circuit issues were found. Further investigation revealed that all of the VFDs had been programmed incorrectly, with KW and HP settings swapped and other critical parameters set improperly. This finding suggested that the previous fan failures might have stemmed from the same programming issue rather than hardware faults.
After consulting with the drive manufacturer, our technician reprogrammed all the drives with the correct settings. We also implemented an advanced drive function that induces a DC voltage to the motor while it's off to keep it warm and prevent dew-point condensation.
The client appreciated the resolution of the ongoing issue. The new HVAC system had already incurred significant repair costs from the multiple fan replacements, and had MSC not identified the VFD programming errors, these issues could have persisted indefinitely. This project underscores that expert knowledge and experience is essential for effective troubleshooting in complex HVAC systems, as initial
symptoms may not always point to the root cause of a seemingly obvious problem.
