Beware VFDs on Make-up Air Units

Many owners of residential buildings are installing variable frequency drives on their make-up air units to save energy.  This is a very effective way to save money, however, it often creates unexpected problems.  Unfortunately, when you reduce the flow to, say, 60% of design flow, you do NOT get 60% of the flow out of each grille in the system.  Instead, you may get almost 100% of design flow at the grilles near the fan, and almost no flow at distant grilles. This means that floors distant from the fan are starved for fresh air.  This is causing air quality complaints as well as significant condensation problems. Not providing adequate fresh air also contravenes Property Standard bylaw requirements.

The problems characteristically presents with condensation on the lower floors of the building (if the fan is on the roof).

The building owner could attempt to balance the system at the lowest flow setting, but this may not be possible, as the pressure may not be high enough to get 60% of design flow at the furthest grilles.  Also, balancing at 60% flow (if it is even possible) means that when the fans run at 100%, the velocity will be very high at the top floors, possibly causing the air to blow right by the grilles (starving these floors) and almost certainly creating noise concerns.

A reasonable middle ground might be possible by balancing at some intermediate flow level (perhaps 80%) and foresaking some energy savings for better air quality.

A good starting place is to see if the original design provided excessive air flow. In this case, an across-the-board reduction in air flow might be possible, avoiding the need to cycle the unit up and down altogether while still generating energy savings.

Also, some systems are scheduled to reduce flow at night however, at night, the building is almost fully occupied, so this is the wrong time to reduce flow.  Occupancies are likely to be less during the day rather than at night.

In summary, using a VFD on a make-up air unit presents a host of problems.  Owners should be wary when proceeding down this path.  While the energy savings might be compelling, the related air quality and condensation problems might outweigh the benefits.