Commercial Air Duct Cleaning: Return Grilles and Supply Diffusers

As a commercial air duct cleaning company in the Twin Cities, we frequently get calls from customers who work in or own office spaces that show signs of dirty air ducts, which leaves the occupants concerned about indoor air quality. In many cases, it’s the overhead return grilles and supply diffusers that exhibit the first indications of an HVAC system in need of cleaning.

Return grilles and supply diffusers in Eagan MN.
BEFORE: Dust clinging to a return grille in the office. Since these ducts suck up air into the RTU, they are often the dirtiest part of the system.
A return grille after cleaning in a warehouse in Eagan MN.
AFTER: The return grille has been removed, washed, and reinstalled, giving confidence to building occupants that the system is clean.

Return grilles and supply diffusers

In the case of this particular customer, the area in question was a small office inside a warehouse. A lot of forklift activity took place in the warehouse, and those in the office were noticing a buildup of dust on the desks, and also in the return grilles and supply diffusers in the ceiling. Dirt on these components is the hallmark of an HVAC system in need of cleaning.

Because return ducts draw air into the HVAC system—in this case up into the RTU (rooftop unit)—the return grilles, which roughly resemble egg crates, are often the dirtiest part of the system, since they are positioned before the filter. Supply diffusers, which distribute conditioned air into the inhabited space, are located at the part of the system after the air filter, so they don’t tend to be as dirty. When supply diffusers and return grilles are equally dirty, debris is getting past the air filter, an indication that the filter is overdue for replacement.

Dirty supply diffuser in a warehouse in Eagan, MN.
BEFORE: A supply diffuser in the ceiling of a warehouse office. The gray/black dust was concerning to the building occupants.
Supply diffuser in Eagan MN after cleaning.
AFTER: The diffuser has been removed to the outside, washed thoroughly with an industrial-grade solvent, and reinstalled.

The cleaning process

Typically one of the first steps in cleaning a commercial HVAC system is to remove all grilles and diffusers, usually to the outdoors, and wash them with an appropriate degreaser or other cleaning product. In this case both the supply diffusers and the return grilles were so dirty they had to be cleaned with an industrial-grade solvent, presumably due to the operation of machinery inside the warehouse.

The RTU (rooftop unit) that serves the office area, and all of its associated ductwork, was thoroughly cleaned with HEPA vacuums, air whips, and an industrial containment system.

Wrapping up the project

Once the entirety of the HVAC system has been cleaned, the newly washed return grilles and supply diffusers are reinstalled. Since these are the most visible components of the HVAC system, their cleanliness gives building occupants reassurance that the HVAC system itself is clean, and that indoor air quality is favorable.

In order to maintain system cleanliness for as long as possible, the project manager recommended to property management more frequent replacement of the air filter in the RTU. This would help prevent the dirt from the return ducts from contaminating the supply part of the system.

Moral of the story: Dust clinging to return grilles and supply diffusers—the most visible components of the HVAC system—is trying to tell you that the part of the system that you can’t see is in need of cleaning.

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