H.L. GRAVES AIR CONDITIONING,Inc. “We provide results, not excuses ”

You will notice from time to time, black dirt around the supply air diffusers and light troffers that supply air to the space.  The dirt will be on the ceilings near the diffuser and on blinds or draperies if they are near the diffuser.  It looks like dirt is being discharged from the diffuser or troffer and impinging on the diffuser, ceiling and anything near it.  Seeing this dirt would make one think that the filters were never changed and the system and the ducts were filthy.  This is often what duct cleaning companies that are trying to sell you duct cleaning service, will point to as an example of how dirty your ducts are.  Do not buy it until you have read this chapter.

You may have a problem, but chances are, it is unrelated to the dirt we are talking about here.  If there is an Indoor Air Quality problem in your building, it is not likely to manifest itself with dirty diffusers and ceilings.  Most of the time IAQ problems are odors and microscopic organisms (bacteria, molds and spores).  Most sources of pollutants that contaminate an HVAC system are from first, the outside makeup air, Second, air returned from the building, third, poor filter maintenance, fourth, and most likely, poorly maintained condensate pans under the cooling coils. 

With few exceptions, well maintained HVAC systems do not contribute to IAQ problems, and certainly do not cause dirt around diffusers.  Proof of this statement is in a closer examination of the area served by a system or a zone (VAV) box.  Two offices of the same size, served by the same zone, balanced to receive the same amount of supply air, will display different amounts of dirt at the diffuser.  One may show a large amount of dirt while the other will show little or no dirt.  How can this be if they both receive the same amount of air from the same source.  The first obvious conclusion is that the source of the dirt cannot be the HVAC system. And this conclusion would be correct.  Then how did the dirt get on one ceiling and not the other?

This dirt is called "aspirated dirt" and the source is the space itself.  It is from air bourn particles of lint from clothing, paper dust, dandruff, dust that we kick up out of the carpet and all the dust and dirt we generate within our workspace.  The office that has a high  degree of activity will have more air bourn dirt generated than an office with little activity.  The high traffic, high activity area will display more dirt around the diffuser and on the ceiling than a low traffic area.  This is particularly noticeable in a corridor when compared to quiet offices off of the corridor that are served by the same zone.  Work rooms with copiers, sorters and other paper handling activities will also display more dirt at the ceiling and the diffuser than quite offices.

As air rushes out of the diffuser at some acute angle to the ceiling into the space, the motion of the air creates eddy currents with the surrounding air causing the room air to move across the ceiling and swirl in a circular motion as it gets caught up in the rush of supply air.  This action increases the velocity of the room air right at the diffuser where the supply air is leaving the diffuser.  The increased velocity of room air ladened with air bourn dirt causes the air bourn dirt to impinge on to the ceiling and diffuser.  This dirt builds up until we notice it and begin wrongly to condemn the indoor air quality and the HVAC system.  Fig. 2.

The next time you see dirt on the ceiling around the supply air diffuser, start looking at other diffusers in areas that have less use.  You will notice the phenomenon called "aspirated dirt".  Be pleased to see this dirt because it is telling you this is an area where a lot of work is getting done.  If it bothers you, the best way to deal with it is to have someone get out the vacuum and clean it.