Duct Dynasty: Why Dirt Streaking Occurs Around Vents
Turbulence, Unsealed Openings, Room Debris are the Usual Suspects
|Isolating the source of dirt, when encountering dirt streaking, is crucial to providing a real solution to the problem.|
Have you ever noticed dirt streaking out of the sides of any of the supply vents on the HVAC systems you service? Dirt streaking on supply vents is a common problem HVAC professionals encounter on a daily basis. The underlying cause is often blamed on the HVAC system’s air filtration and is never considered again. In this scenario, no solutions are truly offered, and the streaking often continues. Is it possible there is more to dirt streaking around supply vents than just the quality of the air filter being used?
To keep it simple, I am using “vent” as a generic term to cover all the varieties of terminal devices that could cover a supply branch outlet. Vents are really broken down into the three categories of diffusers, grilles, and registers. Each of these has its own characteristics and definitions that I won’t get into in this article.
A Call for Help — We Have Streaks
My first run-in with dirt streaks occurred about 10 years ago. We received a call to examine dirt streaking on an expensive ceiling. The building was approximately two years old, and the original HVAC company could not determine what was causing the issue. The contractor’s technicians added 1-inch pleated filters in an attempt to better filter the air and solve the problem with no luck.
The HVAC system was equipped with four-way diffusers, which left a nice multi-directional pattern of dirt streaks coming out of the edges. Due to the style of the ceiling and the purposes the building was used for, this issue had to be solved. It wasn’t acceptable for these dirt streaks to be running across the ceiling, as they were pretty noticeable looking up from the ground.
The original company recommended extending ducted sleeves from each diffuser to move them away from the ceiling. This would prevent the dirt streaking and allow the ceiling to be cleaned up. This must have been a good idea, as a couple of other companies made the same suggestion. The only problem with this was it would be more of an eyesore than the dirt streaking. The building owners weren’t buying this solution.
After some digging around, my suggestion for correcting the dirt streaking was a bit more simplistic. When advised they could correct this issue with a case of caulk and a caulk gun, I got a lot of odd looks. It was too simple of a repair. Once I explained what was occurring, and how the caulk could cure it, they were delighted. It was also a solution a member of their staff could easily correct, once shown how.
So, what was the source of this dirt streaking, and why would caulk fix it?
The Source of Dirt Streaking
One of the main contributors to dirt streaking is the turbulence of the conditioned air coming out of a supply vent. When air leaves a supply vent, it is coming out at a high velocity due to being squeezed through the louvers of the vent. This action is similar to placing your thumb over the end of a water hose to increase the distance water can spray.
These particles of dirt often come from two different sources. One common source is from within the room itself. Depending on what is occurring in the room, the duct streaking can have various contributors, such as carpeting, candles, pets, and people. These dirt contributions become entrained in the conditioned air coming out of the supply vent and stick to the surfaces the conditioned air is moving across.
The second common source is unsealed openings where supply boot penetrations are made, such as through drywall or a subfloor. When this occurs, insulation particles or other particulate from the unconditioned space is pulled into the airstream. As these particles from the unconditioned space become entrained in the conditioned air, they are also distributed across the surfaces the conditioned air is blowing on and leave a trail of streaking.
Depending on the conditions of the living space, dirt streaking may be amplified when certain factors exist. For instance, excessive moisture in the room air can allow particles to develop stickiness, while a lack of moisture can cause the dirt particles to be attracted to the building surfaces. The Coanda Effect is another variable that needs to be considered here, as it has a direct influence on how well air is projected across a surface.
Dirt Streaking Solutions
The first step is to determine the source of the dirt. This begins with a visual inspection of the system and the environment it is operating in. One of the first things you should do is examine the equipment and the ducts feeding the vents that have streaking issues.
If the equipment, coil, and ducts feeding the vents that have streaking issues are clean, you can eliminate filter bypass issues or duct leakage as a contributing factor to the dirt streaking. This leaves you with determining if the source of dirt is from particulate within the room itself, or if it is being brought into the room through unsealed boot penetrations.
A common cure in commercial buildings, where the dirt sources are internal, is anti-smudge rings around the vent penetration. You will see these rings on many restaurant vents as sheet metal collars that protrude down from the vent approximately 1.5 inches. These rings break up the action of the air as it exits the register and forces it downward instead of across the ceiling.
When the dirt sources are external, such as from an unconditioned space, sealing the boot penetration to prevent any gaps between the conditioned space and unconditioned space usually does the trick. This was the recommended correction for the issue of dirt streaking I mentioned previously. There were massive gaps around the boot penetrations in the ceiling of this building that were allowing attic air containing cellulose dust to be entrained and spread across the ceiling. Once the boots were sealed, the dirt streaking stopped.
Understanding air properties can make you a real rock star when it comes to solving issues associated with the buildings you service. These issues might not have anything to do with your system, but they appear to. Take a look at the building side of the duct system, and understand the cause-and-effect relationships that influence your systems and the environments they condition.
Publication date: 1/26/2015