Duct Dynasty: Dirty Vents


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    Duct Dynasty: Dirty Vents

    Why Dirt Streaking Occurs Around Vents

    by David Richardson

    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 that HVAC professionals encounter on a daily basis. The underlying cause of this occurrence is often blamed on the air filtration of the HVAC system and never considered again. In this scenario, no solutions are offered and the streaking continues. Is it possible there is more to dirt streaking around supply vents than just the quality of air filter being used?
    To keep from getting too technical, 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 having their own characteristics and definitions that I won’t get into here. Look for the differences in these in an upcoming entry.

    A Call for Help – We have streaks!

    My first big run in with dirt streaks was about ten years ago. I was called out to a local commercial building to examine dirt streaking on an expensive ceiling that had to be corrected. This building was only about two years old and the original HVAC contractor was at a loss for what was causing the issue. They added 1" pleated filters in an attempt to better filter the air with no success.
    The HVAC system was equipped with four-way diffusers which left a nice multi-directional pattern of dirt streaks coming out of the edges of the diffusers. Due to the cost 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 twenty feet from the ground.
    The original contractor recommended dropping down ducted sleeves from each diffuser to move them away from the ceiling. This would of course prevent the dirt streaking and allow the ceiling to be cleaned up. This must have been a good idea as a couple of other contractors 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 it.
    My suggestion to the group for correcting the dirt streaking was a bit more simplistic. When told 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 that a member of their staff could easily apply 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. Due to this turbulence, particles of dirt can become entrained in the conditioned airstream and become deposited across the surfaces the air is blowing across.
    Air velocity increases as conditioned air leaves a supply vent
    The particles of dirt often come from two different sources. One 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 into the conditioned air coming out of the supply vent and are now distributed across the surfaces the conditioned air is blowing.
    The second source is from 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 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 the particles to develop stickiness to them 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 parallel surface.

    How to Solve Dirt Streaking

    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. The first place I typically start is an inspection of 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 or if it is being brought in 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 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 repaired the dirt streaking stopped.
    Understanding air properties can make you a real rock star when it comes to solving issues associated with your systems. 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 the impact of your systems on the environment they condition.
    About the Author
    David Richardson is a curriculum developer and instructor at the National Comfort Institiute (NCI). 
    This article is used with the author's permission.
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