Many people have seen dirt on registers and the ceiling around the register openings. Oftentimes the HVAC system will appear to be clean. So what could be going on?

The “Coanda effect” could be the cause.

The Coanda effect is named in honor of Henri Coanda, a Romanian aerodynamics pioneer and aircraft inventor. The Coanda effect occurs when airflow is closely projected to a parallel surface, such as a ceiling, or the walls of a duct system. When this occurs, the airflow is affected by the parallel surface it is flowing with. In simpler terms, air clings to surfaces as it moves.

When the airflow leaves the diffuser it produces a lower pressure area between the diffuser surface and the ceiling that causes the air to “cling” to the ceiling. HVAC construction engineers call this the Coanda effect.

Any disturbances to the airflow pathway have a dramatic impact on this extended airstream being maintained. The air moving with the surface needs a smooth, obstruction-free pathway to maintain the Coanda effect.

On a ceiling, items such as light fixtures, sprinkler heads, trim, or ceiling variations, are obstructions and will interfere with the airflow being maintained. Changes in the direction of airflow also have an impact on the Coanda effect. Airflow always takes the path of least resistance and wants to maintain straight line movement.

When airflow is projected into a room from a supply register, it begins to expand almost immediately. The conditioned air flowing from the supply register also sets a large amount of room air into motion with it. This is one of the primary reasons that supply registers have more of an impact on comfort than return grilles. 


As the air travels further from the diffuser, the air velocity drops, gradually causing the air pattern to lose its “cling.” It then falls toward the floor. This results in good mixing of the cool or warm air from the diffuser with the room air. This also results in the room air being pulled up toward the center of the diffuser.

dirt around diffusers

Dust and dirt in the air often collects around registers and ceiling diffusers. Picture courtesy of Robert Rizen.

The Coanda effect is exploited to increase the throw of a ceiling-mounted diffuser. Because the Coanda effect causes air discharged from the diffuser to “stick” to the ceiling, it travels farther before dropping for the same discharge velocity than it would if the diffuser was mounted in free air, without the neighboring ceiling. Lower discharge velocity means lower noise levels and, in the case of variable-air-volume air conditioning systems, permits greater turndown ratios. Linear diffusers and slot diffusers that present a greater length of contact with the ceiling exhibit a greater Coanda effect.

If you’re not a mechanical engineer, why should you care about this?  Your client may be asking: Why are the registers dirty? Why is the ceiling dirty around the registers?

The answer may be one of three sources: primary air, room air dirt particulates or unsealed openings.

The primary airstream starts at the air handler. Outside air is introduced and mixed with the return air. This mixed air is filtered, and then travels through evaporator or heating coils. When the unit’s air filters aren’t efficient, don’t fit properly or are missing, they don’t remove airborne dirt.

The airborne dirt becomes deposited on the coil sets, fan and drive assemblies, and is carried through the supply ductwork, and exits through the diffuser, causing ceiling and vent smudging.

This would be the obvious reason for the dirty registers: dirty HVAC components.

Coanda effect


The HVAC system may be clean, the supply ducts feeding the registers may be clean, the main trunk lines may be clean, and the air handler and coil sets may be clean. So why is there dirt buildup? 

Room air particulates are the most common cause of smudging. Dirt particles (dust, fine dirt) in the room become part of the air circulation created by the diffusers and are drawn to the ceiling by the creation of the low velocity pocket (Coanda effect).

Some of the dirt in the air stream will be deposited on the ceiling as a smudge. This may be exaggerated by surface condensation of the register.

How airflow under the “Coanda effect” looks in a room.

How airflow under the “Coanda effect” looks in a room.

The third source is from unsealed openings where ductwork supply boot penetrations are made such as through drywall or a subfloor. When this occurs, insulation particles or other particulates 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.

Examples of where this is visible is in high-traffic areas, high-volume retail spaces, office buildings, heavily occupied work areas, copy rooms, smoking lounges and in or adjacent to manufacturing plants — anywhere there is a large amount of particulate in the air.

Some things to look for when inspecting:

  • Visible dirt on the horizontal surfaces in the areas — window sills, cubicle and partition tops, bookcases, etc.
  • Carpet — it attracts and holds onto a large amount of dirt.
  • Particulate-generating activities — cutting, welding, foot traffic.


What should you do? Inspect the HVAC system, paying close attention to the outside air intake points and filter rack. Verify filter fit to the rack, change to a more efficient filter if possible, install proper gaskets, and check air-handler door gaskets and damper settings. The dirt is coming from somewhere. 

Dirty fan assemblies and coil sets mean dirt is being brought into the occupied spaces. Have them cleaned. Even if the filtration is adequate and fits properly these assemblies still get dirty. Inspection is key. Coils may appear clean on the surface, but what’s going on deep inside?

Look for and seal boot penetrations into the occupied spaces — stop unwanted air from being pulled into the space.

If the system is complicated or you don’t find the obvious cause, you may want to employ the services of a certified ventilation inspector to find the answers.

Additional housekeeping may be needed:

  • Dusting, vacuuming, wet wiping and more frequent cleaning of the occupied spaces.
  • Have the carpet, chairs and cubicle walls cleaned.
  • Evaluate the cleaning techniques used by the cleaning staff — utilize high-efficiency filtered vacuums where possible and use wet wiping instead of dry dusting. Try to minimize the amount of particulate in the space.