A concentric diffuser system is typically straightforward to install (for a basic overview, see our article: “What Is an HVAC Concentric Airflow Diffuser System?”). However, several site conditions can complicate the installation process. Below are five of the most common obstacles installers face—and what you should be aware of before beginning your job.
Obstacle 1: Welded Curb Supports
When working with a flex duct transition, installation is simple—flex duct adapters can be placed directly on the curb’s duct supports, and flex duct is attached without issue.
However, installations using hard duct transitions require full access to the curb opening. In these cases, duct supports often need to be removed. If the curb’s supports are welded in place, removal becomes more challenging.
Recommendation:
Be prepared with the right tools—specifically a Sawzall—in case the welded supports need to be cut out to allow proper transition clearance.
Obstacle 2: Curb Adapters with Internal Transitions
Many existing curb adapters include angled internal air transitions and plenums. These angled passages direct airflow from a new rooftop unit (RTU) to the existing curb footprint. Unfortunately, those internal angles often prevent:
- Concentric transitions
- Insert adapters
- Flex duct adapters
From being installed correctly.
Possible options:
- Identify the original RTU model the curb was designed for.
- Remove or replace the existing adapter.
- Order a concentric package sized for the original curb.
- Install the new curb adapter over the existing curb to trap the concentric package in place.
- This approach can restore compatibility and ensure proper airflow.
Obstacle 3: Vibration Isolation Curbs
Vibration isolation curbs use springs to “float” the RTU and reduce vibration transfer into the building structure. While effective for noise and vibration control, they pose challenges for concentric installations.
The movement from the upper section of the vibration isolation curb (where duct supports are located) can cause:
- Stress on hard duct sections
- Separation at duct connections
Workable solution:
Flex duct systems can be used if:
- The diffuser section is securely supported
- Enough slack is provided in the flex duct to accommodate movement
Proper support and play in the flex duct are essential.
Obstacle 4: Adjustable Pitch Curbs
Adjustable pitch curbs use upper and lower sliding sections to create a level platform on pitched roofs. When the curb is adjusted to compensate for a steep pitch, the upper assembly may extend too far into the interior opening.
This can obstruct:
- Concentric transitions
- Flex duct transitions
- Proper duct alignment
If the curb wall protrudes too far inward, installers may be unable to angle or seat the transition correctly.
Workable Solution:
Level the duct transitions within the curb opening before mounting your flex or hard duct sections. By ensuring that the transition is aligned and properly seated inside the curb, you can maintain clearance, achieve optimal airflow direction, and complete the installation of your flex/hard duct and diffuser without interference.
Obstacle 5: Improper Airflow (Air Balance Issues)
This is not a physical obstruction—but a critical performance issue. Poor airflow balance is the most common cause of occupant complaints, including:
- Hot spots
- Cold spots
- Drafts
- Air noise
Tip: Always complete a proper air balance after installing a concentric diffuser.
Too much airflow velocity can cause noise and cold drafts. Too little airflow can result in:
- Hot spots
- Reduced comfort
- Short cycling
Short cycling occurs when insufficient supply air causes return air to quickly reach the RTU, triggering premature shutdowns and reducing system efficiency.
If air distribution is critical:
Choose a concentric diffuser with adjustable air blades to help direct airflow where it is needed.
Need Assistance?
If you have questions about diffuser selection, compatibility, or installation challenges, contact MicroMetl’s Customer Service Department. We’re here to support your field installations and provide the right solutions for your application.
What Is an HVAC Concentric Airflow Diffuser System?
An HVAC Concentric Airflow Diffuser System is designed to deliver supply air into a space and return air back to the rooftop unit (RTU) using a single diffuser assembly. Instead of relying on traditional ductwork with multiple ceiling grilles or registers, a concentric system allows both supply and return airflow to pass through the same opening. This creates a clean, efficient, and compact installation — ideal for open areas or spaces just below a ceiling surface.
How a Concentric Diffuser Works
A concentric system delivers conditioned supply air from the RTU into the space, while simultaneously drawing return air back through the same diffuser. These systems are commonly used in:
- Large open spaces
- Retail environments
- Warehouses
- Areas without extensive ceiling infrastructure
Because the supply and return are combined in one assembly, the diffuser is located close to the ceiling and requires only a short duct run.
Hard Duct vs. Flex Duct Concentric Systems
Two primary types of concentric systems dominate the market: Hard Duct and Flexible Duct systems. The duct connection method dictates the design, components, and installation requirements.
Hard Duct Systems
(Figure #1)
Hard duct concentric systems use rigid sheet metal ducts, typically shipped flat for field assembly. They can be configured in two ways:
1. Duct-in-Duct
- The outer wall delivers supply air downward into the diffuser.
- The inner duct collects return air and directs it back to the RTU.
2. Side-by-Side Ducts
- Separate rigid ducts for supply and return
- Installed parallel to one another
Advantages:
- Durable construction
- Clean air pathways
- Excellent airflow control
Flexible Duct Systems
(Figure #2)
Flex systems use short flexible duct sections to connect the rooftop unit to the diffuser. Two or more flex ducts direct supply and return air independently.
Advantages:
- Easier to route around obstacles
- Faster installation
- More forgiving in tight or uneven spaces
Both hard and flex duct options are effective — the correct choice depends on installation conditions, performance needs, aesthetics, and available clearance.
Key Components of a Concentric System
A complete concentric system involves several integrated components:
Roof Curb
The roof curb is purchased separately and supports the rooftop unit. For concentric installations, curb supports or deck pans may need to be removed temporarily or permanently to allow the transitions or duct sections to fit properly.
Transition Adapter Sections
Transition adapters (Figure #1) are sheet-metal assemblies that install inside the roof curb. They direct supply air from the RTU into the hard duct or flex duct sections. These adapters almost always ship in a knockdown configuration and must be field-assembled.
Adapter Inserts
Adapter inserts (Figure #2) have round or ob-round openings designed for flex duct connections. They sit inside the curb and provide the connection point for the flexible duct sections.
Drop-Type vs. Flush-Mount Diffusers
The diffuser is the final point of air distribution. Two primary styles are used in concentric systems:
Drop-Type Diffuser
(Figure #3)
- Disperses supply air from the entire perimeter
- Adjustable blades allow directional control and better airflow distribution
- Provides longer air throw and quieter return air due to larger return openings
- Common in open areas without drop ceilings
- Extends below the ceiling surface when used with grid systems
Flush-Mount Diffuser
(Figure #4)
- Designed to sit flush with ceiling surfaces
- Fixed blades and a more compact face area
- Generates slightly higher static pressure
- Provides shorter air throw compared to drop diffusers
- Ideal for suspended ceiling applications
Final Thoughts
MicroMetl concentric diffusers deliver supply air in a full 360-degree pattern with consistent velocity around the perimeter. Diffusers are sized according to the CFM requirements or tonnage of the rooftop unit to ensure proper airflow, comfort, and performance.
Selecting the correct diffuser and system components is critical. Improper sizing or mismatched components can lead to poor airflow, noise, or installation challenges.
If you need assistance choosing the right concentric system for your application, please contact MicroMetl’s Customer Service Department — we’re here to help with design, selection, and installation support.
