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