The installation of a Concentric Diffuser is usually straight forward (See The MicroMetl Blog “What Is A HVAC Concentric Airflow Diffuser System?” For A 101 Primer), however, obstacles can arise.
Obstacle One: Welded Curb Supports
When installing a flex duct system (Illustration #1), you can simply lay the flex duct adapters onto the curb’s duct supports and attach the flex duct. However, hard duct transitions require that you remove the duct supports and utilize the entire opening (Illustration #2). If the existing curb’s duct supports are welded, be prepared to bring a Sawzall to the Jobsite.
Obstacle Two: Curb Adapters
Too often Curb Adapters contain internal air transitions and plenums. The angled openings allow airflow from the new rooftop unit to the existing curb and will not permit concentric transitions or insert adapters with flex duct to be installed. Depending on the direction and extent of the internal angles you might get lucky!
If you can determine the original rooftop (RTU) model that had fit on the existing curb, you could remove or replace the existing curb adapter, order a concentric package to fit the existing curb, and place the new curb adapter over the existing curb trapping the installation.
Obstacle Three: Vibration Isolation Curbs
The Vibration Isolation Curb uses springs to “float” the HVAC rooftop unit, The movement from the top section of the curb which contains the duct supports will cause a hard duct section to stress, and pull apart. It is feasible when using a flex duct concentric system… if the diffuser section is secured in place and adequate play is remaining in the flex duct connecting the curb to the diffuser.
Obstacle Four: Adjustable Pitch Curb
The Adjustable Pitch Curb is designed with an upper & lower section. If the curb is set to a deep angle, the upper section may extend too far inside of the curb. Therefore, a transition or flex duct section may become obstructed by the protruding curb wall, preventing the field installer from either obtaining the desired angle of the curb or cause the transition to improperly fit into the curb.
Obstacle Five: Improper Airflow
This is more of an airflow, air balance issue. However, it is critical. Hot spots, cold spots, and air drafts are the greatest complaints of room occupants. It is crucial to complete each installation with an air balance. Too much velocity will cause cold spots, air noise, and drafts, too little will cause hot spots and short cycling. Short cycling is the result of too little air velocity through the supply side. It occurs when cool or warm air is too quickly returned through the return air section to the RTU. If it is critical to direct the air throughout a room… Therefore, choose a concentric diffuser with adjustable air blades.
Please contact our MicroMetl Customer Service Department with all your diffuser questions and needs.