What is a Curb Adapter? — For anyone new to the HVAC industry, this is a valid question.
Let’s Start With A New Installation:
When a building is constructed, prior to the installation of the HVAC Rooftop Unit (RTU), the roof is cut, and a metal structure called a Roof Curb is installed. The roof curb is designed to support the weight of the unit, and to offer a means for contractors to connect the supply & return air duct work.
The Curb is secured into place, and roofing material is extended all the way up the sides of the curb. The building’s supply and return duct work is run up to the curbs openings and secured into place, then sealed to prevent air leakage.
Alternatives When Updating The Rooftop Unit
After many years of service, an upgrade of the HVAC system will be inevitably required. 
Most often the new HVAC rooftop will be different enough in size and construction so the existing curb will not properly facilitate the new HVAC size & design.
The building owner is faced with one of two decisions… remove the unit, and the existing unit’s roof curb, install a new curb, transition the duct work to attach to the new curb, and set the new RTU in place. This decision inherently causes significant downtime and expense to remove the existing curb and re-roofing a new one, not excluding the risk of creating a roof leak.
The best alternative is a Curb Adapter. Curb Adapters are designed to properly seat a new (RTU), then redirect the new supply & return openings to match the duct work fastened to the existing curb.
Curb Adapter Strategies – Aligning The Supply & Return
Nearly every HVAC manufacturer uses different roof curb layouts specific to their HVAC rooftop units. Most manufacturers will strive to keep the same cabinet height, width, and length dimensions, as well as the precise positioning of supply air and return air opening located on the bottom of their RTU. This overall consistency is designed to entice loyalty. Upgrading with the same manufacturer most often reduces overall installation costs when the same roof curb is re-used.
However, due to the design differences of rooftop manufacturers; changing CFM/Tonnage requirements, or just (most often smaller) more efficient technology, the existing curb will not work… so now what?
Here we see the RTU layout of two popular HVAC Rooftop manufactures looking down through from the top, and their associated roof curbs.
Fig. #1 Carrier RTU Supply/Return Layout
Fig. #2 Trane RTU Supply/Return Layout
Fig. #3 Associated Carrier Roof Curb
Fig. #4 Associated Trane Roof Curb
Figure #1 displays the supply & return air openings of a Carrier unit, Figure #2 is a Trane unit. As you can see, not only is the length and width different (Figures #3 & #4), but the supply and return openings are of different sizes, and placed in different locations.
A well designed curb adapter strategically positions the new HVAC rooftop supply & return air openings to the existing curbs supply & return openings so to obtain minimal air resistance (Static Pressure).
Choosing a proper Curb Adapter can be challenging at times. Feel free to ask any questions you may have here, at our MicroMetl Open Forum, or with our MicroMetl Technical Services Group.
