Furnace large custom duct collars for large ductboard return

According to North Carolina building codes, there are specific requirements for constructing duct collars when building a large ductboard supply duct into the furnace:

1. All connections, including the duct collar, must be sealed with water-based duct mastic1. This is critical for meeting the new North Carolina Energy Conservation Code (NCECC) requirements.
2. Tape can be used during installation, but should not be relied upon as the primary method of air sealing ductwork1. Mastic should be applied over the tape to ensure airtightness.
3. The duct collar should be properly sized and well-sealed where it meets the air handler1. This is particularly important due to the large size of plenums, as poor connections at the air handler can lead to significant leaks.
4. When connecting the ductboard to metal components like the furnace, a metal collar or sleeve should be used to ensure a secure and airtight connection4.
5. The duct collar must be constructed to meet the pressure class requirements of the system4. Each size in a pressure class has minimum specifications for joints and reinforcements.
6. Proper support for the duct collar and adjacent ductwork is required. Metal straps with a minimum width of 1 inch (25 mm) and equivalent to or heavier gauge than the duct material should be used for support8.
7. The duct collar should be designed to allow for proper airflow and minimize pressure drop in the system4.

Remember to consult the specific North Carolina Mechanical Code and SMACNA guidelines for detailed requirements and best practices when constructing duct collars and connections.