Duct tape is useful for repairing just about anything, but ironically, not for ducts.
Although the name might be misleading, general purpose duct tape was not designed for use in HVAC ductwork at all. In fact, if you tried using “regular” duct tape on your ductwork, it would more than likely fail. The adhesive used in general purpose duct tape is not built to withstand the conditions that HVAC-specific tapes are tested to perform in. Cold and hot temperatures can cause this type of duct tape to flag and fall off, leaving leaks in the system that can result in energy loss and expensive heating and cooling bills.
While general-purpose duct tape is not suitable for use on HVAC systems, there are some cloth duct tapes that are specifically designed for use in certain HVAC applications. These tapes are usually recognizable with packaging that indicates its approval for usage in HVAC applications, and if the tape is UL Listed, it often features a printed backing that lets inspectors know that it meets the code. The benefits to an HVAC-specific duct tape for applications such as seaming and sealing Class 1 Flex Duct include adhering to rough or dirty surfaces, being easy to tear, and offering high holding power.