Which type of duct tape is not permitted as a sealant on metal ducts?

Unlisted duct tape is prohibited as a sealant on metal ducts because it does not meet the specific performance standards and safety requirements established by the codes and regulations governing HVAC installations. Duct tape that is labeled as "listed" has been tested and certified to ensure it can withstand the conditions typical of HVAC applications, including exposure to heat, air pressure, and environmental factors.

In contrast, unlisted duct tape may lack the necessary durability, adhesion, or resistance to fire and other hazards, potentially leading to an insecure seal. This could result in air leaks, reduced efficiency, or even safety concerns over time. Therefore, using only listed duct tape ensures compliance with safety regulations and helps maintain the integrity of the duct system. Other types of tape, such as heavy-duty tape or fiberglass tape, may also have specific applications, but they still require proper listing or approval to ensure they meet code standards for use on metal ducts.