It's a violation of building codes to use duct tape on ducts.

Despite its name, duct tape is actually rarely intended for use on sealing heating and ventilation ducts. Manufacturers recommend special, other kinds of tape. Research conducted at Lawrence Berkeley National Laboratory Environmental Energy Technologies Division concluded that duct tape should not be used to seal ducts because it can become brittle and fail. 

Furthermore, duct tape can burn violently, producing toxic smoke, and in California, it is actually prohibited by law to use on ducts! Most other locations in the US also prohibit its use by building codes. This might make one wonder what it's actually intended to be used for. Duct tape is taken on every NASA mission for a variety of emergency situations where it is needed. 

A variant of it is also used in the US military for its resistance to oils and greases, specifically on submarines. Despite this, let's not forget duct tape can be used to put together a car! 

(Source)