Vents from your building gas appliances should be installed with an understanding there are requirements on clearance and types of materials to be used. Proper sizing is important due to the fact ducts will get hot and could potentially start a home fire. Combustible materials should have a proper clearance with recommended sizing on duct run. You can see at the top of this run the duct is closer than the dimension recommended. The sleeve should be pushed back up into place and secured.  Also having this run in a closet you should pay special attention to not letting stored items come in contact with the vent run.