While rerouting the power cord for our newly installed front air conditioner We (one of the guys in FCRV that has been helping me with my mods) didscovered that the exhaust hoses coming out of the furnance were very prone to leakage or loss of hot air. The two small ducts coming out the side (one goes to the throne room) we only held on with one "tab". We put 4 self taping screws in around the perimeter and then tape.
The main duct going out the top had one corner of the flange bent at about a 45 degree angle. The gentleman constructing my unit tried to fix this by taping the offending flange down. The fix didn't last long. - We installed a doubled over piece of aluminium to hold the bent corner flush against the duct and then taped over the edges.
Bill