I have 4" duct running 15 feet off main trunk to a 12x12 bedroom with no return line. Would increasing it to 6" make a huge difference? It would require opening up a drywalled cealing and lots of labor repairing plaster. The other option would be a plug in heater but would not solve my a/c problems during the summer. Temp differences from the living room to bedroom are 70 degrees vs 60 degrees. Half my basement is unfinished and I ran 6" to other rooms with dramatic improvement but those rooms have a return.