I have a 1974 2-story house with a half basement that is heated with an oil-fired forced hot air system. There are 3 bedrooms on the second story. Two of them can get ridicculously hot while the third is rather cool. The 2 hot bedrooms have 2 registers each while the 3rd cool room (the smallest of the 3) has 1 register. I have tried closing off some of the registers in the other bedrooms and downstairs as well to push more hot air upstairs to the cool bedroom, but that hasn't helped. BTW, all the windows have been replaced with high-efficiency models.
Any thoughts? Could the ductwork going to that 3rd bedroom be clogged?