Yong Huang wrote in news:238ca902-6a55-4281-a150- snipped-for-privacy@s31g2000yqs.googlegroups.com:
Can you install an extra duct from the furnace to the far bedroom with an extra register? That will route more warm air to the far room than anything you can do with register balancing. If you can't install a new duct then replace the old duct with a larger one and use a larger register. Also, insulate the ducts to that bedroom so the air will be as warm as possible when it gets there. Any insulation will work - I use fiberglass batts - wrap around and use supports for the bottom.
You may also need to install an extra return air duct from the cold room. You can't get more air into a room unless you take more air out. Usually there is enough capacity to let the extra air out of the room but if there is a tight door and the walls are tight then more return air capacity will be required to get the benefit of the new warm air duct. It is a system and all the parts need to work together. Cheapest solution may be to install a grate in the door or interior wall of that room to let the extra air out. Or just cut off 1/2 inch from the bottom of the door.
Register balancing is difficult because when you restrict the air flow into one room the excess air then goes equally to all other rooms. The excess air won't go to a particular room unless you run around the house and restrict all the other registers. That will take a good long while to get balanced and then sooner or later someone will come along and undo all your hard work by opening up some or all of the registers. If you restrict any registers make a mark or scratch on each one so you can easily see if it has been moved and can re-set them without repeating the whole exercise.