1100 foot square home with an unfinished basement and a forced air furnace. All the upstairs vents are fed by 6" round ducts from square mains. Two bedrooms are fed from an 8 X 9 square duct. To the side third bedroom is fed directly from the above the furnace. In the other direction, the main living area is fed by a 14X8 duct with 1 6" round branch, then tapers down to 8 X 9 where a further three 6" branches come off. At the end of each of two runs I cut a vent directly into the duct to get some heat in two of the the corners of the unfinished basement.
So I now have heat in two corners of the basement. If I wanted to add more heat can I put a splitter in some of the 6" runs to the upstairs vents without loosing to much heat up there? Or run new 6" round ducts to each end of the trunks. Or what about splitting the run to the third bedroom that’s fed directly from the furnace (which always gets too hot anyway having only one exterior wall and a lot of computer equipment, and the I close the vent all the way in the winter). All the basement vents are shut during air conditioning season because it stays cool down there without them.
Been looking online and it seems there’s no simple answer to "how many specific sizes of vents can I have on a specific size of duct.
