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continue our conversation.First off the existing return is way undersized even if it would work the way the installer claimed. Of course it won't since all it would take is closing a door between the return and the room for the single return to be useless. Also just 2 6" branch ducts to handle 4 registers is also grossly undersized. I have no Idea why a downdraft unit was used since it's not suitable for this application installed on the main level. Have you contacted the general contractor and/or the HVAC contractor. This entire ducting arrangement needs to be reworked and they should foot the bill. If you don't have any luck with them your next step might be to contact a lawyer since the work required to fix this is probably going to cost more than the small claims limits in most States.