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Hi Bob, It is the SA (supply air) register design that puts the air into the room where it's needed called the "throw" of a register. A system design with the correct register throw design can have a ceiling return. Alot of the newer homes are now running balanced HVAC systems much like the commerical HVAC systems. This means that total CFM flow of a system is distributed evenly throught the duct system to the heating or cooling demand of each room. Placement of the registers in this type of system is critical to correct system orperation. With the above in mind a HVAC contractor most likely designed your system to only handle the finished living space at the time of construction but sized the indoor air unit so a new fan or fan motor could be installed at a latter time when the basement was finished. To add additional SA registers off a existing balanced system would starve the total system of air by about 11% or a 11% drop of SA cfm in each room. Here in AZ you would not feel comfortable in a room with a 11% drop in cfm during the peak cooling season on a typical balanced HVAC system as most contractors undersize system requirments to begin with. T_Bone
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