We have a new house (<2 years) and the basement had several supply side heating ducts installed off the main line. There are four in total, one for each quadrant of a generally square basement. I believe they are 6 inch diameter, and at present use a very simple inline damper to stop flow (not even close to stopping the flow, almost 1/2" gap all around, but that will be replaced with a normal round damper & diffuser once finished.) The whole house at present is one zone, thermostat is on 1st floor of 2 floor house (with basement will be three).
I am wondering if air return lines are needed when we finish the basement?