Thanks for any help you experts can give. I have a 2 story house approx. 1900 sq ft total. I have supply vents on both floors, two returns on the first floor, none on the second. All of the bedrooms are upstairs and have approx 1 1/2" gap at the bottom of the door. I'm experiencing hot temperatures upstairs in the summer and much (5-10 degrees) cooler in the winter. I'm wondering if installing return vents upstairs will help with this temperature variation. I understand that I may need to reduce the opening area of the return vents downstairs in order to keep the system balanced. Thanks for everyone's input.