I have a newly constructed two-story house with a Lennox system. It's been fairly hot lately, in the high 80s. The system keeps downstairs at demand temperature. As you climb the stairs to the scond floor, you can feel a temperature change about half way up the stairs. The bedrooms upstairs are very warm. There is a return upstairs and the vents are blowing good cold air, but it is still too warm there. Should vents be closed downstairs? What's the solution?