The recent and awesome HVAC thread by ZW17 reminded me of this so I figured I'd see what arfcom has to say...it seems no one can give me a solid answer or, if they do, the reasoning never seems sound.
I have a 3100 sq ft rancher (upstairs) with a ~3000 sq ft, unfinished, uninsulated walkout basement (so, ~6000 total sq ft) - all but a small portion of one wall in the basement is underground. It stays pretty cool in the summer and warm...ish in the winter.
The upstairs is big enough for the whole family so I'm the only one who spends any significant time down there for the most part. What I can't determine is whether or not I should be opening or blocking the vents in the basement and/or the returns in the basement. My goal is simply efficiency - I don't need to control the basement climate for comfort-sake.
My theory is that, in the summer, my returns should be open and the vents should be blocked. My reasoning is that I will be pulling naturally cooler air from the basement and cooled air that has seeped from the top level into the system lightening the load while providing maximum output to the upper level. My winter theory is based on the same reasoning and is, therefore, just a reverse of the summer theory - open the vents in the basement and block the basement returns so that (by my reasoning) only less cool air is entering the system.
Any thoughts?