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?

In general I agree.

Cold Air sinks, so in the summer pumping all the cold air upstairs makes sense and the basement will still be comfortable.

In the winter the warm air will rise, so there is less need to pump it upstairs....  I basically do that on my 2 story plus basement and it is comfortable on all floors



One concern about the returns.... you don't want to block off the returns a create a situation where the fan is working too hard to move air around.  I think I'd leave the returns alone and leave the return system properly balanced.


ETA:  Your really pumping heat into or out of the house.  So from an efficiency standpoint cooling cool air to push it upstairs is not really hurting you that much.  The difference is not that great to impact how much heat transfer you are getting out of the coils.

Leave the returns open.

You can shut off the lower vents in the basement to force more air upstairs. The vents in the basement are most likely closest to the unit if it's installed in the basement as well. So, air wants to go the path of least resistance which would be your basement vents. Close them down some and force more upstairs.

Ideally you would have a test and balance done by the HVAC contractor, but that's rarely done. It's common in the commercial world to set the proper airflows to each space.

In the summer, I move the supply vents to an almost-closed position to keep the basement from getting too cold.
They're open enough to keep the air circulating, but not enough to freeze you out.
In the winter, I open up the supply registers.
I also run a dehumidifier when heating season is over and turn it off when it starts back up.

Same here

Thanks for the replies. At this point I feel more confident I'm not doing anything too dumb. I'll just leave all the returns unblocked, then.