Makes a difference if you have a two storey house with the unit in the basement. It works fine for heat (warm air rises) but not for cooling. My furnace couldn't push much of that cool heavy air up fifteen feet to the second floor, and they told me it would wear out the blower motor trying. I ended up putting another a/c unit (but no heat) in the partly finished third floor attic, to cool the second floor. It is mounted horizontally, lying right on the attic floor. Works great. BTW, I was told that it was all right to close the vents, but make sure to NEVER block the return air registers.
No problemo on the upstairs unit, since the return air is on the CEILING.
Lindy