Mike S. wrote:
I have central air in my house and the unit and everything is about 20
y/o. I realize it should be updated but I have no control over that.

The problem I have is that if I set the thermostat to to 70 degrees
fahrenheit, then the downstairs stays about 70. But the upstairs will
be anywhere from 76-80, depending on the room and how hot it is
outside. So the problem is that people complain that it's too cold
downstairs and too hot upstairs. Ideally, I would like it to be about
76-78 downstairs and about 74 upstairs.

Why does this happen and is it normal? I know that a lot of air blows
out in the vents downstairs and not a lot blows out upstairs. I shut
off all the vents that aren't needed and close the doors to rooms that
aren't used. I have the fan set to "auto". I don't know if leaving it
on auto is better than setting it to "on". I don't like to mess with
it because it seems to cause problems with it.

Any opinions or suggestions (other than buy a new unit)?

You are on the right track. Follow the advice Ed provided. I will only
add this. The real answer you have ruled out in that you may need changes
to the distribution (supply and return ducts) system and maybe increased air
handling capacity. That requires some measurements and math. There are
standard formulas for that. There is only so much you can or should do with
closing off vents. Close too many can you can cause damage to the system.

You may be in a no win situation as the lack of the ability to make
serious changes to the system may rule out the only real answers.

--
Joseph Meehan

Dia 's Muire duit