We have central air, and the upstairs stays around 85+ (in 100%
humidity) despite the temperature downstairs staying at 75 or less (ie,
whatever the thermostat is set at). IThe upstairs has two fully-open
vents through which little air blows, and what appear to be 2-3 vents
for the warm air to exit. Can anything be done to help cool the
upstairs? The curtains/blinds are kept closed, and the door leading to
the upstairs is kept shut to prevent any warm air downstairs from
moving upstairs.

Also, attached to our air conditioning system in the basement is an air
cleaner that doesn't work, and when you remove the cover from that, a
ton of air is pulled in. I've noticed that this causes a lot more cold
air to be forced through the vents, substantially cooling the
upstairs. However, we asked someone who would know about these things,
and he said you shouldn't do that (for some reason). Any information?