On Sunday, September 29, 2019 at 7:49:36 AM UTC-5, trader_4 wrote:
[...]
Another factor, you say the upstairs is the problem. I assume
that means that downstairs the temp is OK? If there are two units
and the one for downstairs isn't running, the temp there is higher,
that will increase the load on the upstairs. Another common
problem is that with one system for a two story house, it's common
for installers to screw it up, eg not enough returns for upstairs,
which makes it impossible to cool it properly. Without two
systems, I think about a 3 deg temp diff is typical during the day,
when it's hot.


Well, I think that is part of the problem. The upstairs gets hotter
and so the upstairs AC runs far more than the downstairs AC, however
the downstairs AC is definitely a larger unit. The downstairs of the
house does have more square-footage but there is an open staircase
so I'm sure some warm air flows upstairs. This makes me wonder if
the upstairs AC should be the larger unit.

The upstairs AC unit also suffers from the disadvantage of the
ductwork being in the unconditioned crawlspace attic.