On Sunday, September 29, 2019 at 5:10:48 PM UTC-4, Davej wrote:
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.


I've seen exactly that problem here in new construction, large McMansions.
You'd think by now they could correctly size these, there are programs
they are supposed to use where you enter a lot of detailed info, like
type of construction, insulation, number/size of windows, which direction
they face, etc. But I think many of them, maybe most just wing it.

Having the AC in the attic is a disaster too. They use those flex ducts
that only have maybe 1.5' or so of insulation, they get hung with straps
or whatever that eventually crushes them. Would be a good idea to check
up there for any of that, for leaks around connections, etc. It's
possible the system isn't charged correctly too, you could have it checked.
It's possible they could run larger ducts or add returns etc, without
a lot of work or eqpt change. It's also a good idea to have a pan under
the AC so that if the condensate drain gets plugged for either the furnace
or AC the water will go in there and to have a float switch in there that
will cut off the eqpt if it fills with water. They have battery water alarms
for ~$10 that you could put in there too.

One quick rough check you can do on the AC is measure the temp delta between
the air coming in and going out. About 18F or so is normal, but it also
depends on the blower speed, humidity, etc. If you're seeing 10F,
then something is likely hosed.