On Sun, 29 Sep 2019 15:06:09 -0700 (PDT), trader_4
wrote:

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.


Attic installation is the norm here and you are right, flex duct sucks
but you can make it somewhat better if you blow in insulation around
them. It is also standard practice for good HVAC guys to "mastic" all
of the joints. (Sealed with a goo that gets firm but not brittle/hard)
They slather it on pretty well. The other thing is keep the runs
straight and not squeezed in any way. The best solution is to lay
1x12s down and run the ducts on them across the truss chords so you
don't have a roller coaster for the air to go over. Running with the
truss is OK if it is flat. Best is to build plenum boxes everywhere
the ducts make transitions or 90 degree turns. I knew a guy doing his
own that just used flex to sleeve round metal duct that he assembled
in the attic and a box at every turn. He said it allowed him to use a
smaller blower to get the same air flow and the payback was less than
one season.
The aux drain pan is code here.