On Sunday, September 29, 2019 at 7:21:58 AM UTC-4, Davej wrote:
On Saturday, September 28, 2019 at 1:25:15 PM UTC-5, rbowman wrote:
On 09/28/2019 10:11 AM, Davej wrote:

If an HVAC system was "appropriately" sized, what maximum duty
cycle should be seen? [...]

Does it maintain the desired temperature on hot days? If it can't
it's undersized.

But what is a "reasonable" desired temperature? Is 20F less than
outdoors considered "reasonable?" So I should be able to maintain
75F indoors at 100% duty cycle if the outdoor temp is 95F?

Yes. Like Gfre said, it should be able to keep the house at a comfortable
temp on the hottest day that you could expect. NYC area for example, that;s
about 100F, so it should be able to keep the temp at say 76F when running
100% of the time. I'd want it sized a bit more than that, and typical
installers are going to make sure they err on the side of it having a bit
more capacity instead of less. The benefit to them is they won't have
customer complaining, it would require changing eqpt to fix. For the
customer there is benefit in that if the system has been off and the house
is hot, it can cool it down faster. So, oversizing it a bit isn't bad,
but if you oversize it a lot, then it won't run enough to even out temperatures
in the house, it puts more stress on the eqpt with more starts and stops, etc.

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.

What's going on in the attic? Properly ventilated and lots of insulation?
IF that's not right, fixing it, adding insulation could be the best money
spent. There should be adequate exit and entry for air, eg ridge vent and
soffit vents. Make sure the soffit vents aren't blocked with insulation.