Davej wrote:

I hate to interrupt the ongoing political trolling but...

It's ok, as long as you occassionally chime in so we know where you
stand...

If an HVAC system was "appropriately" sized, what maximum duty
cycle should be seen? In other words if my upstairs AC runs
continuously -- that probably means it has a problem or is
undersized.

Air conditioning is a funny thing.

My AC can run continuously if the outside temp is 72 and my interior set
point is 70 but the outset humidity is 65% and I want my house humidity
to be 40%. In that situation my interior temp might be 71 and the air
blowing out of the outside condensor unit is luke warm.

I'm pretty sure that the higher the temperature the air blowing out of
your outside condensor unit, the harder your AC is working. It also
seems that the lower the temp difference between outside and house air,
the cooler will be their air blowing out the condensor unit.

I'd say that half the time (from May through to end of September) I use
my AC to lower the humidity in my house vs bring the house air
temperature down. I think that a proper or useful HVAC control system
(ie wall thermostat) should give me the ability to set a set-point for
humidity control to turn on and off my AC in addition to the thermostat
to set set-point for temperature. I've never really looked, but I get
the sense that no off-the-shelf residential thermostat allows for that.

A variable-output furnace is more clear-cut in terms of judging size /
efficiency vs duty cycle I'd say.

So my take on your AC being sized correctly is to ask the question when
your house air temp is not at your desired temperature set-point AND the
air blowing out of your outside condensor unit is really really hot. In
that case I'd say yes, your AC is probably undersized for that
condition.