During my Home Inspection career recently, when I inspected a lot of new construction, I noted several different condensate arrangements. It kind of depended on the builder's spec, perhaps in concert with discussion with their plumber and or HVAC subcontractor, since the subs that actually install these things ONLY do what the builder has specified. So, here's what I've seen. Remember this is in answer to your "modern" construction question.
1.
Primary drain hard piped to outside, just a raw dump (there's that muck situation). This works with gravity so if the equipment is in the attic, great. In a crawl space, generally with a condensate pump. The pump always has a switch so if its small tank is full and cannot pump, the equipment shuts down.
2.
Secondary drain from a drain pan generally is piped to just above a window on the 2nd floor; the ostensible reason for this is you'll notice a dripping out of that drain, and then note your primary drain is not functioning. My guess is less than 1% of homeowners know about this. Many homes don't have this secondary drain, nor do they have a drain at all from the pan underneath the equipment. In these cases they'll have a float or other water-detecting switch to shut down the HVAC in case water is in the pan. My own opinion is this is kind of dicey, I've seen older switches totally non-functional, and also placed where it would take a LONG time for water to reach it. Not a great plan. I've also seen really haphazard hard piping from this pan, bad enough that it would not function as intended. Red Flag.
3.
Primary drain 2.0: this is when the equipment is in the attic, and the laundry room is on either the first or second floor. There is hard piping to
just above the washing machine drain standpipe. Condensate then drips into that; "just above" means it has the "air gap". In my work as a handyman I've heard people complaining about the dripping noise...
In my own home, the first floor HVAC is a condensing unit so it's always making water, and that originally drained to outside. That has the switch built in, so in case of failure of the pump, the HVAC shuts down. The second floor unit was originally hard piped to outside also, but no pump needed. As
@cgilley points out, that side of my home never dried out--ever. There was water dumping there year round. So my solution, reported here a few years ago, was to bring the upstairs drain to a condensate pump; re-route the downstairs unit to a drain trap assembly I added to my home's primary drain system (it actually ties into a smaller branch used for the washing machine drain). I used a 1.5" PVC trap into the main drain line, and put an air gap unit on top of the the trap. All condensate goes through the air gap, into a trap, and into the home's drain. Is it totally legit? Don't know, don't care. It works perfectly, it's trapped, it's got an air gap, and the side of my home is dry. The water flow is nearly constant when the downstairs unit is operating in any mode, and also constant when the upstairs unit is in a/c mode. It's not a LOT of flow, just a trickle, but it is constant. The second condensate pump was installed when the crawl space was encapsulated, and takes water from the dehumidifier. That pump sits in a plastic tub with a water alarm. Problem solved.
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