I just replaced two 23 year old Rheem systems with Carrier Infinitys. I was very impressed with the install but the water drain from the coils got me thinking. The original system had closed PVC lines but the new system has an inline switch and an open pipe at the top. The switch turns off the unit if water backs up and the open pipe was explained as an overflow rather than have water back up in the coil unit. Sounded logical at the time, but now I see that the open pipes (and I guess the old closed pipes that vented to outside) pump out conditioned air. On the downstairs unit, I guess that's okay because at least that conditioned air eventually finds its way into the living area but the upstairs unit is between the ceiling and roof, so the vented conditioned air is wasted.
Seems strange with all the emphasis on efficiency these days that there isn't a way to not pump conditioned air where it does no good. I'm not too concerned since I now realize the old systems were both pumping that air outside, so the new layout is at least some improvement in that regard.