If there's one thing you can do that will guarantee reduced efficiency, higher maintenance, and shortened equipment life—well, OK, there are numerous ways, but let's ignore the others for the moment—it's to put your air conditioner in a blast furnace, that is, in the attic. How hot does it get in an attic on a hot summer day? Well over 100° F, sometimes even 150° F! How cold should the air be coming out of your air conditioner? About 55° F. 
Oh, but that's why we have shiny insulation on ducts, right? To keep that air cold as it travels perhaps 50 feet or more through that blazing hot air, the building code requires insulation with an R-value of 8. Well, the truth is, putting an air conditioner in an attic is one of the worst places you can put it.
