Dual-hose portable air conditioners bring fresh air in, cool it, and return hot air outside through a separate hose. Single-hose systems use one hose for both intake and return air, so they’re not as efficient. The Midea Duo is a dual hose unit that uses a special hose-in-a-hose design to simplify installation and increase energy efficiency and cooling capacity. All of the other units on our list have a single hose system.