I'm planning use a window type A/C on my little attic room. My A/C unit is just .5 HP, just enough for the small room. The problem is there's no proper exhaust area like the standard Window type. There's an empty space on the side that can be used as space for its backside/exhaust. The empty space is a above the ceiling of the house (not the attic) (the attic is on the 2nd floor used for some storage). The empty space is enclosed within the roof, so no constant "fresh air" coming in. It's used for maintenance way to the roof.

Split type AC are not an option.

I just wanted to use this spare A/C unit. The downside is the exhaust is above the ceiling of the house, but the space is the ceiling is so huge. Even 10+ times bigger than the room. Is this possible? or any downside? I mean the best option was to have a split type Aircon, but for now I want to use the extra A/C for the attic.

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    How do you plan to handle the condensate? – longneck Sep 10 '15 at 12:37
  • You either need a window or a hole in the wall "like" a window to use a window-type air conditioner - and the other end of that hole needs excellent airflow and a route for water to leave without damaging the house, which generally means it needs to be outside, just like the outside of a window. A split-type AC is not only an option, it's your only option, unless you have a window leading to the outside you can put your window AC in. – Ecnerwal Sep 11 '15 at 3:47
  • there is inside "pre existing" gutter for water to handle the water coming from the AC , it was used previously as a catch for the leak in the roof. – sharky Sep 11 '15 at 13:25

I think you'd find that the heat built up and leaked right back into the living area, unless the attic area was actively ventilated. That's where a lot of the heat in your unit is coming from in the first place, after all. And pumping heat with an A/C is less efficient if you're pumping it into an even hotter space.

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