Having only a hot air exhaust means the A/C is sucking a lot of outside
air in through air leaks here, there and everywhere. All that hot outside
air has to be cooled, so it really loads the system. A/C's are usually
set up to recirculate on the interior (cooling) side, not to draw from
outside like an automobile A/C. The unit needs to have an outside air
intake duct along with the hot air exhaust duct.
Al Grayson