Cool air flows downward. Thus many people wonder whether they might be able to install a non-split ductless system on their second floor and allow that air to just flow naturally downwards. This would conceivably save the expense of having to install a mini-split system that has different outputs on the two floors. In theory this might be true, but in practice there is likely to be too little flow to make the downstairs comfortable.
There are a few situations where this non-split ductless setup might work, such as:
1. A largely open second floor that is more of a loft than a true second story so that there is a wide area for the air to flow downward – in these set-ups a long railing often exists on the second floor that overlooks the first.
2. A second floor that is connected by a stairway that is open and leads to the primary area you want to cool on the first floor. This may not be ideal but might at least provide cooled air where you want it most.
3. A home where the daily activities mostly take place on the second floor – sometimes called “upside down houses”.
4. A home where the first floor is actually fully or partially underground so that it tends to stay cool.
Most other homes that have second floors, and where you want your ductless system to fully cover both floors with comfortable central air conditioning will require a ductless mini-split system. It is likely that as opposed to leaving the window boxes in one floor you will save money with a mini-split system in the long run.
(Some people also wonder whether they can strategically position fans on the floor with ductless in order to send the cooled air to the other floor. This tends not to work unless you are merely magnifying the effect of one of the more favorable layouts described above.)