My three story city rowhouse (think large, old and leaky) has two CAC high velocity systems that were installed by the previous owner. The larger system cools the first two floors and seems to do so adequately. The smaller system cools only the top floor and it's always hot up there (never seen it below 83 degrees, even when the CAC has been running full time and set at a lower temp for hours and hours and hours.

It has been serviced and the filter is regularly changed. The CAC guys say there's nothing wrong with it.

I am wondering if between the hot air rising to the top of the house and the heat pounding down on the flat black roof above, it simply can't keep up. Since most of time there's no one up there, I am wondering if a practical solution would be to simply turn the system off and put a small window unit in my son's bedroom that he can turn on and off as needed.

Is there any reason to worry that the result would be to overtax the main system cooling the first two floors? Or anything I'm not considering here?