Is there any reason for a florida room to be 10-15 degrees colder than the room next to it? It does have 3 walls that are not connected to the house. These 3 walls are almost all windows. There are 3 floor vents in the room. The house is 10 yrs old. During the winter, we had to keep the doors closed because of the amount of cold air coming into the living room. The return vent for downstairs is about 20 ft away in the foyer. Any ideas?
Well you nailed it. without any further explanation, the fact that 3 walls are mostly windows is the reason this room is colder. You never mentioned the type of construction or the windows.
If this is a "seasonal" room, there may be no insulation in the walls at al and it is not really intended as conditioned space. This will be indicated by whether or not the windows are insulated glass or just single glazed.