We have a two story house that is about 1,100 sf downstairs and 1,200 sf upstairs which includes a 12'x20' bonus room. We have a nine year old Carrier Comfort Zone system. The system has one outdoor 3 ton unit and 3 zones (upstairs, downstairs, bonus room). The house has very little shade and gets sun most of the day. When the outside temp is around 90, the system runs all day (from about 10am to 10pm) maintaining 2-3 degrees over the thermostat setting. Does this sound normal? I've had the charge checked and the coils have been cleaned. Also the bonus room is over the garage and has a attic/storage that runs parallel with the room. This room gets all of the afternoon sun and is very difficult to cool. It has its own return but only has one vent which to me seems awful small for this size room. So my second question, can bonus rooms like mine be difficult to cool and should I look at other cooling methods for this room ( a friend recommended a hotel style a/c)? Thanks in advance for any input.

Location: Denver, CO - current daytime temps in low 90s, low humidity. Central air unit is icing around the large line, both inside at the cooling coil and once outside at the compressor housing. Cleaned cooling coil, though there wasn't much there. Replaced filter. Unit turns on, runs and can be turned off just as if nothing is wrong, but will run all day because it just isn't getting the job done. Turns off at night, but I suspect that is because the ambient temperature drops here once the sun goes down. The duct work above the furnace feels very cool when it is running. The small line feels very warm, and of course, the iced up line is very cool. Unit is 10 years old and has never had a problem cooling the house even though the airflow has never felt particularly strong. House was new when the unit was installed. Window coverings in 2 upstairs rooms were recently removed and those rooms, doors and cooing registers, have been shut off pending receipt of new window coverings. Does the icing symptom indicate any normal failure, or is that upstairs area just getting too hot for the unit to cool the rest of the house?