We have a Carrier 38CKC060300 a/c unit and our house is approx 2900 sq ft with 10-11 foot ceilings. Our house has not cooled below 78 degrees since June and our utility bills have been between $700-$800 each month. We had it serviced and were told that the unit is fine and that it's just been too hot... is this correct? We're melting!

Well if it checks out to be good and it has been too hot,next thing to look at is the insulation, is it good or better than good,was the duct system inspected to insure no loss of air?
If not needs to be looked into