I have been having problems with Trane AC unit cooling my 1277 square foot-2nd floor apartment. It cools it during the day, but it gets up to 80 degrees at night when it is 90 degrees outside and my thermostat is on 74. The people underneath me do not have a problem and their bill is about 260 during the summer while mine is about to be around 500. By the way they keep their unit on 75. I have two vaulted ceilings but they are not the high vaulted. The serial # is Y18212817. From previous postings that I read it was made in 1985! The apartments claim their is nothing wrong with my unit (2.5 ton), the insulation is fine, filters have been changed, digital thermostat was installed. Ceiling fans installed in the living room and one of the three bedrooms. Curtains are up on all of the windows except for the little kitchen window and my sons bedroom window (1). I am not sure what else to do to get the apartments to take care of their equipment. By the way they made me pay for my own ceiling fan.