I bought a Rheem 3.5 ton unit about a year ago for my 1880 sq. ft. house. Today it is 90 degrees outside and I can't get it below 73 in the house. The AC has been running continuously for about 8 hours. I've got it set to reach 72, but it can't get there. My old unit (~25 years old) that was replaced could cool the house to the mid 60's pretty quickly. The new unit seems a lot less powerful. Does anyone know if this is normal for the newer units, or does it sound like I've got a problem?