i am in living in my parents old house in IL. it is 900 sq ft. has a whirlpool a/c from 1976 or 79 looks like a horseshoe. it cools but not that well (i looked at the a coil and it needs to be cleaned)is it worth cleaning or better to get a new unit? will a new unit cool better then a old unit? i think it will be more effiencient for i am looking at American stander 12seer 1 1/2 ton . i used(hvac cal residential 4.0) to get heat gain and got a heat gain of 17954 (1.5 ton) is this a good unit and the best thing to do? thanks for any input jim