I have a 1200 sq ft ranch house here in Kansas City with a 25 year-old Carrier unit that has given me no problems in the 6 years I've owned the house. My highest electric bill last summer was $130 but the age of the system is concerning me. I've got one estimate to replace the A/C with a Lennox XC14 16 SEER unit (and add a service disconnect) for xxxx. I plan on getting a few more estimates just to get an idea on what this is going to cost me. Should I be that worried about the age of the unit now or just take a chance and wait until old faithful dies...any ideas?