Hi. I have a 4 year old, 2 1/2 ton Trane air conditioner that is running constantly when I set it at 76 degress or lower. It really had a hard time cooling when temperatures hit 100+ last week. I have a 22 year old Carrier furnace. My house is a little over 1300 square feet. I had a service technician tell me that the problem is with the old furnace, not the air conditioner. Do I need to have the furnace replaced as he suggested? Also, if I need a new furnace, does it have to be a Trane or can I use another, less expensive furnace.