I have an old 3 ton AC unit on the roof. My house is about 1750 sq ft. Last year, the AC unit stopped working. It could never cool the house down once temperature shoots above 80. I had an AC company come by and added refrigerant thinking that's the problem. It seemed to get worse after the refrigerant was added. Now I have a quote from an AC company for a new AC unit and the cost of replacement is about $3000 which I think is reasonable. He also said if I replace the coil inside my furnace, I would get better efficiency from the new unit. Is that necessary? Do I have to replace the coil along with the AC unit? The AC unit is about 40 years old. The coil may only be 20 years old. Thanks!

Today the minimum efficiency you can buy is 13 SEER. Yes, you need to replace the coil.