If I have an older system, can I save money by just replacing the outdoor unit?
Probably not. Most older systems use Freon R-22, which has been phased out and replaced with Freon R-410A, also known as Puron. Puron is more environmentally friendly and more efficient. In addition, the minimum SEER rating or efficiency that we can use today is 13 SEER (most older systems are 10 SEER and below). The refrigerant and efficiency rating of your indoor coil and outdoor unit must be the same, so you have to replace the whole system at once.