The answer to this question often depends on the equipment you have, how well it was installed, and how well it has been maintained. Home builders often use lower grade “builder models” offered by otherwise well-known manufacturers (including Trane, Rheem/Ruud, and Lennox). Builders’ models may not last much more than 8 to 12 years, whereas premium models from these companies will typically last much longer. If you believe you may have a builders’ model, you may want to consider taking advantage of rebates and warranties offered with a new systems once your equipment is 10 or 12 years old. Oftentimes, utility bill and repair savings alone will more than offset the cost of a new high-efficiency system.
Regular professional maintenance is an important factor, too. Even lower quality equipment that has been frequently serviced and maintained can last 20 years or longer. Many times the proud owners of such equipment will decide to replace the equipment in order to obtain equipment that offers improved efficiency or new features, not because the equipment is failing. Because of our extreme Central Texas weather, premium equipment that has not been maintained will typically last between 12-18 years. However, because of efficiency improvements that are continually made by manufacturers, once a system is 10 years old it may pay to replace the system because of the higher efficiency (and lower gas & electric bills) and rebates available for newer equipment.
Finally, how well your system was installed plays a major role in how long it will last. Systems left with even small amounts of water vapor in the refrigerant lines or ones that have too little return air flow will often have serious malfunctions before they’re even five years old.