Most HVAC systems start telling you they're done well before they actually quit. Bills creep up, the system runs longer to hit the same temperature, repair bills stack up, and rooms that used to be comfortable aren't anymore. Once a system is 12–15 years old and starting to need real money in repairs, replacement usually pays back faster than people expect — both in monthly bills and in not being without AC the next time it fails on a 95-degree day.