This can be very difficult decision without being able to see the future. The short answer is to replace it before repair costs become exhorbitant and reliability becomes inconsistent. The only thing worse than replacing your system too early is spending too much on repairs and then replacing your HVAC system too late. Reading the following guides will help:  Knowing When It’s Time to Repair or Replace Your HVAC System and Common Problems that Usually Mean It’s Time for a New HVAC System