The answer to 'when to replace zone control system' is that you should consider replacing your zone control system when it is over 10-15 years old, or if it is no longer functioning properly. Zone control systems have a typical lifespan of 10-15 years, after which they may start to experience issues like inconsistent temperature regulation, frequent breakdowns, or incompatibility with newer HVAC equipment.
If your zone control system is older than 10-15 years and you are experiencing problems, it's generally a good idea to have a professional HVAC technician evaluate the system and recommend whether a replacement is needed. Newer zone control systems can offer improved efficiency, better temperature control, and more advanced features that can enhance the comfort and energy usage of your home.