The answer to 'when to replace thermostat' is that you should replace your thermostat every 5-10 years, or if it is no longer functioning properly. Thermostats can wear out over time, especially if they are older analog models. Newer digital and smart thermostats tend to last longer, around 10 years on average. However, any thermostat that is not accurately controlling your HVAC system or is difficult to program should be replaced, even if it's less than 5 years old.
Replacing your thermostat can improve the efficiency and performance of your HVAC system, helping you save on energy costs. It's also a good idea to replace a thermostat if you are upgrading to a more advanced model with features like programmable scheduling, WiFi connectivity, or energy monitoring.