The answer to 'when to replace thermostat' is that you should replace your thermostat every 5-10 years, or sooner if it's not functioning properly. Thermostats can wear out over time, especially with regular use, and may become less accurate or responsive. Replacing an old thermostat can improve the efficiency and performance of your HVAC system, helping to keep your home comfortable and save on energy costs.
It's a good idea to have a professional HVAC technician inspect your thermostat and provide recommendations on whether it needs to be replaced. They can also help ensure the new thermostat is properly installed and integrated with your heating and cooling system.