What type of batteries does the TH5110D use and how often should I replace them?

The TH5110D uses two AAA alkaline batteries. You should replace the batteries once per year as a precaution to prevent the thermostat and heating/cooling system from shutting down due to lack of battery power, even though the thermostat has a Replace Battery indicator that flashes two months before batteries run down completely. Always use fresh AAA alkaline batteries, as non-alkaline batteries do not last as long and can leak, causing damage to the thermostat. (Page 13)