How often should I replace the batteries in my CT3355 thermostat?

You should replace the batteries once a year to prevent the thermostat and heating/cooling system from shutting down due to lack of battery power. As an additional precaution, change the batteries before leaving home for longer than a month. Only AA alkaline batteries should be used, as non-alkaline batteries do not last as long and can leak, causing damage to the thermostat or wall surface. (Page 9)