What type and size of batteries does the TH5110D Thermostat require?
The TH5110D requires two AAA alkaline batteries. Non-alkaline batteries should not be used because they do not last as long and can leak, causing damage to the thermostat. Always use fresh AAA alkaline batteries for best performance. (Page 13)
From the official Honeywell TH5110D Home Programmable Thermostat manual, page 13
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