What type and how many batteries does the TH8320U thermostat require?

The thermostat requires three AAA alkaline batteries, which are included with the unit. The batteries are installed in the back of the thermostat, and you must make sure the positive and negative terminals are oriented correctly as marked on the device. Batteries are only needed if you are not using a 24 Vac Common wire to power the thermostat. (Page 7)