I have a maple chase thermostat about 20 years old. The batteries (2 AA) used to last over a year. Noe only 2-3 months. What's wrong?

Short or corrosion is draining the batteries. Tstasts are inexpensive and after 20 years I would replace.

My old Maple Chase has spaces for 2 batteries but only uses one. The second space is so you can put in a new battery before removing the old in order to prevent the stat's program from being lost. The instructions specifically said not to install two batteries (for what reason, I don't remember). Your stat may be a bit newer & requires both.