A white-rodgers generic cheapie that came with the new furnace 3-4 years ago, specifically. Takes AA x2. New duracells (with use-by dates several years in future) seem to go flat in 3-4 months, and the thing loses all the settings. Doesn't seem like there should be a lot of juice used driving a couple chips and that tiny LCD.
Given that power is available from the furnace, why do these things need more than a button cell for memory backup, anyway?
-- aem sends, adding another item to the shopping list for tomorrow...