Why in the world are thermostats battery powered?

[Paperdoc]

In the units I have had and seen, all furnaces (with or without A/C) have a small transformer to supply 24 VAC power to their relays for their controls.That same power source is sent up to the thermostat, which switches the power onto return wires to call for heating or cooling. Thus the thermostat HAS a 24 VAC power source it can use. And it DOES. It uses its own electronics to derive a small low-voltage DC power supply for its circuits to operate AND to maintain power to its programming and clock circuits. The ONLY function of the battery is as a backup for those clock and programming circuits, maintaining their data when the AC supply from the furnace fails. In those conditions the furnace cannot respond to any calls for action from a thermostat, anyway, so the thermostat actually does NOT operate when the furnace fails to supply 24 VAC power - it only maintains its data (via battery power) until external power is restored. It used to be that the batteries used in the thermostat for this were AA or AAA units because they are so easily available and cheap, and they have significant storage capacity and hence lifetime in this low-use application. More recently some use other smaller batteries like the "coin" style for their small size, and to match better the voltage required in many current printed circuit applications. But the basic power supply design concept is unchanged.

WIRELESS thermostats are a different matter, of course. The units NEED a power supply that is NOT fed from the furnace - the whole idea is NO wires. So, consistent with that, there are no wires between the thermostat and ANY external power source. Hence, battery power only. PLUS this style must operate a transmitter to send out signals, so that impacts the power required.