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Ralph Mowery

In article ,
says...

a) Imagine a programmable thermostat that wiped-and-reset at each power failure.
b) Furnace/boiler power can vary from micropile DC up to 24V AC.
c) Imagine an expensive device that could be spiked very easily from many sorts of causes.

Hence the need for batteries. All the on-board relay does is switch the AC to the unit. Furnace/Boiler power is entirely isolated from the thermostat power.

Not to mention the additional complications with 5 and 7 - wire heat-pump systems.

Peter Wieck
Melrose Park, PA

If designed correctly the thermostat should hold all its memories during
a power giich or fail without depending on the batteries. It should
have some short back up battery or 'super capacitor' to keep the time
for a few hours or more.

I would want it powered by the furanace for the problems stated above.
Dead batteries and none in the house late at night or snowed in.