To be fair, most "dumb" thermostats still require an external power source to continue to operate.
Very few actually pull operating current from the 24v C wire if it even exists on the given system. If it doesn't, R (the switched 24v power for Heat and Cold signals) isn't guaranteed to continually have current. Only when your Heat is turned on (probably a standard toggle lightswitch on the side of your furnace) will there be current on the Rh line, and only when your AC is enabled (possibly a breaker shunt on the side of your house near the condenser unit in a small box) will there be current on the Rc line.
Nest tries to recharge it's battery by trickling the C wire, if available, and if not it will try to charge off of one of the R wires, either during normal operation, or it will try and "pulse" the heat signal to pull a little bit of current to keep going. Thermostats were designed at a time where they didn't even consume any electricity on their own. We're trying to retrofit computers into signaling system, not a circuit.
The GP is right: most new thermostats don't take power from the 24VAC line. That surprised me when my heat wouldn't come on one morning because the battery was too weak to pull in the relay for more than a few seconds. That's what I get for ignoring the "low battery" warning! All my previous electronic thermostats only used the battery as a backup.
In any case, are you really saying that using a toxic metal (mercury), or an imprecise bimetallic strip is really an improvement over a simple $10 electronic thermostat?
I should have been more specific by stating the difference between a dumb digital thermostat as I was describing, and a truly analog thermostat like the Honeywell T87.
A dumb digital thermostat is just a thermocouple and a relay, which you could rig together with very little EE knowledge and a weekend with an Arduino.
Very few actually pull operating current from the 24v C wire if it even exists on the given system. If it doesn't, R (the switched 24v power for Heat and Cold signals) isn't guaranteed to continually have current. Only when your Heat is turned on (probably a standard toggle lightswitch on the side of your furnace) will there be current on the Rh line, and only when your AC is enabled (possibly a breaker shunt on the side of your house near the condenser unit in a small box) will there be current on the Rc line.
Nest tries to recharge it's battery by trickling the C wire, if available, and if not it will try to charge off of one of the R wires, either during normal operation, or it will try and "pulse" the heat signal to pull a little bit of current to keep going. Thermostats were designed at a time where they didn't even consume any electricity on their own. We're trying to retrofit computers into signaling system, not a circuit.