As others have mentioned, there can be issues with running without a C wire.  Their &quot;power stealing&quot; technique did not seem to be effective for keeping my first-generation Nest from running out of battery.  (Although there may be some variables in my situation that contribute to the problem.)  I blogged about it here: <a href="http:&#x2F;&#x2F;cafbit.com&#x2F;entry&#x2F;nest_learning_thermostat" rel="nofollow">http:&#x2F;&#x2F;cafbit.com&#x2F;entry&#x2F;nest_learning_thermostat</a>