A:Answer A C-Wire is not required in the Nest thermostat (as it is required in the Lyric Thermostat) because Nest has managed to have the unit pull power from one of the other wires ... in other words, Nest has found a way to get just enough power from the existing setup without a C-Wire. Although having a C-Wire makes it very easy to install and operate. The Lyric, on the other hand, requires a separate C-Wire connection, or you must use highgrade, and expensive, lithium AAA batteries, which can be costly and have to be replaced at varying rates depending on your heating/cooling system. I had the Lyric and had to replace the battery every 3 weeks. It was annoying and expensive. The Nest, though, has worked with no problems now for three months straight using the existing wiring on my home system and without a C-Wire