I recently jumped into the wifi smart thermostat game and ended up installing a Honeywell RTH9580WF. The thermostat requires a C wire to be hooked up to your thermostat and your 24V AC transformer in your furnace. I initially installed a Nest thermostat, but after a week of use my Nest base unit shorted the red and green causing the fan to run non-stop. I’ll talk more about this in another blog post.

I spent a ton of time trying to figure out what exactly should be happening on the C wire. It is called C because it is a common connection. What that means is it provides a path to ground on the transformer. The thermostat grabs power from the red wire which is the “hot” lead on the transformer and it uses that power to power the thermostat. The Thermostat needs this to power the wifi and the always on LCD screen, standard AA batteries are not enough.

If you put a meter on the C post on your furnace you will not get a reading unless you are testing the red along with it. It is not hot. At first I thought the C should be providing power but finally stumbled onto a diagram that explained things to me and made sense.


As you can see the C is a common or a ground. Even my Honeywell thermostat instructions stated the C provided power to the thermostat… give me a break… to me providing power is a a hot lead not a common ground!

Anyway hopefully you have up to date wiring coming from your furnace, otherwise you will need to update your wiring to have the C wire available to you. If you have five wires you should be all set for most common heating and cooling systems used today.