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.
Do you have a likely diagram for rth 9580 wf
With C K RC R WO/B Y G
I’m not following you. What diagram are you talking about?
From the blog
What is the C wire that is needed for a wifi ” smart ”
Posted on january 24, 2014
Someone stumbled onto a diagram with this title,
Thermostat / HVAC control unit,
and someone said ” give me a break, to me providing power is a hot lead not a common ground.
I just bought a RTH9580wf
and I have an oil furnace combined with an A/C, and I want to know where those terminals go to.
Does this help
Not really, but thank you.
I wish I could be inside my smart thermostat and undestand how he thinks.
The owner of these premises is asking for cool air, so let’s start the A/C through terminal O/B or Y … or else.
That is what I wanna know.
That guy on the bottom of the page mentioned this
I would connect the fan center R and C to the stat R and C.
Connect the stat Rh and W to the furnace T and T terminals.
Remove the jumper between Rh and R at the stat.
Otherwise I bet these guys could help you here
can you show me a video for setting the c-wire for Honeywell RTH9580WF because I’ve just bought one, please?
my instructions said connect C to power
Now I understand R is power RED
C is for Common