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Heating & Cooling

Hooking up the C wire to my heater/fan

I was looking at getting a smart wi-fi thermostat but that more or less needs a c wire. We have a blue wire but it's not hooked up at either end. We also have a white wire that's only hooked up to the heater/fan in the attic but not the thermostat. Looking at the attached picture, what screw would I need to hook up the blue wire to? Is it just the C screw where one of the red wires is currently hooked up? Our A/C unit is located outside the house, do I need to do any additional wiring to that?

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Posted 2018-11-15T22:16:12+0000  by primehalo primehalo

Hello primehalo.  Welcome back!


The pictures you posted are excellent, and helped me greatly in figuring out how your system is currently wired.  I do wonder if you would be willing to recheck that white wire at your thermostat.  I believe it is connected to the W terminal.  If not, then your furnace would never run a heat cycle.  The fan and air conditioner would work fine though.


This may be a bit longwinded, but bear with me.

Inside your “Heater/fan in attic” there is a transformer that provides 2 taps.  One goes to the R terminal and provides continuous 24 volt AC power.  The other is a neutral, (often mischaracterized as a ground), which is connected to the C terminal, as well as the internal fan and heat relays.


There are 2 cable sets attached to your “Heater/fan in attic”.  The lower one has only 2 wires and it goes to your air conditioning unit.  The A/C unit needs a neutral to complete its relay circuit, so you will see that the red wire is connected to the C terminal.  The white wire carries power sent from the Y terminal which actually comes from the thermostat using its yellow wire.  It might be easier to decipher if the wires going to the A/C were yellow and blue/black, but it is what it is.  So to answer your last question, no wiring changes to your air conditioning unit will be necessary.


Let’s look at the thermostat wiring now.  What the thermostat does is sense house temperature and send appropriate instructions to various HVAC relays which then provide full electrical power to their components.  The thermostat gets continuous 24 volt AC power from the R terminal on your furnace panel.  That power travels up the red wire to, (in your case), the RC terminal.  There is also a brown jumper wire on the thermostat that provides power to the RH terminal.  These correspond to RC=cool, RH=heat.  When the thermostat senses a need for heat, it passes that power to the W terminal, which is the white wire going back to the white W terminal at the furnace.  In addition, it will also send power to the green G terminal which activates the fan through its relay.  Y does the same thing for the air conditioning relay, and is the yellow wire.


So now you can see that your new thermostat already gets continuous power, but has no way to complete an independent circuit without causing some component in your HVAC system to activate.  That’s where the C wire comes into play, and fortunately you have one available.  When you install your new wireless thermostat, you will connect that unused blue wire to the C terminal.  At the furnace, as you gathered, you will connect that end of the blue wire to its C terminal with the red wire that’s already there.  All the other thermostat wires should go to their same respective terminals.  Please check to see on your new model whether or not you will need that jumper wire from RC to RH.  Some models just do that internally and will just have an R terminal for power in.




Posted 2018-11-16T15:44:55+0000  by Chris_HD_CHI
Thank you, that was a very clear explanation and you were correct, the white wire is attached to the thermostat, it was just positioned in such a way that it didn't look like it was connected. From your explanation it sounds like I should cut off the power at the breaker before attempting to attach the C wire to the furnace.
Posted 2018-11-16T19:22:32+0000  by primehalo
Yes.  Please read and follow the instructions that come with your new thermostat.  I am sure that one of the first things mentioned is to cut power to your furnace.



Posted 2018-11-16T19:39:08+0000  by Chris_HD_CHI
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