Update 02/02/2019 I appreciate all the responses and help but unfortunately i must have not seen the emails stating there were responses! I am now looking into a NEST thermostat and was curious if they too, use the same connections. Lastly, looking for a quick recommendation on which WiFi Compatible therms are preferred by the memebers that respond! Thanks again your knowledge is highly appreciated! Bob

I Purchased the Honeywell WiFi Prog Thermostat to replace my Round Digital Honeywell Thermostat. I have separate Heating and Cooling (Heating is a Weil McLain Boiler for Hot water baseboard and Hot water(no tank) and Cooling is a separate unit that was added later.)


RC Cool
R Heat
W Heat
Y Cool
G Cool

3 wires from AC unit and 2 from Heater.

If I were to go the separate transformer route to power the thermostat, where would I land the 2 wires?

C and ???


8 Answers 8


You guys down voted the only answer that is correct and up voted one that is completely wrong!!! You CANNOT use just one wire from a transformer and expect to get it to do a thing for you. The simple answer to your question is that if you add a transformer you want to tie the lines from it to Rh and C. Most modern thermostats I know of expect to pull power between Rh and C. You do NOT want to tie Rh and Rc together since your heating and cooling are on separate transformers and you don't know if they are truly floating.

The complex answer is that you should check to see if the control board of your heating system doesn't, in fact, have a common terminal and nobody bothered to run the wires for it. If it does, it might be simpler to just pull 18/3 wire from the heater to the thermostat. Depending on the run, you can use the old two conductor wire to pull the three conductor wire through.

  • There are "c-wire adders" available, also. You connect it to the R, C and W wires at the boiler, and to the existing 2 wires in your wall. On the other end (thermostat side) you connect the receiving box, which converts the 2 wires back into the 3: R, C and W. Adds a point of failure, but it's a great option if you can't easily run a new wire.
    – apraetor
    Commented Nov 1, 2017 at 20:18
  • I'd point out that I checked my furnace with a multimeter yesterday, and the common on my 24VAC side is tied to the the 120VAC common. That's a "common" thing to do, as these aren't isolation transformers. Also, it explains why stealing JUST the C-wire works -- they're effectively only using the neutral wire from the receptacle into which their new transformer is connected. In fact, since the xfmr common is wired directly to the AC neutral by definition, all they really need is to connect their household neutral to the C wire directly. The transformer adds no benefit or protection.
    – apraetor
    Commented Nov 2, 2017 at 16:21
  • And since there HAS to be 120VAC feeding the original transformer, if they have 3 wires, but only 2 are connected, and they see no "common" lug -- just connect the C wire directly to the 120VAC neutral on the transformer high side. But MAKE SURE YOU KNOW WHICH LUG IS NEUTRAL.
    – apraetor
    Commented Nov 2, 2017 at 16:24

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You will wire the cooling transformer to RC and the heat transformer to RH


I have no 'C' wire and I just installed a Honeywell RTH9500. My furnace is a six zone hot water with radiant floors. Each thermostat on has a red and white wire. I purchased a 24VAC transformer and was able to put it into a closet that is behind the location of my new thermostat. I connected the R and W wires as you would expect and ran another set of wires to the transformer and connected each wire to each 'AC' terminal. I did not use the Ground terminal. Lastly I connected on of these wires to the 'C' and one to the 'Rc'. I then pulled the jumper for 'R' and 'Rc', turned the power back on and everything works as expected. Hope this helps... it was easier than I expected. Love the Wi-Fi connection!! (5 more to go)


I had 2 wires only, with a gas fired radiator furnace, (NO AC SYSTEM). Honeywell Wi-Fi RTH6500WF Thermostat. I Found confusing advice & fixes which with each post got more confusing.

I finally have it working like a charm, so I wish to share what I did for easier installation.

I had Red & White Wire from furnace, but I needed power to the thermostat. I bought a 24 volt transformer

I ran transformer wires up to the location of the thermostat. Attached the Red Wire to the R Lug & the White Wire to the W Lug on the New plate than Attached one of the transformer wires to the C Lug & the other to the RC Lug. I did not remove any jumper wires.

Plug in the transformer & BINGO follow the directions to Wi-Fi Connect Enjoy it is awesome.


I am replacing my old thermostat with a Lyric T5 Wi-Fi thermostat. I added an additional Transformer after speaking to Honeywell support, as my control board has no C terminal. I first only ran one side of the transformer to the T5 and connected it to the C terminal(as suggested in an answer above). Nothing happened the thermostat would not power up. I had to run the other side of the transformer up as well connecting it to the Rc terminal. I also moved the switch in the terminal block to the down position. Unit powered up and works perfectly.


I had the same issue as above. I worked out the same solution, but was concerned that I might damage either the furnace control board or the separate AC system.

I sent email to Honeywell support, and they replied a schematic showing the C wire from the furnace. I had R and w from the furnace. I had Rc, Y, and G. From the AC. Used the jumper and everything worked.

When conversing with the technician he kept pointing to the fact that the C wire needed to originate at the AC system. That the jumper should not be used. I still question why if you can use an external 24 volt transformer you can’t use either the heating system or separate ac as a source for the C wire.

The result is I have a $150 thermostat in the basement where we only require heat. Note in this configuration you must run R to Rc. If you use R with no jumper it won't work.


It is a conspiracy. Contractors don't want DIY to know how this works because it eats into their livelihood.

That said, you do have to be careful and aware that control wiring for home furnance and AC are not intuitive.

Here's my take. Typical combination forced air furnace / AC configuration will source power when T-stat "calls for heat or cooling". When you hear the T-stat click, a relay is closing and the Stat is closing a circuit back to furnace. When the temperature meets Stat setpoint (stat is satisfied) the circuit opens and you hear another click. The stat is no longer sourcing power.

WIFI stats need to have power "all the time" to enable the connection to your home WIFI router. The "C" terminal is COMMON or Neutral back to the power supply on the furnace. The stat is now "stealing" power from the 24V source at your furnace. In a lot of cases, the transformer is used by the complete heating and cooling system - common to both. In some cases, when the furnace is "off" and you're in cooling mode, you may have a separate cooling system with its own dedicated power supply. This would be when you'd see indvidual RC and RH wires. Sourcing cooling at RC / heat at RH.

Goal is to have the C to a common or "always energized" power supply at your furnance/boiler, space pac, so on, so forth. This is why it is not simple or typical. There are many ways to apply HVAC to homes and many manufacturers have their unique methods.

Do your home work and figure out what kind of furnace, boiler, AC you have. Do some reading on that end of the system and your success with T-Stat install will be much higher.

  • 2
    Less commentary and more answer is appreciated on this site. Other sites push doing homework we try to help.
    – Ed Beal
    Commented Dec 15, 2016 at 0:34
  • This is definitely the reason, to be fair. There's no reason to leave the C wire disconnected except in anticipation of the inevitable service call when the homeowner installs smart thermostats. How many millions of AAA batteries are in landfills due to this, I wonder? @user63876 speaks the truth! Commented Jul 5, 2022 at 18:07

Bear with me on this story. I believe the above "recommendations" are wrong. Simple reason is you need to have a complete circuit as you stated. I purchased a Honeywell RTH6500WF for installation. Much to my surprise, I found out you had to have a "C" terminal. The HON adviser stated I needed to add an additional 24 volt transformer - which I did, a HON AT72D 1683. I ran it off the 120V for the furnace so that it is off when the furnace was off. Heat and wireless worked great. That was a year ago. Summer came. I turned it to AC and had nothing. The "fan on" also never worked. Using the "R" from the furnace and touched it to the "G" and the furnace fan went on. Touched "R" to "Y" and the outside condenser went on. Problem is in the HON Thermostat!

So I call HON again. Seems that they are not really focused on helping us DIYers. First adviser said he could only help me with programming. Electrical work must be done by an experienced HVAC person. I understand, but I NEED AC. So hung up and called back. Next adviser said basically the same thing. Have your HVAC person add a "C". I said I have a "C" but where does the "Load" or "R" connect. Sorry cannot help you.

So we moved to trial and error based on various videos on U-Tube. The HVAC call out fee is 250 USD so it is more than a new thermostat and it is only 24 V. So here is what I found. The "C" lead from the added 24 volt transformer goes to the "C" on the thermostat base. I then to the "Lead" or "R" from the added 24 volt transformer to the "R" block. I hooked the "R" from the furnace and hooked it to the "Rc" on the thermostat base. Nothing on the thermostat. (Hooked up the other way around gave me heat but no AC). So I said my prayers and found the jumper. I jumped "R" and "Rc" on the thermostat base and EVERYTHING WORKS. I have AC, manual over ride on the fan and heat. Good luck on getting your system up and running.

  • 1
    -1 This appears to be a story about how you're repurposing other wires to lose functionality rather than a description of the proper method to add a new C wire. Stories rarely make a good answer to the OP's question.
    – BMitch
    Commented Jul 7, 2013 at 18:11

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