The current installation does not have a C wire. So I have run an extra wire and am now confused as to where to connect it. There is a 24VAC connection and a common connection. Comments say that the C wire provides 24VAC. Does that mean it connects to the 24VAC at the TACO or because it is common, it connects to Common.

Connecting to 24VAC shows 24VAC between R and C and 0 VAC between W and C. Connecting to common shows 0VAC between R and C and 24VAC between W and C.

Color me puzzled

Any guidance? Thanks

  • C is usually common (similar to a ground) and uses a black wire. Are you sure you don't have a C wire? – wallyk Aug 30 '14 at 20:20
  • This answer might be helpful. This one too. – Tester101 Aug 30 '14 at 21:03
  • Have you checked the wiring diagrams (likely found inside the furnace/air handler/condensing unit, or in the documentation for said equipment)? – Tester101 Aug 30 '14 at 21:15

The 'C' terminal connection & wire that the Honeywell instructions are talking about provides power for the thermostat's electronic control, while the R and W terminal connections simply connect to the loop in your furnace's circulation pumps.

As with you, my old thermostat only needed 2 wires, and the wiring to it was a harness that only had 3 (three) wires in it, 1 of which wasn't being used. In order to upgrade, I needed to (a) pull new wire through my wall which had 5 wires in total (although only 4 of the 5 are being used with the new installation, 1 pair (R&W) for the circulation pump loop, 1 pair ('C' and 'Rc" for the power from a 24 VAC step-down transformer.) I also purchased a Honeywell AT72D 1006 24 VAC 40 VA, which serves as a persistent power supply for the thermostat, available of the shelf at HomeDepot.

Since I am not using a 'C' wire off of the rely for the circulation pump, but instead I am using the external 24 VAC transformer, It was crucial for me to remove the jumper on the thermostat's terminal connections that had been bridging the 'R' and 'Rc' connections.

Since there are 3 heating zones in my house, the 24 VAC transformer can function as the power source for all 3 thermostats, but I can't comment on the limit of how many thermostats are able to function from the single unit.


I suggest you look at either diagram in the URL below. You will note that "R" is on the LEFT, while "W" is on the RIGHT; not that that makes any difference in operation, since we're working with 24 VAC.


  • Hello, and welcome to Home Improvement. Thanks for the answer; keep 'em coming. And, you should probably take our tour so you'll know how best to contribute here. – Daniel Griscom Nov 2 at 12:01

I tried to connect a Honeywell RTF9580 WiFi Thermostat to my Taco SR506 temperature control box which has 24VAC terminals. I had a 2 wire thermostat; disconnecting the thermostat I noticed that there were 2 extra wires wrapped and unused. It was 4 wire with colors of red, white, green and blue. I tried to use the green wire as the "C" wire, so I connected the green wire to the 24V terminal then connected the red & white wire to the R & C terminals powering the thermostat screen light but was constantly blinking. Then I decided to disconnect and use external 120/24VAC, 200mA transformer Stencore P8395 which can be purchased from Allied Electronics for about $10.00. The transformer is small enough to fit in an electrical outlet box. I have installed two thermostats this way and it works very well.

  • Wait... You connected the unused green wire to the 24V terminal, but then connected red and white to the R & C terminal? Shouldn't you have connected the red and green wire to R & C? – Tester101 Mar 25 '15 at 15:43

There is a diagram on the taco web site that describes a simple configuration that lets you connect a 3wire WiFi thermostat requiring a "c" terminal without external 24v power supplies etc. The thermostat terminals on the SR-504 Are labeled "X" and "X", suggesting it makes no difference which terminal you put red or which terminal you put white on. However in the three wire configuration, it is essential that white is on the left and red is on the right. Then attach the "C" wire to the "common" terminal on the SR – 504 and you should be good to go

  • Do you have a link to that diagram? Sounds useful – DaveInCaz Nov 2 at 11:08

I installed a Honeywell RTH6580WF thermostat. You will need at lease three wires. My controller is a Taco 6 zone controller. W wire from the controller to the W terminal on the thermostat.Red wire to the R terminal on the thermostat. The third wire will go from the common terminal at the 24VAC terminal in the controller to the C terminal in the thermostat. Honeywell's customer service gave me a different configuration that did not work. This setup worked like a charm.

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