Wiring schematic Corrosion

This one seems to be a little different from previous questions on the subject. Here is the dilemma, I don't see a transformer. I have included a few pics.

There are Three wire bundles. For ease we will call them "Bundle-A", "Bundle-B" and "Cannon-Plug" - see schematic diagram for Cannon-Plug.

Bundle-A has two wires in it, Red and White. This bundle comes down the wall from the second story. There is another HVAC upstairs, maybe it runs there?

"Bundle B runs through the wall to the Thermostat. I think it is a 10 wire bundle, but they cut back all of the wires except the 4 wires that are in use at the thermostat.

Here is how it is wired:

Bundle-A-Red is connected Cannon-Plug-Blue.

Bundle-A-White is connected to Bundle-B-Yellow

Bundle-B Red is connected to Cannon-Plug-Red through an overflow-shutoff wire (black in this photo).

Bundle-B Green is connected to Cannon-Plug-Green

Bundle-B White is connected to Cannon-Plug-White

Also notice how the Red/Blue wires are also bridged (with a capacitor? resistor?)

Any thoughts on which wire I tap for "C-Wire"?

On another note, look at the corrosion image. How bad is that? How long before leak and repair - scale 1 to 10?

Thanks Ya'll


  • 1
    According to the partial schematic you've shown, the blue (BL) wire should be C. However, it looks like there's some uncommon wiring, so it's difficult without more information to be sure what's going on.
    – Tester101
    Apr 27, 2015 at 1:21
  • That is a resistor.
    – Tester101
    Apr 27, 2015 at 1:22
  • 1
    Bundle A might go to the condensing unit for the A/C.
    – Tester101
    Apr 27, 2015 at 2:52
  • 1
    Also, if you have two questions, you should ask two questions.
    – Tester101
    Apr 27, 2015 at 2:53
  • I thought C was for "common ground" and thus green would be it.
    – ssaltman
    May 28, 2015 at 20:10

2 Answers 2


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This is a Heat Pump wiring diagram, if you only have an AC unit then the AC unit outdoors will only require the Yellow and the Common wire as far as the 24 volt control circuits go. Common is the side of the 24 volt control circuit that every 24 volt circuit returns to, to complete the circuit, this is why it is called Common, cause every circuit terminates there. Every 24 volt device has a 24 volt hot leg and a Common, the AC unit will have the Yellow 24 volt hot leg of power and the Common side of the 24 volt control power.

The furnace or air handler has low voltage terminals R/ red, W/ white, Y/ yellow, G/ green and C/ common. The circuit board will have them if no low voltage terminal strip is seen. Follow the 24 volt wiring to see where the terminals are, the pics you gave show nothing of any use, its hiding the components, google 24 volt hvac transformer, or 40 VA HVAC transformer to see what one looks like. Red is the 24 volt hot leg of power White is the Heat circuit Yellow is the Cool circuit Green is the Fan on circuit Common is the side of power every 24 volt circuit terminates upon. 1 of the 24 volt wires going to the AC unit will be Common.

  • Thanks for taking the time to reply. I will look for the transformer and see if I can find it.
    – Tom
    Apr 29, 2015 at 2:26
  • I am heading home tomorrow, and I will look at this more closely, but I have been thinking about what you said. "if you only have an AC unit then the AC unit outdoors will only require the Yellow and the Common wire as far as the 24 volt control circuits go" given the white wire running to the condensor is connected to the yellow wire, the other wire (red) running to the condenser must be connected to common. If so that is the BL wire. Yes?
    – Tom
    May 1, 2015 at 4:11
  • So, is my understanding correct that C is just a 24V ground wire? I have a similar problem: I have a control board, but it doesn't list a C terminal or wire in the component diagram. I do see a wire which looks like the ground, however.
    – Kir
    Jun 2, 2015 at 14:44
  • @Kir -- post a separate question please Nov 25, 2015 at 4:12

I cannot be 100% sure, but from the schematic on the label and the manual of what seems to be a closely related system, it appears likely that the Blue wire coming from the cannon plug is indeed your C (Common) wire; that, and the fact that your thermostat cable has extra wires in it, should make this a relatively easy job.

In fact, your full thermostat color code is as follows:

  • Blue -- Common (C)
  • Red -- 24VAC (R) via the overflow shutoff (I wouldn't have wired it this way, but it will work with most smart 'stats that have a battery backup)
  • Yellow -- Compressor/Cool (Y) -- Bundle A goes to the compressor/condenser unit
  • Green -- Fan (G)
  • White -- Heat (W)

As to that mystery part? It's a resistor -- a 360 Ohm resistor capable of handling a couple watts of power, to be exact. What it's doing from your W wire to your C wire, though, is not at all clear! (It shouldn't harm much by its presence other than wasting a bit of power on the 24VAC side whenever you call for heat, though.)

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