Understanding the C-wire in a thermostat system


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Old 11-17-22, 06:26 PM
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Understanding the C-wire in a thermostat system

Hi, I have a couple of basic questions about the C-Wire in a thermostat/Heating-Cooling.

1. Am I correct to say that the C-Wire is the common/negative wire on a 24V circuit?

2. When I look at the wires by the thermostat on the wall, I don't see the C-Wire but I see it in the 24V circuit board in the furnace. By the furnace, I see a set of two wires (red and white, see arrow) that don't go where the thermostat is located. It almost looks like these two wires go outside the condenser, is it possible that these wires could be used somewhere outside where the air conditioning condenser is located?

Picture related to question #2


By the way, the main reason for my question is that I'm trying to install a thermostat that requires the C-Wire and I'm trying to determine if I would need to buy a 24V adapter that I could plug into a 120V outlet or if there is a chance that the C-Wire could be stuck somewhere inside the wall and I just don't see it or it really goes to the condenser.
 
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Old 11-17-22, 08:16 PM
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1. AC is different from DC (where you have defined positive and negative polarity). In this case, the 2 wires coming from the AC transformer are 'defined' by the voltage between them. People refer to the wire opposite the "R" wire as common; but it is not a 'ground', rather it simply completes the circuit. Older thermostats/wiring didn't include a C wire because there was no need for active 24V power as the thermostats were either mechanical or battery-powered.

2. The red/white pair indeed goes to the condenser. It serves as a low-voltage signal at the contactor which completes the high-voltage circuit for the compressor/fan. It is the same R and C used at the thermostat.

Your issue is you have an old 4-wire thermostat bundle. Your options are:
1. Run a new thermostat wire, if possible use at least 18/8.
2. Use an add-a-wire kit (PEK for ecobee, Nest has power connector).
3. External 24V adapter (less desirable)
 
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Old 11-18-22, 05:56 AM
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Thank you very much for clarifying my question about the C wire, makes sense (positive/negative on DC). I was confused because of the fact that people refer to it as the common wire.

Also, thank you for confirming that the condenser does in fact use the C wire; I see those two wires going in the direction where the condenser is, makes sense.

A couple of questions.

- Can you please elaborate more on why you're suggesting a set of 18/8 wires and not just the 18/5 wires?

- Also, is there any limitation that you know of when using an add-a-wire kit? I have heard that in some cases you lose the ability to turn the fan on directly.

Thanks a lot for your help, I really appreciate it!
 
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Old 11-18-22, 09:17 AM
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It's always good to run spares. You need 5.... 8 is better.
Yes... add a C modules work but are not totally reliable.
They don't eliminate the fan mode.

If you use the 4 wires you have to run a smart stat.... you would lose independent fan control.
 
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Old 11-18-22, 10:02 AM
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- Can you please elaborate more on why you're suggesting a set of 18/8 wires and not just the 18/5 wires?

- Also, is there any limitation that you know of when using an add-a-wire kit? I have heard that in some cases you lose the ability to turn the fan on directly.
1. Like PJ said, more is better. But the cost difference between 18/5 and 18/8 is negligible, and whether the labor is easy or involved you only want to do it once. More wires also leave room for equipment changes or accessories in the future.

2. True add-a-wire kits shouldn't affect functionality/control.
 
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Old 11-19-22, 05:41 AM
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Thank you all very much for your help!
 
 

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