I am wanting to replace my old round mercury filled Honeywell thermostat with a new wifi smart Honeywell thermostat, the only problems is I don't have a "C" wire on my current thermostat and need help finding my "c" wire. I looked at my wiring diagram and it appears to me that the "c" wire is just not being used. I have attached a picture with blue arrow pointing to what I feel is the "c" wire on the wiring diagram and what i feel is the "c" wire in the actual wiring. I am hoping that someone with more HVAC experience can verify this.
Finding “C” wire on old heat pump HVAC unit
thermostatthermostat-c-wire
Related Solutions
According to the schematic, the B terminal does appear to be connected to the transformer.
I'd actually trace the wiring in the unit just to confirm, but based on the schematic it looks good.
You'll want to make sure the transformer has enough power to supply the additional load of the new thermostat, but typically it isn't a problem. If you hook up the new thermostat, and the fuse inside the unit blows. That would be an indicator that the thermostat is drawing too much juice.
You'll want to focus your attention to the wiring at the bottom of your last photo, that's where all the control wiring is.
Don't touch the wiring on the primary side of the transformer, as it's at line voltage and could cause a nasty shock.
If you clip the zip tie holding the bundle of wires together, you should be able to get a better look at what's going on. I can't tell exactly what's going on from the photo, so I'll explain what would typically be seen.
You should see a cable with a red, blue, green, yellow, white, and possibly brown wires. This is the cable that runs between the air handler and the thermostat. You should see another cable with a red and white wire (it may include other colored wires as well), which runs between the air handler and the condensing unit. According to the schematic, there should be a red and brown wire coming from the secondary side of the transformer.
The red wire from the transformer should be connected to the red wire from the thermostat, this is your Rc wire. The yellow wire (Y) from the thermostat cable, should be connected to a the red wire from the condensing unit cable. The blue wire (C) from the thermostat cable, should be connected to the white wire from the condensing unit, and the brown wire from the transformer. The green wire (G) from the thermostat cable, should be connected to the black and green wire from the air handler.
At the new thermostat you should have two cables, one from the air handler and one from the boiler.
- Remove the jumper between
RhandRc, if one exists. - Connect the red wire from the boiler cable to
Rh. - Connect the white wire from the boiler cable to
W. - Connect the red wire from the air handler cable to
Rc. - Connect the yellow wire form the air handler cable to
Y. - Connect the green wire from the air handler cable to
G. - Connect the blue wire form the air handler cable to
C.
NOTE: This is all based on typical wiring, your wiring may vary.
NOTE: Some thermostats may only use the Rh and C wires for power, so supplying an Rc/C combination may not work.
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Best Answer
The highlighted wire is NOT the
Cwire.If you read Note 3 it says "Transformer factory wired for 240 volts. Use red and blue leads for 208 volt applications". The only time you'd use that blue wire, is if the system feeding the furnace was a 208 volt electrical system.
If you look at the other side of the transformer in the schematic (labeled 24V), you'll see two yellow wires. One is
R, while the other isC.