Nest thermostat not seeing the C wire but it's there...

[Ron_Jeremy]

...or maybe it isn't?

Here's the wiring setup of the thermostat I just removed. Keep in mind I cannot confirm if it was ever wired correctly to begin with but the old thermostat chewed through 3 x AA batteries very quickly.

Here's the wiring of our new Nest thermostat:

However, the Nest says the C wire is not present:

More Nest wiring info:

Wires at the AC unit. I doubled checked all the connectors and everything looks good to my untrained eye.

I cutoff an inch from all the wires going into the Nest and exposed fresh copper but it did not fix the issue.

I have a multimeter available: is there something else I can check? If so, please provide instructions as if talking to a golden retriever.

 

[WorthFlorida]

Ruff ruff;

The small white wires under the wire nut, where is the other wire going to? It's strange that someone used what looks like #14 or #12 wire for thermostat wire. The white C wire is small so it was added when the previous thermostat was installed. If the thermo went through batteries pretty quick, the wht wire has opened some where. The batteries is a back up only with the old thermostats. New WIFI thermo's do not have battery backup.

The photo of the NEST base, is the wht wire is pushed in all the way? Inside the air handler check these wires. The white wire may have broken off at the terminal. On a terminal board and it may be a low end controller board, there will be a similar strip as the old thermostat. It easily can happen that it may take years for a wire to break from a knick in the copper after it was stripped.

The brown cable appears to be coming from the air handler. Where these wires terminate to the white wire eventually connects to one side of the 24v transformer. It might be through a land on the circuit board.

Is there air conditioning?

 

[Sponsor]

 

[Reach4]

Can you take voltage measurements through those holes? If so, what voltage do you have between R and C?

Could it be that the C wire is not inserted sufficiently? The other wires don't have that bare section showing.

 

[JerryR]

You probably don’t have an active c-wire if the previous radio thermostat went through batteries.

You have to remove the cover from your air handler and see where the white wire that comes from the thermostat terminates on the AH control board. There should be a C- terminal on the control board. From your pictures you can’t tell what it’s connected to.

 

[WorthFlorida]

Here is a simplified HVAC thermostat wiring. I added a C wire to show what is does. WiFi thermostats need constant power to power the electronics and screen.

 

[Ron_Jeremy]

Thank you @WorthFlorida @JerryR and @Reach4 for offering to help -- I'll try and answer all your questions.

1. The AC/Heatng unit works with the Nest plugged in -- setting it to cool does just that, and same for heat.
2. I measured the voltage between the white C wire and the red R wire (Klein MM300 multimeter at the 200VAC setting) and got exactly zero volts. Note: I was unable to measure the voltage while the wires were inserted into the Nest (probes would not fit in holes) so I pulled the wires to measure.
   
3. The white wire looks like it is not pushed in all the way because I stripped off additional insulation to be sure it was making a connection. It is pushed in all the way.
   
4. Below is a picture of the air handler. The wires from the thermostat are entering from the top. Do I remove the top or bottom panel to source the white wire?

 

[WorthFlorida]

Remove the cover by removing the 5/16" hex head bolts. At the top section you'll see a nest of thermostat wires.

 

[WorthFlorida]

You have a Goodman Air handler. The control board might look like this. Older units may not have one but a relay board and wire nuts connections.

 

[Reach4]

I measured the voltage between the white C wire and the red R wire (Klein MM300 multimeter at the 200VAC setting) and got exactly zero volts. Note: I was unable to measure the voltage while the wires were inserted into the Nest (probes would not fit in holes) so I pulled the wires to measure.

So you want to measure between the other ends of those same wires.

Note the WorthFlorida schematic in #6. The secondary voltage of the transformer should be across those two wires.

 

[Ron_Jeremy]

I will get on this tomorrow — thank you all!

 

[Ron_Jeremy]

Update: I do not see the white wire actually enter the air handler, well at least to my eye it doesn't. The white wire that comes from the thermostat seems to continue onwards to the heatpump/ac unit that sits outside.

Here's a look inside the air handler (I hope I'm using these terms correctly):

Thoughts?

 

[SShaw]

Show us the labels on the lugs on the left side of that circuit board. The C wire from the thermostat needs to go to xfrmr-c, or whatever it's labeled.

Looks like it might be that blue wire on the bottom left of the circuit board.

If so, you can get a double spade connector, put it on the end of the C wire from the stat, connect it to the circuit board, then connect the blue wire to the extra blade on the connector.

https://www.amazon.com/DZS-Elec-Con...=double+spade+connector&qid=1619806806&sr=8-3

 

[WorthFlorida]

The blue wire, common for the transformer looks like it has a fuse holder which is good to have. Twist it to open it and check it. The wire nut check the connections.

That is a replacement transformer, not the original. The wires might have been too short in its original location. The two holes to the left of the relay board may have been its location. It's looking like whomever connected the old thermostat just did color to color match.

The transformer wiring? According to the label on it the orange wire to the transformer is for a 240 v single phase input, white is the common and the orange is connected to a white wire? If that is going to the heat pump, a heater may have been added outside. For heat pumps the outside coil can freeze under certain conditions. Or when it gets cold enough these heaters allow the heat pump to work. The orange wire is probably putting out 120v. I would remove it from the Nest and not use it for now.

The wires going to the heat pump; red for 24v ac, yellow to operate the relay contactor that brings power to the compressor and fan. The brown controls the reversing valve. No voltage it's in heat pump mode, voltage applied to the brown places the system in cool mode. The white? You'll need to look outside to see if it is connected to anything. It might be a common. The green wire looks like a spare.

The blue wire from the transformer is the common and it appears to be connected to white wires. Get a better picture and trace these wires. The white to the C terminal of the Nest needs to be connected to this blue wire.

 

[Reach4]

Looks like it is wired for 240v on the primary.

 

[Ron_Jeremy]

Show us the labels on the lugs on the left side of that circuit board. The C wire from the thermostat needs to go to xfrmr-c, or whatever it's labeled.

Looks like it might be that blue wire on the bottom left of the circuit board.

If so, you can get a double spade connector, put it on the end of the C wire from the stat, connect it to the circuit board, then connect the blue wire to the extra blade on the connector.

https://www.amazon.com/DZS-Elec-Con...=double+spade+connector&qid=1619806806&sr=8-3

Photo as requested. The gray wire (or very light blue) goes to the C connector on the board. It is not directly connected to a wire coming from the thermostat, only the blue wire going to the outside AC unit.

Side note: I tried removing the spade connectors but none will budge even when pulling on them very hard.

 

[WorthFlorida]

I’m just more confused on this. If I was there I would have it in a heartbeat. To remove those stubborn connectors you a long nose pliers. The diagram under the transformer, if you can take a picture move as much of the wires as you can.

 

[Ron_Jeremy]

I’m just more confused on this. If I was there I would have it in a heartbeat. To remove those stubborn connectors you a long nose pliers. The diagram under the transformer, if you can take a picture move as much of the wires as you can.

LEt me kow if this photo works for you or if you want another one taken.

 

[Jadnashua]

See the thing labeled TR and the squiggly lines? One side says 24V. One lead (of the secondary) of that transformer goes to your thermostat as the R lead. To complete a circuit, you need the C(ommon) lead from that transformer connected to your thermostat. It's not in tight focus, so I can't read what it says, but it will probably make sense up close. To verify, if you use your multimeter and place it across those two leads, you should read a nominal 24vac (note, it's often higher than that and might even get slightly over 30vac). So, follow that through the wiring mess and if you can trace that and get it hooked to your Nest thermostat, it will be happy.

On some systems, the Nest thermostat can live without the C wire, but it will work better with it. That thermostat has an internal, rechargeable battery, but it can't be constantly charged without the C wire. Depending on the use factor of your HVAC, there may not be current running through the thermostat (only when it's trying to turn something on) can the internal battery be charged. The output of that transformer is the supply, and to make a complete circuit, power has to travel out (on the RED), go through whatever, and back to the common to complete that circuit so current can flow. To charge and run the Nest, it gets that power on the R wire, runs the electronics and charges the internal battery, and the C wire back to the transformer completes the circuit...without the C wire, you do not have a complete circuit which is why you read zero volts when you measured at the thermostat...it must show it's powered.

 

[WorthFlorida]

That wiring diagram is for if heating strips were added for electric heat. It’s probably on one of the covers, usually the one for the blower.

Have you checked the blue wire with what looks like fuse holder? I’m suspecting it’s good. What you need to do is get the blue wire from the transformer to the C terminal. If the other end of the white wire from the thermostat where you believe is going to the outside compressor unit, it might be going to a common connection outside. Trace the xformer blue wire.

 

[SShaw]

Looks like it might be that blue wire on the bottom left of the circuit board.

If so, you can get a double spade connector, put it on the end of the C wire from the stat, connect it to the circuit board, then connect the blue wire to the extra blade on the connector.

As I said above, it looks like the blue wire on the lower left is the 24V C (common) wire from the transformer. Note that it says "SEC BL-COM" on the transformer, which indicates the blue wire is the common for the secondary side (24V) of the transformer.

Check and see if you get 24VAC between R and C on that circuit board. If so, then connect R and C to the thermostat.