NEST Battery Issue - Do I have a C wire?

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Old 01-08-18, 05:50 PM
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Question Do I have a C wire?

This is NOT a NEST specific question so skip down to below about the C wire

My furnace and condenser are about 27 years old, running strong, Amana units. I'm very happy with them overall.

I have had NEST gen 3 for about 6 months until 2 days ago I had no issues. 2 days ago it gave me E73 due to severe cold here (-7F for a few days). I disconnected it and it has been working fine.

But this entire researching of what causes E73 lead me to learn about the "C" wire and how one should have this C wire. Indeed, the Vin/Vout and Battery values of my NEST is rather low:

Battery fluctuates around 3.78-3.87

While heat is on:

Vin: 7 to 9V
Vout: 34V

While unit is on rest:

Vin: 30-31V
Vout: 34V

Which I think means there isn't enough power going in.

Which brings me to my actual question about the C wire

I removed the nest and inside my drywall, I can see an extra two cables rather in a strange fashion. I see a brown cable and I see a blue cable. I also see that they are taped together with black tape.

Furthermore, I opened the furnace and I can see there is a connected wire in C terminal but there are 2 C terminals (why is this?) and the second one is empty.

Anyways, I took a few pictures, pictures of the furnace board, the pigtails of connections right next to furnace, picture of a transformer of wire that goes to A/C unit, and picture of the NEST wires.

Now here is what is very strange about this, please bear with me:

- Two separate cables exit the furnace. One is a thick brown cable, one is a smaller brown cable. The smaller brown cable has the red wire (which is connected to Y terminal) and the white wire (which is connected to C terminal). When you look at my pictures, you will see a connection between white and red cables and blue and brown cables. This is NOT from the cable that is leaving the furnace! So dont let that confuse you. The cable that connects to the furnace's board's C and Y terminals leave the furnace and go directly to the A/C.

- The cable that you see with white and red wire connecting to the blue and brown wires actually is an ENTIRELY SEPARATE cable that connects to some transformer by my main breaker box.

So to sum up:

Furnace board ====> 2 cables. Let's call them BIG and SMALL cables

BIG CABLE ====> Goes upstairs
SMALL CABLE ====> Goes to A/C outside

The BIG CABLE has 2 conductors (blue and brown) that joins to ANOTHER SMALL cable's white and red conductors which are connected to a transformer by my breaker box.

Now the question is, are either of these blue and brown wires upstairs that connect to the transformer, can be used as a C wire?

If the answer is NO, how can I run a C wire and also what is the role of that transformer?

This is very confusing for me!
 
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Old 01-08-18, 06:08 PM
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A transformer by the breaker box is for the doorbell.
The transformer for a furnace is always inside the furnace.

Big cable = thermostat wires
Small cable = A/C condensor (outside)
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At your thermostat I see.....
Red = Rc
Yellow = Y1
Green = G
White = W1

I see the same four wires at the furnace board on the same basic terminals.

Look at that transformer and see what voltage it is. Either someone ran that for a powered thermostat or it was for something else upstairs.

If the wires aren't used inside the wall..... you can disconnect them from that transformer and pickup the C at the furnace board.

FYI..... the nest only charges when the furnace is NOT running. With this extreme cold..... furnaces are spending more time running which equals less time to charge. The C wire will eliminate that problem.
 
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Old 01-08-18, 06:17 PM
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Originally Posted by PJmax View Post
A transformer by the breaker box is for the doorbell.
The transformer for a furnace is always inside the furnace.

Big cable = thermostat wires
Small cable = A/C condensor (outside)
Attachment 89317

At your thermostat I see.....
Red = Rc
Yellow = Y1
Green = G
White = W1

I see the same four wires at the furnace board on the same basic terminals.

Look at that transformer and see what voltage it is. Either someone ran that for a powered thermostat or it was for something else upstairs.

If the wires aren't used inside the wall..... you can disconnect them from that transformer and pickup the C at the furnace board.

FYI..... the nest only charges when the furnace is NOT running. With this extreme cold..... furnaces are spending more time running which equals less time to charge. The C wire will eliminate that problem.
I have two transformers by my breaker box. One is this one and another (much smaller) one is the doorbell one. The doorbell wire goes upstairs whereas the transformer's wire (the picture) goes to the furnace area where the white and red wires join to blue and brown wires.

Given this, do I have a C wire?

Also, do I simply touch both wires on the transformer with multi meter? I dont want to break anything.

Lastly, I was under the impression that Nest only charges when the furnace is actually running not the other way around. In fact, this is why they have a power steal hack that quickly turns the furnace on/off when the power levels are too low, no?
 
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Old 01-08-18, 06:31 PM
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I have to look further into the power stealing. Typical power stealing works when the device is in series with the source. When the heat is off..... the stat is in series. When heat is active.... the wires are shorted together.

It looks like someone ran a transformer to run a powered stat. IF the wires in the wall are not being used..... disconnect them from the transformer and use on for your C wire.
 
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Old 01-08-18, 06:47 PM
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If I disconnect them from transformer, where will they draw their power from?
 
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Old 01-08-18, 07:04 PM
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Your nest already has 24v power...... it just needs a C connection. Pick one color and connect it to the C terminal along with the white wire that is already there on the control board.
 
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Old 01-08-18, 07:29 PM
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Ok that makes sense except why would there be a transformer outside of the entire circuit. I just dont get it. You are saying the furnace wires already provide 24VAC. So why would anyone need to install a separate transformer?

my unit is almost 30 years old so I dont want to fry something by accident
 
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Old 01-08-18, 07:57 PM
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I don't know why that transformer is there. Just guessing based on the wiring.
Did you see what voltage is stamped on it ?

The nest doesn't require much to charge. I've never seen one draw enough to damage a transformer.

If that transformer is 24v and you want to use it.... here's how to connect it.

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