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I recently purchased a Nest thermostat. After couple weeks of usage, the Nest is constantly shut down due to low battery. I search online that adding a common wire is the go solution for this. However, I am not too familiar with electrical stuff, So I hope someone can help me on this. Unfortunately, I could not locate the circuit board, hopefully some of below pictures might be helpful.

Update: I have reorganize the picture to match the number with scheme. Please let me know where I can connect my unused red wire to pick up the 24volt and used as common wire.

1. My thermostat

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2. The box where I found my thermostat wires along with other wire bundles.

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3. The switch found when I follow the red white blue green bundle going to the furnace.

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4. Honeywell box.

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5.

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6. Transformer on the furnace.

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7. Control

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8. Scheme

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    You shouldn't have taken that cover off. All the thermostat wiring is low-voltage and is outside the box. The high-voltage wiring (which is dangerous) is inside the box. That's why the transformer is on the box lid, to keep the high voltage in and the low voltage out. – Harper Dec 31 '17 at 2:16
  • @Harper thanks for your comments. Without opening the lid, I would't be able to discover the unused red wire. How would you connect the unused red wire to make it a common wire for the thermostat? – Random Dec 31 '17 at 4:14
  • The Nest runs on 24V, not 120V. You have 120V wiring inside that junction box. I turned up the brightness on that first photo... does that cable coming off the outside of the transformer, does that loop back into the inside of the junction box???? From the other photos, it appears it does not. You need to follow that cable. If it did loop back into the box, that would be illegal and dangerous. – Harper Jan 1 '18 at 4:08
  • Hi @Harper, there is a hole on the other side of box (blocked by the box in picture 2 and 3), seem like it's going to the second floor. Also, I think there is another transformer (I will take picture) on the furnace as well, where it has a black and white wires attached to it. – Random Jan 3 '18 at 1:06
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    Another transformer! That and what you said settles it, all your thermostat wiring is outside that box. Follow the wires which are coming off those 2 transformers, good chance one of them is a doorbell and the other for furnace controls. – Harper Jan 3 '18 at 1:10

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