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Nest Thermostat Gen 3: Wiring with separate heating and AC wires

JAWatt
Community Member

‎08-03-2024 05:41 PM - edited ‎08-04-2024 08:44 AM

Nest Thermostat Gen 3

There are two separate wires arriving at the thermostat. The the furnace cable has two wire (black and white) , with a voltage difference of 27V, when unconnected. The A/C cable contains three wires (black, white, and red), with a voltage difference 12.9V across the black and red, and white and red (0.2V across the white and black), when unconnected. Presumably, the red wire is the source.

Currently, the furnace wires are conned to W1 and Rh, which seems to have worked during heating season. The A/C wires are connected as white to Y1, red to G and black to Rc. The A/C does not operate. In addition, the difference falls to 1.4V between black-red and white-red when connected. Additionally, the unit seems not to be charging (red flashing light), nor reboot.

 

The question is what is the correct wiring configuration with these wires, for a conventional forced air furnace and A/C? An additional issues is that the battery does seem to be charging.

 

 

 

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pereztzum
Bronze
Bronze

‎08-06-2024 07:33 PM

Hi there!

I understand how frustrating it can be when your Nest Thermostat isn't working as expected. Let's work through this together.

The red flashing light on your thermostat typically indicates a low battery. To address this, I recommend charging the thermostat for at least two hours. There is a micro USB at the back of the display.

However, for optimal performance and to avoid future battery issues, adding a C wire (common wire) is highly recommended. This wire provides a constant power supply to the thermostat, ensuring it works reliably and efficiently.
We know that these installations can be expensive so I recommend you to purchase a Nest Power Connector, this can be cheaper than a C wire and it will help you to solve your power issue.

If you're unsure about wiring or installing a C wire, I strongly suggest contacting a Nest Pro installer. They have the expertise to correctly set up your thermostat and wiring, ensuring your system operates smoothly.

 

Regards,

Brandon.

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