How to know if the blue wire is the C wire
Upgrading to a smart thermostat often leaves you dealing with an unlabeled, disconnected blue wire in the wall and an unclear connection at the furnace control board. This makes it difficult to confirm if the wire is actually the C wire or how to hook it up, causing the thermostat to report a lack of power.
What people tried
Every workaround mentioned in the threads below. We haven’t tested any of them — and nobody here is claiming they worked.
- 1Connecting the loose wire to the C terminal on the furnace control board.
- 2Taking pictures of the control board connections inside the furnace to trace the wiring
- 3Stealing the fan (G) wire to use as a common wire at both the thermostat and furnace board.
- 4using a C-wire adapter
- 5inspecting the control board connections in the attic
In their words
Unedited, most upvoted first, each linked to the thread it came from.
“I installed a Nest thermostat and during installation it said C wire had no power.”source ↗
“When I checked the furnace Control Board I noticed: 1- there's a red wire on the C terminal 2- I found a blue wire disconnect. I'm assuming is the C wire that goes to the thermostat If so should I connect it to the C terminal together with the Red wire?”source ↗
“The old thermostat has red and white connected, I need a C wire for the ecobee.”source ↗
“Past owners did some splicing and connected two different wires, the so bundle at the thermostat is different from at the furnace.”source ↗
“Am I right in thinking I could splice the green or black wire at the furnace, with the corresponding colour from the other bundle, and there is my C wire.”source ↗
“I'm interested in buying an Aqara W200 smart thermostat but I'm unsure whether or not I need a C-wire adapter to make sure it has constant power to use it.”source ↗
“Previous thermostat did not use a C wire. Attempting to confirm unlabeled blue wire in wall is the C wire. Any help?”source ↗
Where this came up
People with this problem also raised
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