Where does the blue wire go on a new thermostat?
Older setups sometimes jumped the blue wire to the red wire at the thermostat instead of using it as a common wire. Guessing the new placement—such as putting the blue wire in the O/B or C holes—often results in a furnace that blows hot air continuously or refuses to turn on. Extra unused wires behind the wall plate make it even harder to figure out whether to copy the old jumped wiring or use the blue wire directly.
What people tried
Every workaround mentioned in the threads below. We haven’t tested any of them — and nobody here is claiming they worked.
Found another way around this?
Share your steps and trade-offs so fellow engineers don't have to reinvent the wheel. No account required.
Where does the blue wire go on a new thermostat?
- 1Using trial and error to guess which terminal the wires belong inDid this help you?
- 2Posting photos of the wiring and furnace board on online forums for helpDid this help you?
- 3Taking pictures of the old thermostat wiring and furnace control boardDid this help you?
- 4Asking HVAC forums for help matching the old wire configuration to the new oneDid this help you?
In their words
Unedited, grouped by where they were said, most upvoted first within each place, each linked to the thread it came from.
“I’m replacing my thermostat, 3rd picture is the old one, but I don’t understand why the blue wire was not used for the common but yet it’s jumped to the R. Makes no sense to me, would I hookup the new smart thermostat the same way as this old one or use the blue wire?”source ↗
“Basically on the new stat, no jumper with blue wire on c and the remaining wires as they were on the old stat?”source ↗
“In the new thermostat, I've run the wires [like this](https://imgur.com/JTQ9xrv) \- Red to R, blue to C, white to W - but the furnace wont kick in and I can't figure out what I've done wrong.”source ↗
“Maybe a clue: originally I had the blue wire in the O/B hole ([right side, second hole from the top](https://imgur.com/JTQ9xrv)). The furnace turned on and was continuously blowing hot air no matter the temperature I set on the thermostat. That prompted me to move the blue wire to the C hole but now it doesn't turn on no matter what I set the temp to.”source ↗
“It wasn’t and it also had 2 other wires not used behind the plate. That’s what confused me, the furnace has the blue wire on the common but it was jumped at the stat off of the red wire”source ↗
Where this came up
People with this problem also raised
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- 13How to know if the blue wire is the C wire
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- 4What is W3 on a heat pump thermostat?
- 2How to operate an unfamiliar commercial thermostat
- 3How to know if a replacement temperature controller is compatible