# Installing new thermostat - need C wire

**URL:** <https://boards.straightdope.com/t/installing-new-thermostat-need-c-wire/676734>\
**Category:** Factual Questions\
**Created:** [December 19, 2013, 4:34pm UTC](https://boards.straightdope.com/t/installing-new-thermostat-need-c-wire/676734 "2013-12-19T16:34:31Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![gotpasswords](https://avatars.discourse-cdn.com/v4/letter/g/c57346/32.png) [@gotpasswords](https://boards.straightdope.com/u/gotpasswords)\
**Post date:** [December 19, 2013, 4:34pm UTC](https://boards.straightdope.com/t/installing-new-thermostat-need-c-wire/676734/1 "2013-12-19T16:34:31Z")

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I want to replace a dumb non-programmable thermostat in my home with a programmable one, but the one I’m looking to get requires a C wire.

At the moment, there’s no C connection at the stat, but there is an unused blue wire in the cable. Can I just connect this wire to the C terminal on the furnace and be done?

The existing wiring looks like the top (1 cool 1 heat) diagram [here](http://www.thermostatforums.com/showthread.php?23-Thermostat-wiring-diagram), except for the no connection from C to the stat.

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**Author:** ![minor7flat5](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/minor7flat5/32/258_2.png) [@minor7flat5](https://boards.straightdope.com/u/minor7flat5)\
**Post date:** [December 19, 2013, 4:55pm UTC](https://boards.straightdope.com/t/installing-new-thermostat-need-c-wire/676734/2 "2013-12-19T16:55:25Z")

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Yes.

This is exactly what I did. There was a big obvious “C” post in my furnace that was unused and a spare wire in the cable.

I hooked the spare wire to “C” in the furnace and on the thermostat.

By the way, I got one of those wifi-enabled thermostats. I never imagined how convenient this would be, figuring it was more of a gimmick. It is so neat to be in bed and be able to whip out my iPhone and adjust the temperature.

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**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\
**Post date:** [December 19, 2013, 4:56pm UTC](https://boards.straightdope.com/t/installing-new-thermostat-need-c-wire/676734/3 "2013-12-19T16:56:03Z")

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Sure.  
Just make sure you document the wiring, and leave a printout at both the thermostat and the furnace. You’ll appreciate this when it comes time to upgrade again.