can't find C wire at thermostat


Old 11-01-19, 07:19 PM
Thread Starter
Join Date: Mar 2019
Location: Canada
Posts: 99
Received 1 Vote on 1 Post
can't find C wire at thermostat

I'm about to install a smart thermostat. I removed the old thermostat cover and see 4 wires connected to the thermostat. The old thermostat is powered by two AA batteries.

Green Wire connects to G
White Wire connects to W
Black Wire connects to Y
Red Wire connects to R

The C connection is empty on the thermostat.

When I go downstairs into the basement and remove my furnace cover it appears that there is a wire connected to the C. So shouldn't this wire also be going to the thermostat? I've looked behind the old thermostat, but I don't see a 5th wire anywhere. Just a bundle of 4 wires. Am I missing something or do I have to call an HVAC technician to install a C wire? Thanks

The pictures below show the thermostat connections and the furnace circuit board.
Attached Images   
Sponsored Links
Old 11-02-19, 01:27 PM
Geochurchi's Avatar
Join Date: Nov 2012
Location: United States
Posts: 4,871
Received 78 Votes on 73 Posts
Hi, the C at the furnace is probably going out to the condenser unit for your AC., unfortunately you will need to get another wire up to the Tstat, there are other ways but I believe this is the best.
Old 11-03-19, 11:22 AM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Northern NJ - USA
Posts: 59,690
Received 1,173 Votes on 1,088 Posts
Nope.... you are seeing correct. You'll need to replace the 4 wire cable.
When doing that..... add extra conductors for future.
Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Thread Tools
Search this Thread
Ask a Question
Question Title:
Your question will be posted in: