Jump to content
Rebecca

Common Wire Connection at Furnace

Recommended Posts

We have an old Wesco furnace circa 1979ish. Trying to install a Sensi touch screen thermostat. I had no problem taking the mercury thermostat off the wall and finding extra wires behind. When crawling under the house, I cannot figure out where I should be connecting the wire of the same color for the common wire. The cream color and red wires are the same as the two on the thermostat upstairs.

48EAF647-D7EB-4DC4-A1B0-2CE6633BA9D4.png

Share this post


Link to post
Share on other sites

This is the standard I've followed for decades:

White is heat

Yellow is cool (A/C)

Green is blower

Red is 24 volt supply

Black is common

Orange or sometimes purple is the reversing valve on heat pumps.

DIYers get them mixed up all the time.  If they are, you might have to trace them and figure it out.  Spent many a service call doing that.

Edited by Marc

Share this post


Link to post
Share on other sites

It appears that you have old 2-wire thermostat wire. If so, I suspect it's not going to be compatible with the new thermostat. A 24 volt transformer may be needed.  Look at the install directions for using old 2 wire thermostat wire.   

Edited by Trent Tarter

Share this post


Link to post
Share on other sites

Rebecca, 

It's going to be tricky to figure out. You're also going to want to install the green wire to get control of the blower. 

Take a picture of the schematic and post it here. You'll find the schematic pasted to the back of the furnace door. 

 

Share this post


Link to post
Share on other sites

Did you remove the metal panel that covers the electrical compartment? It's usually held in place by two or three hex-head screws. You might find a schematic pasted to the back of that cover. 

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


×
×
  • Create New...