Help me add a C-wire

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2,150
Location
WNY
[Linked Image from i.imgur.com] I have an older Janitrol forced-air furnace in my house. I have an old Honeywell programmable battery powered thermostat that has two wires going to it. Red and white. I want to hook up my Nest thermostat and know that it's best to have a common wire. I have tried it without the common and my Nest just short-cycles my furnace. I ran a new 7 wire (to future proof and potential add of A/C) from the thermostat to the furnace. Anyone have any idea where to add the common wire? In addition to this wiring, I have (but do not use) an air humidifier that is tapped into a water line. I have to look again, but in this Pic, you see the red wire tied to a black wire. I think this goes to the air humidifier so I can probably remove that. The other wire here besides the white is probably the red. Thanks.
 
Messages
24,880
Location
Upstate NY
You need to find the transformer that supplies the 24V. One lead goes to your current thermostat. The other goes to the furnace controls. You want that lead as the common.
 
Messages
2,790
Location
USA
Does the cable with the yellow tag go to the gas valve or to the humidifier? On a sticker inside the furnace there should be a schematic of the stock wiring, get a picture of that.
 
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Messages
5,897
Location
Texas Hill Country
The honeywell smart thermostat I purchased included an adapter to provide power when no C wire is present. I did not use mine, however my brother has an older furnace with no C wire, and he used his just fine. The adapter deal is installed inside the furnace, and it was fairly simple to install, he is non technical. Shut the power off and read the directions I suppose. I would get a C wire adapter on ebay or amazon.
 
Messages
3,283
Location
Madiganistan/ChiRaq
If you ran a new cable from furnace to thermostat you don't need an adapter, just use a multimeter on furnace control panel to find a constant 24v with furnace at idle, tie in and send that to your nest
 

M56959

Staff member
Messages
723
Location
AZ, USA
Please refrain from posting if you cannot be helpful and civil. I've removed the posts that weren't helpful.
 

redhat

Thread starter
Messages
2,150
Location
WNY
Thank you all for the info. I will be looking at this hopefully tomorrow morning and I'll post back. I have a multimeter and will check too if need be. Thanks.
 
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