Hello all
I am thinking in buying a new WiFi thermostat but it looks like it requires a "C" wire in order to function properly.

I do have an extra blue wire on the cable that I can use as "C" however checking my furnace motherboard I having a hard time to figure out if the "Com 24V" I have in my board is the same as the "C" I need.

Any help will be appreciated.
Thanks in advance!

 

It would be a help if you posted the make and model you are looking at.
The blue "C" or common wire usually connects from a ground at the furnace to your T-stat.

 

Sure Thanks for answering. I am planning to install as Ecobee3 Wifi thermostat on a Carrier weathermaker furnace and to make things more fun I realized I could also control my Aprilaire 600 humidifier with the ecobee which would be great since my actual humidifier dedicated controller is acting up on me and needs replacement.

I currently have a 18/5 wire runing from the thermostat to the furnace but i believe if I am going to use the ecobee to start the humidifier I will need at least 2 more wires so I think I need to replace the wire with a 18/7 or 18/8 but still trying to put together what i need to connect.
I see on this manufacturer page https://www.ecobee.com/compatibility/
that after I have the C wire ready that's is the connection I need for the furnace only but if I am adding the humidifier I will need ACC+ and ACC- as well?
Thanks again!

 

That com/24v connection is fine for C use.

You only need one wire from the Ecobee to the humidifier. That would be the ACC+.
The com/24v will supply the C connection.

 

Awesome Pete thank you, just confirming I keep whatever I have connected with the apprilaire 600 right now but run a wire from the ACC+ to one of the solenoid leads??

The way I have now the furnace board provides 24V and i believe ground to the HStat board and than I have another two wires out from the HStat board to the solenoid.
I seen on Youtube a guy keeping the HStat on max and I believe using the Ecobee's ACC+ just to trigger the solenoid but I wanted eliminate the HStat if possible.
I can take some pictures of that setup.
Thanks again!!

 

The Aprilaire 600 has a solenoid with two yellow wires.
Connect one to C and the other to ACC+ on the stat.

 

I'm new here so apologies if I should start another thread. My furnace, a Burnham 205NCL-TEI2 does not have a "C" terminal so I was thinking of just running a wire from the 24v transformer that is connected to the two t'stats and control valves, back to the t'stat. The goal is to install a new Wifi t'stat. I have two zones so obviously I would run a wire to each. Is this feasible and would I connect the load or common? I assume common but I have to ask.