Best Thermostat no C wire - c wire installation instructions - vents with automation

I live in a house built in 2013, no c wire

i tried with the lower honeywell and even had a free (maybe not best) electrician come to help

i have seen some kits online when i google the issue which claim they can replicate the wire without having to run it … but read reports those are no good

i want a fairly high end thermostat - I CANT STAND NOW HAVING CONTROL OVER THIS, nicer honeywell would be nice.

i know im out of the loop, but what is the big advanatage of nest? the interface just gives the impresssion of not much functionality.

ideally id like to be able to add additional temperature sensors. I currently have one indoor one from netamo which smartthings can read…

anyone have advice??

-single female obsessed with technology but getting stuck…

one more thing has anyone seen the vents for AC which auto shut /close depending on temperature? (example an upstairs room is hot yet downstairs is cold, system shuts that vent). That seems like it could independently operate and not need any app… but what if i was hot from working out, and even though a room was cold according to set standards, i wanted to close vents to other rooms remotely and force AC to room where i was working out?

just thinking out loud

Are you referring to Keen Home - Smart Vents?

If there is an extra wire in the wall (behind the thermostat) that is unused, you could hook that into the C terminal on the HVAC unit.

How many wire do you have for your thermostat?? Most newer homes would have a 5 wires for the thermostat. If you have both AC and heat you only need 4 even though all 5 may be connected atm. Which means you have a spare.

In theory I had “no c wire” either but had 2 power wires, 1 for AC and 1 for heat. After one year with this connection and one particular annoying issue, I found out the Nest thermostat only needs one (or the other) connected. At the furnace end both power wires were connected to one terminal. Which meant I had a spare which I ended up connecting to the C terminal at furnace and thermostat. Why would I do this??? Google “Nest thermostat banging furnace” :smile:

FYI, the “C” wire is just the unit’s common ground. There’s nothing magical about it.


The C wire, or “common wire” enables the continuous flow of 24 VAC power to the thermostat.

The ecobee 3 is a very nice high end thermostat and it comes with an adapter to alleviate the need for the c-wire. It has very good instructions on how to install it without the c-wire.

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I have the Lowes brand CT-101 that paid $90 for and I do not have a C wire. I read the response may be a tad slower (I have not noticed) and you will have to replace batteries more often but other than that its been working fine.
I also read you can rig up an AC adapter that when plugged in will work as the C wire, I am eventually going to try this.

Hope this helps,

[quote=“jodyalbritton, post:5, topic:11447”]
The C wire, or “common wire” enables the continuous flow of 24 VAC power to the thermostat.
[/quote]Yes. It’s a common ground. It allows continuous flow of 24v in the same way an in-wall switch uses a neutral to enable continuous flow of 120v.

If you have an HVAC unit with a proper “C” terminal on the control unit, you should see that it’s connected to ground. Quite a few, oddly, don’t have a “C” terminal. In those cases, you can use ground (the same ground that HVAC uses - not just any ground.)

Edit: Here’s a link that explains t-stat wiring:

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I just received 2 CT100’s today and waiting for the HVAC guys to wire it up this weekend. Let’s see how it goes although I have to set it up once wired and basics are done by them. This is for heating only. For the AC unit, have to wait it out for spring and need a little flashier one… :wink: the price on CT100’s have gone far below 100 dough on Amazon.

That was the distinction I was trying to make. You need to complete the 24v circuit provided by your ac unit.

I’m using the Honeywell Z-Wave thermostat. It came with a “wiresaver” that eliminates the need for a “C” wire at the thermostat. I’ve had no issues so far (about 8 months).

Well, it “magically” elimainated the banging my furnace was making before I connected the C wire :smile:

Be careful . . . . a lot of HVAC equipment is designed without a grounded control circuit. In this case, one side of the transformer secondary winding is labeled C or common but that’s arbitrary. I wouldn’t utilize voltage sourced from the thermostat wires and referenced to ground, even if you know there is no potential between this ground and ground at the equipment. If there is no way to get C to the thermostat, use of the battery operated models.

Which model did you get? @allison?

We went with Peq thermostats that we got from Best Buy. They can use batteries, so you can use it without power (which is what you get from the C wire). The thermostats are re-branded Centralite Pearl thermostats if you want to check them out.

How many wires do you have?

Here’s the thing… you can get by without a C wire if you really have to. Though it’ll cost a bunch in replacing batteries. The CT100 is a good stat. Though not as sexy as an Ecobee3 or Nest. You could also grab one of these AC adapters that substitute for a C wire and run an ugly wire down your wall. This video explains how to use it and potentially hide most of the wire…

But again… the thing…
Smart thermostats are not going away and they’re getting better and more feature-rich. And they are all going to require a C wire moving forward. If it’s possible, hire an HVAC expert to come snake some new thermostat wire. If your house was built in 2013, I’m sure the furnace/boiler will have a C terminal. Even if it doesn’t, you could use the above AC adapter or something similar in your basement to make it happen. Anything is possible. I just wouldn’t mess with that type of wiring without an expert on hand.

The Nest “works” without the C wire, and is advertised to do so, but be careful, this doesn’t work so well and can result in Nest going offline intermittently when not “charging”

I am so scared of these thermostats. I am sitting here with two CT100’s for the heating and waiting on the HVAC guys to hook it up if they show up at all (bad weather in NorthEast again). No C wire in my late sixties home.

But for the AC, I am so looking forward for a fancy one… There are too many wires on the Lennox vintage thermostat and hoping that one of them may be repurposed as a C wire. Then again I have no clue about these thermostat wirings. I am going blindly and respectfully with @jody.albritton’s suggestion of ecobee 3. Nest makes me nervous.

Allison, what specific model do you have ? Thanks

What is a good thermostat that doesn’t require a C-wire? I don’t mind using a battery on it, I can check it no problem.

Does anyone have experience using it w/o a c-wire?