How Do I Add A C Wire To My Thermostat

Back in the olden days, thermostats were uncomplicated on/off devices that didn't demand their own continuous power supply. Mod thermostats with Wi-Fi and backlit brandish, by contrast, need a steady supply of juice.

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

Technically speaking, power flows from the R (red) wire, but not continuously (not on its own, anyway). To make information technology continuous requires a common wire to complete the circuit. When the circuit is complete, 24V energy will flow continuously.

If you're considering purchasing a smart thermostat, yous're probably thinking of doing the installation yourself. Later on all, if y'all're able to change a light switch or receptacle, you're skilled enough to install a smart thermostat – assuming your organization already has a C wire.

If your system has a C-wire, it might exist in use or just tucked abroad behind your current thermostat.

If your system doesn't have a C-wire, you lot'll demand to run a new cable from your furnace to your thermostat to install most of the modern smart thermostat models.

Thermostat wire is merely a big spool of arranged, color-coded wires. The "xviii" refers to the gauge and the "v" refers to how many individual wires are inside the cable.

Thermostat wire comes in diverse weights. If you lot have rut and Air conditioning, yous'll need eighteen/5. If you've but got oestrus, xviii/3 is sufficient but yous may want to run 18/five anyhow for a flake of hereafter-proofing.

Closeup of 18/five wire. Pull back the plastic sheath to reveal the individual wires. These are what gets wrapped effectually your thermostat's terminals.

Identifying Your Thermostat'due south Wires

In that location are 2 means to find out what wires your HVAC system has.

Method 1: Look at the wires behind your thermostat

Detach your thermostat from the wall and await at the wires connecting to it. If you've got a wire connected to the concluding labeled "C", you lot're (probably) good to become with a smart thermostat installation.

If you don't see a C-wire here at your thermostat, don't assume y'all don't have ane just yet. It might exist blimp into the wall, which is what some installers practice when the C-wire is nowadays simply non needed.

You lot can also look inside your furnace (see method two). If y'all find a C-wire in your furnace, then the other end is probably stuffed into the wall backside your thermostat.

Method ii: Look at the wires within your furnace

Disable ability to your furnace and pop the cover off. (The ease of doing it this fashion will vary past furnace and installation.)

Look for a row of screws labeled R, C, W, W2, G, Y/Y2, like so.

Looking inside my 2010 Rheem gas furnace

Every bit you tin can see in the photo above, this furnace does not have a C-wire hooked up. To make this HVAC organization uniform with a smart thermostat, information technology would need either:

a new bunch of wires run between the furnace and the thermostat, or
the Venstar Add together-a-Wire, or
a smart thermostat like the ecobee3 which comes with a Power Extender Kit for systems without a C-wire

If the idea of poking around in your furnace or thermostat makes y'all uneasy, consider hiring a professional person for the installation of your smart thermostat.

There are no standards for wire color!Any wire can be used for whatsoever purpose. A previous possessor or handyman may take been "creative", so what you lot find behind your thermostat might vary from what you read about online or here. These photos are from my own home, merely every domicile is a unique snowflake.

Here are mutual uses for wire colors:

Blueish or Black – C –Common wire, may be unused by your existing thermostat. Enables continuous power flow from the Cerise wire.
Red – R – 24VAC power from the furnace's transformer
Red – Rc – 24VAC (defended to heat call)
Red – Rc – 24VAC (dedicated to cooling call)
Dark-green – G – Fan
White – W – Estrus
Yellow – Y – Air conditioner

Practise I Actually Need a C Wire?

Yeah, you should install a C-wire if you are upgrading your thermostat.

Nigh all modern thermostats, smart or not, require a ability source, and that doesn't look likely to change any time soon.

Nest Owners Non Exempt

The popular Nest thermostat claims to work without a C-wire, in that location are some caveats. Without a C-wire, the Nest gets its power from your heating or cooling system… bold information technology's running. When it's non running, the Nest still needs to get power. The Nest will "pulse" the rut wire, turning on the furnace to pull a bit of power to go on itself going.

In some systems, this is unnoticeable, but in others, the furnace responds every bit if being told to plow on and and so immediately off.

Read one frustrated Nest owner's review of his feel with the "pulse" trouble here.

Nest'south manufacturer updated its literature to warn that the Nest may be incompatible with some single cycle, no-C-wire systems, but the reality remains that outdated wiring is going to become more and more of a pain in the butt to bargain with.

Hereafter Proofing

Nigh states have outlawed the onetime fashioned "mercury blob" thermostat. Fifty-fifty the almost no-frills thermostats on the market today still demand a ability source. Calculation a C-wire, either through new wiring or an adapter, ensures compatibility with new thermostat technology.

Don't do the "fan wire" pull a fast one on

It is true that yous tin can re-purpose the fan wire every bit a power wire, but so you (and time to come homeowners who live in your home) won't be able to manually turn the fan on. If you're going to spend a couple hundred bucks on a thermostat, spend a little more for an adapter or a technician to exercise the job right.

What to do if yous lack a C-wire

Hire a pro to run new wires between your furnace and your thermostat (or do it yourself)
Grab a Venstar Add-a-Wire, which adds a 5th wire to your iv-wire setup
Go a smart thermostat that's designed to work with systems lacking a C-wire, similar the ecobee3 (it has an adapter for C-wireless systems) or the Emerson Sensi (in many systems, it doesn't need a C-wire at all)