Nowadays, thermostats come with various features. Most run by connecting to the wireless internet and do not require batteries. The C wires give these thermostats constant power and save energy bills so they can run day and night without turning off.
If your thermostat does not have a C wire, the device will lose power quickly and stop working. You can add one using a 5-conductor wire, adapter, and 24V transformers. Alternatively, you can use thermostats with a battery and built-in adapter or one that runs without the C wire.
Be it the batteries or internet connection, having a C wire can save the thermostat and the HVAC system in many ways.
This guide will help you know what to do if the C wire is missing from the thermostat. Along with that, we shall also learn ways to add one if possible.
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What is a C wire?
The C wire is called the common wire in the thermostat.
It provides the WiFi thermostat continuous power to keep it operating day and night.
It will also save energy bills.
C wires are needed for all the Resideo WiFi thermostats.
However, it is not always labeled as the C wire, and the color code may not be the same always.
C wires do not help provide constant power to the thermostats.
The hot wires, labeled Rc and Rh, provide sufficient current to the thermostat.
These wires give a power source of 24-volt from the HVAC control board.
Sometimes, these wires are separate and have separate wires, and the label will be Rh/c.
Without a C wire, the thermostat will lose energy faster and stop working.
For some smart WiFi thermostats, a C wire is a must.
Some smart thermostats can run without a C wire because they have batteries.
But, since they have a lot of features, the power can run off faster.
A C wire can also help you monitor the room temperature, provide the exact reading, and give good heating and cooling.
Constant monitoring will also keep the thermostat energy-efficient due to the accurate readings.
So, it is better to have the C wire in the thermostats.
How to understand that the thermostat has a C wire?
Before you do anything about the C wire, you need to confirm that your thermostat is missing this wire.
If you live in an old house, your thermostat may not have the C wire.
You must check the thermostat to check it.
Here are the steps to check the thermostat for the C wire:
- Switching off the thermostat won’t be enough. You should switch off the power at the circuit breaker.
- Take out the thermostat from the walls. Be careful with the wires, as you don’t want to pull them hard and damage them.
- Look at the wires, count the numbers, and check the terminal labels connected to the wires.
- So be sure to check if the terminal has C marked anywhere. If there is a C terminal and a wire is connected, your thermostat has the common wire.
2 or 3-wire setup
If your thermostat has only 3-4 wires, the fifth one, i.e., the C wire, could be hiding somewhere.
Check in the walls for the C wire.
When a thermostat is a 3 or 4-wire device, the system might run on a low voltage, and the thermostat does not need the fifth wire.
In that case, you will have the blue wire if all other wires are present.
The line is a voltage thermostat setup if the thermostat has only 2 wires.
The wires will be mostly white and red and incompatible with the recent smart thermostats.
In such a case, you can use adapters to transfer the line voltage system into a 5-wire setup and make it compatible with the smart thermostats.
There are also some brands of thermostats that will work in this setup.
You will have every solution to your problem in the next few sections.
What if there is no C wire in the thermostat?
Sometimes, the house owners will not find any C wires in their thermostats.
In that case, you can use some other wire as the C wire, or there are other options.
Here are the options:
Get an Add-a-wire adapter.
This adapter is a device that adds wire to the thermostat and plays the C wire role.
The wire is applicable for thermostats with four-wire setups.
You need one more wire for your 4-wire thermostats.
You can add the Venstar-Add-a-wire for your thermostat.
It is affordable and easy to install.
The cost to install the wire is around $30.
You can also have adapters with 2-wire line voltage setups.
Adapters by Aube technologies will also work with a 3-wire setup and are better than Venster, as Venster works with only a 4-wire setup.
The adapter will cost around $40.
The device will transfer any line voltage system to a low voltage system.
Once it transfers to the low voltage, the setup will become suitable for most smart thermostats.
The complication is that you have to perform certain wire connections.
It will be good to hire a professional to do the job.
Use a 24V step-down transformer.
It is another easy way to make a C wire connection for a thermostat with a missing C wire.
You only need to have an extra outlet near the thermostat.
The transformer will have two output wires.
Connect one wire to the Rc terminal and another one to the C terminal of the thermostat.
Plug the transformer into the outlet near the thermostat and switch on the thermostat to check.
If it works, the C wire is successfully installed.
Get a 5-conductor thermostat.
If the thermostat does not have the C wire and you did not identify the wires coming out of the cables, you can get a 5-conductor wire.
You may have to drill holes for the new wire.
Run this 5-conductor wire from your house’s HVAC system to your thermostat.
If your basement has a boiler, snake the wire from the hole down to the basement.
In this part, you might require professionals to do the job because moving the cables from the basement to the thermostat or vice versa is quite complicated.
Experts can do it better without any mistakes.
You can do it yourself only if you have an electrical background.
Use fish tape to get the wire and pass it to where it needs to go.
Keep the distance between the boiler and the thermostat while connecting the wires.
You will also need to knock a few holes in the wall during the work.
Rewire your thermostat
Pull the existing wire and add the new wires to the thermostat.
You have to connect the new wire from your furnace to the thermostat.
The project can be big based on the distance between the two units.
Since it is difficult and risky, you need a professional for this job.
The professional contractor can charge around $50 to $150 for this project.
The charge may increase or decrease based on the project’s difficulty and the time taken for installation.
Use G wire as a C wire.
If your thermostat does not have any C wire, you can use the G wire as the common wire.
But this wire controls the fan separately.
Suppose you use the G wire as the common wire.
You won’t be able to run the fan independently, especially when the HVAC’s heating and cooling systems do not operate.
Find the unused wire.
The C wire sometimes remains hidden and unused inside the wall for a long time. (Check the “2 or 3 wire setup point.”)
While checking the number of wires in the thermostat, try finding unused wires.
You can remove the screw of the wall plate and take out all the wires to have a better look at the thermostat.
If you get hold of this wire, label it as the C wire and connect it to the thermostat’s C wire terminal.
Use a stripper to expose ¼ inch of the wire.
Go to the furnace, open the cover, find the unused wire’s end, and connect it to the C terminal of the furnace.
Close the cover.
Also read: How To Add C Wire To Thermostat?
Can I replace the thermostat?
If your thermostat does not have any C wire, and you want to avoid adapters, transformers, or extra wires, change it.
Here are some good thermostats to help you deal with the C wires.
Get a thermostat with a built-in adapter.
Suppose your smart thermostat does not have any C wire, and you do not have to add any transformer or adapter separately.
In that case, replace it with a thermostat having a built-in adapter.
Ecobee is a good brand for smart thermostats, no matter the model.
The adapter will make the thermostat installment easier if it does not have the C wire.
The prices of the product model from the Ecobee brand can differ based on the amount of space for the number of smart sensors you wish the unit to have.
Here is the price for 2 good Ecobee thermostats:
Both will need only 45 minutes or less to get installed.
Buy a battery-powered thermostat.
If your thermostat misses the C wire and you don’t want to deal with any adapters, get a battery-powered thermostat.
There are thermostats compatible with such tough setups.
You can use a thermostat with double-A lithium batteries.
The LUX Geo smart thermostat is a good option, priced at $100.
It has multiple features powered by a micro-USB cable that prevents you from changing batteries frequently.
Two good products from LUX are:
You can pair the device with your mobile app for the remote.
It has a programming schedule of 7 days, and you can set it up once and forget about it.
You can also connect the thermostat with the Alexa device, but it does not have built-in like the Ecobee products.
The Lux products also have an air filter monitor, which you can place horizontally or vertically.
Another good feature of this thermostat is its radial geofencing.
It will use your phone’s GPS to trigger the heat or air conditioners when you reach your house.
It will be a great way to save energy and keep the house comfortable.
Get a smart thermostat for a line voltage system.
Line voltage systems are quite uncommon these days.
Most smart thermostats cannot accommodate houses having these setups. But a few smart thermostats will do it.
If you choose this path, you won’t be future-proofed for the thermostat that needs a C wire.
If that is not vital for you, you can choose thermostats for the line voltage system. These thermostats are simple and affordable.
One good thermostat is Casa Energy’s Caleo WiFi line voltage thermostat for electric baseboard heaters.
The cost will be around $90. It comes with two modes; one is fan-forced heating, and another is electric radiant floor heating.
Another good option is the Z-Wave Plus by Stelpro, provided you have a baseboard heater or convector.
The price of this smart device is $80.
The device will need a controller for programming.
What happens if I install a thermostat without a C wire?
The C wire provides adequate current flow to the thermostat.
It guarantees endless current flow by finishing the circuit from the hot wire to the HVAC control board.
If you have a C wire, the thermostat will always run both during the day and at night.
But without a C wire, your thermostat will soon lose its power and stop working, especially if you use a smart thermostat.
Call a professional and get one installed.
You will require the C wire settings in the thermostats with WiFi settings.
These smart thermostats work with internet smartphones.
So they can track your energy consumption and adjust the HVAC system to save energy.
Since these thermostats have multiple features, you need to let them have enough current flow (24 VAC).
The smart thermostats also have R wires that act as a power source but do not provide enough current like the C wires.
That is why you must install the common wire.
Do all thermostats need C wires?
All the thermostats do not require C wires.
Ordinary thermostats can run without a C wire because they run on batteries and do not have as many features as smart ones.
Smart thermostats also have batteries, but their multiple features cannot rely only on them.
However, some thermostats have rechargeable batteries and can recharge themselves.
You have to wait for your house’s HVAC system to get activated.
Once the system activates, the thermostat will divert some portion of the system’s power to the batteries and start charging themselves.
Even if the HVAC system is not turned on, the batteries will still get charged.
The thermostats will turn the HVAC on briefly and draw power from the system’s 24V transformer.
It is called Power Stealing, which will stop only when you turn on the HVAC system.
However, this process is not recommended.
Since the batteries take power from the HVAC, the thermostat keeps turning the system on and off, reducing the system’s lifespan.
You can also hear strange alarming noises from the boilers, furnaces, and fans.
So, even if your thermostat can run without a C wire, it is always safe to install one.
Otherwise, you can use a C wire adapter instead of installing the wire.
Thermostats to install without a C wire
You can use smart thermostats without the C wire to avoid the efforts to put into the existing thermostats explained above.
Most smart thermostats work by WiFi and need a C wire for constant power supply, but some smart thermostats won’t need the C wire for functioning.
These thermostats have rechargeable batteries or additional technologies that eliminate the need for the C wire.
Here is a list of some smart thermostats that will work without the C wire:
- Google Nest E Thermostat: It uses a rechargeable lithium-ion battery that eliminates the need for a C wire. It offers a display that always remains on and shows the current temperature setting.
- Ecobee Lite Smart Thermostat: It allows you to control the heating and cooling system directly from the mobile app and helps to schedule the program efficiently. However, it does not have features of the bells and whistles present in most of the Ecobee thermostats.
- Emerson Sensi thermostats: They work with Alexa. It also works with iOS and Android phones so that you can know the energy usage and control it.
- Line voltage thermostat: This one stands for its mid-range price and stylish design. It is made to work with line voltage heating systems, like baseboard heaters, convectors, and fan-forced heaters.
- Wyze Smart Thermostat: This product works with or without the C wire. It does not have a battery, but you can still use the power adapter included with the thermostat. The thermostat contains the 7-day schedule for home, sleep, away mode, usage tracking, etc.
- Honeywell Home T9: This one is similar to the Wyze thermostat. It comes with an adapter, works with or without the C wire, and is compatible with Alexa, Google Assistant, and Apple HomeKit.
Also read: 12 Reasons Your Thermostat Is Blank+Fix.
A C wire helps supply constant power to the thermostats without increasing the energy bills.
Suppose your thermostat does not have any C wire.
In that case, you can deal with it by adding an adapter, a 24v step-down transformer, rewiring the conductor with a 5-conductor wire, and using the G wire as the common wire.
Sometimes, a fifth wire stays hidden and unused inside the walls.
Please find it and use it as the C wire.
If you want to avoid adapters or transformers, replace your thermostat with other types: a built-in adapter, battery-powered unit, or smart thermostat for a line voltage system.
Though most smart thermostats running on WiFi will require a C wire, all the thermostats do not require a C wire because they have batteries.
There is no need to add a C wire for such thermostats, but adding one can be safe in many ways.