My device won't turn on: Powering the S100.

The most common problem involving powering a smart thermostat is the common wire (C-wire), which most battery-powered thermostats do not use. Read on to learn more about locating and using a C-wire.

What is a C-wire?

Technically, power flows to the thermostat from the R (red) wire. However, this is only half of the circuit. For it to flow continuously, the thermostat needs a C-wire, or “common wire,” to complete the circuit. Older thermostats typically run on batteries and turn on and off as needed, but a smart thermostat needs continuous 24V power to maintain their Wi-Fi connection and run their display.

Quick Check for an Existing C-wire:


Pull off your existing thermostat's face. If it has batteries, remove them. If the thermostat loses power it does not have a C-wire, and you may need an add-a-wire kit. If it doesn't lose power, locate your breaker panel. Flip the breaker for the wall with the thermostat on it. If it loses power, you likely have a C-wire.

Check for a C-Wire Method 1: Behind Your Thermostat.


Pull off your thermostat face and you may see something like this:



Looking closely, you’ll notice that this particular thermostat does not have a terminal labeled “C” and therefore does not use a C-wire.





However, if you look where the wires come out of the wall, you might see a fifth wire curled up and tucked back. This is your C-wire, it’s just not being used and therefore does not have the metal exposed and is tucked away for safety purposes.



Alternatively, you may see something like this when you pull off your thermostat face:



Looking closely, you’ll notice that this particular thermostat also does not have a terminal labeled “C” (though it does have a “not used” terminal).



This particular thermostat definitely does not use a C-wire, but that again doesn’t mean that you do not have a C-wire available. We can still check for a hidden C-wire, but in this case we have to remove the wall plate to get a good look.




NOTE: Since you will be moving wires at this point, it is important to flip the breaker for the wall that the thermostat is on to prevent a potential electric shock. You should also take a picture of the existing wiring for reference.

Remove the wall-plate, and pull out the A/C wire a little further and you might see the fifth wire curled up further back. Again, this is your C-wire.



NOTE: In both of these cases, the C-wire is likely “dangling,” which means it is disconnected at the AC/air handler panel, and will need to be connected at both ends to power the thermostat. View Method 2 to see what the other side may look like.

Check for a C-Wire Method 2: At the AC/Air Handler Panel.

Locate your AC or Air Handler panel. This is likely in either the attic or the basement of your home. If you’re in an apartment, you’ll have to contact maintenance to get access to the panel. Opening the cover, you’ll see something like the picture below. We want to find the labeled terminal block:



Looking closely near the terminal block, you’ll see the AC wires that match the ones behind your thermostat. You may see a fifth wire curled up. This is your C-wire, it’s just not being used and therefore does not have the metal exposed and is tucked away for safety purposes.



Connecting a C-wire.

NOTE: Since you will be moving wires at this point, it is important to flip the breaker for the wall that the thermostat is on to prevent a potential electric shock. You should also take a picture of the existing wiring for reference.

To connect your C-wire, you’ll need a pair of wire strippers and a screwdriver. Remove the thermostat and its wall plate, exposing the wiring underneath.



Uncurl the previously identified C-wire and, using the wire strippers, carefully remove about ¼ inch of wire sheath.



Tuck the wires through the s100’s wall plate, and install the new wall plate onto the wall. Connect each wire to its corresponding terminal.



Install the smart thermostat by snapping it into place. Note that it will not power on until the C-wire is connected at the AC/Air Handler panel.



Now you need to access your AC/Air Handler panel. Remove the door panel and identify your C-wire and COM 24VAC terminal. You’ll be connect the C-wire to this terminal.



Uncurl the C-wire and partially strip it like you did with the wire behind the thermostat and loosen the COM 24V terminal screw.



Connect the C-wire to the COM 24V terminal and tighten the terminal screw.



Reinstall the AC/Air Handler door panel, flip the breaker back on, and return to the smart thermostat. It should be powered on and ready for installation.



Using a Smart Thermostat Without a C-wire.

There are a couple of options to using a smart thermostat without a C-wire. Assuming that you have four wires available at the thermostat (R, G, Y, and W), the simplest choice is to reappropriate the system’s fan wire (G-wire) to be used as a C-wire, and use a jumper wire to tie the Y and G terminals together. The downside is that the fan will only operate when the AC operates, and if your thermostat has tailored fan-control you will lose that functionality. Instructions below (note that you will require an additional 18-gauge wire):

NOTE: Since you will be moving wires at this point, it is important to flip the breaker for the wall that the thermostat is on to prevent a potential electric shock. You should also take a picture of the existing wiring for reference.
  1. Remove the AC/Air Handler door panel to gain access to the control board.
  2. On the furnace control board, locate the wiring terminals – R, G, Y, W, C.
  3. Move the wire at the G terminal to the C terminal.
  4. Connect the additional 18 gauge thermostat wire from Y terminal to G terminal.
  5. Close the AC/Air Handler door panel.
  6. At your thermostat, connect the R, Y, and W wires to their appropriate terminals.
  7. Connect the G-wire to the C terminal.
  8. Connect the thermostat and switch on the power from the circuit breaker.
The better option to retain independent fan control is to use one of the readily available “add-a-wire” kits, such as the Fast-Stat 1000 or the Venstar ACC0410 Add-A-Wire Kit.

Instructions for the Fast-Stat 1000 are located here (page 8 is likely the most useful).
This kit may be purchased here:

A video walk-through for the Venstar ACC0410 Add-A-Wire kit can be found here.
This kit may be purchased here.








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