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.
- Remove the AC/Air Handler door panel to gain access to the control board.
- On the furnace control board, locate the wiring terminals – R, G, Y, W, C.
- Move the wire at the G terminal to the C terminal.
- Connect the additional 18 gauge thermostat wire from Y terminal to G terminal.
- Close the AC/Air Handler door panel.
- At your thermostat, connect the R, Y, and W wires to their appropriate terminals.
- Connect the G-wire to the C terminal.
- 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.