As shown in the photo below, if your old thermostat only has three wires (R, W, G), you will still need a C wire to install the MTS300.
 


#Method ONE

For most HVAC systems, the fan (G) and heating (W) operate simultaneously. Please locate your HVAC control board, find the wires that come to your room thermostat, move the G wire to the C terminal on the board, and then use a jumper wire to connect the W and G terminals. Return to the room thermostat wiring box and treat the original G wire as your new C wire. 



#Method TWO

Install a 24VAC transformer, and follow the wiring below:

- Connect the original R wire to the Rh terminal on the MTS300.
- Connect the original W wire to the W terminal on the MTS300.
- Connect the original G wire to the G terminal on the MTS300.
- Attach one wire from the 24V adapter to the Rc terminal on the MTS300.
- Connect the other wire from the 24V adapter to the C terminal on the MTS300.