How to Install a Smart Wifi Thermostat with no C (Common) Wire

How to Install A Smart Thermostat With No C Wire

Nest Thermostat

There are many new models available now that can control your homes temperature from anywhere and do many other features. However, if you are installing your new thermostat and you notice it needs a power source and have no C wire you do not always have to run a new thermostat cable or buy expense wireless adapters. Common (C) functions as a power source for your thermostat (Often colored Blue). If your old thermostat doesn’t have the wire needed, here is how you connect your Common wire without running new thermostat wire.

Thermostat Wire
Unused Thermostat Wire Wrapped Around the Cable

First a Multimeter is recommend, but no required. Look behind your thermostat base there is usually a few wires wrapped around your thermostat cable. Take one of the unused wire and hook it up to C. Next go to your air handler/ furnace and locate your transformer.

Take a stake on crimp connector and run a wire to the common (grounded) side of your transformer.  The Red wire is usually your R (Hot) wire off your transformer and the other side C (Common) is usually grounded to the Air Handler/ Furnace.

Furnace Transformer

Hook up the unused thermostat wire to the common (grounded) side of your transformer and then you have power.


***Please use caution when working with electricity, if you are not comfortable doing the project contact a professional***

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How to Install a Smart Thermostat with No C (Common) Wire
Article Name
How to Install a Smart Thermostat with No C (Common) Wire
How to power your smart thermostat when you do not have a C (Common) wire without running new thermostat wire.
Publisher Name
ACE Heating & Cooling in Crossville, TN
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