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How to Wire a Smart Thermostat

My current wiring only has 4 wires but I understand I can use the fan wire and make it a common wire. How to Wire a Smart Thermostat, Change the Wiring for a Smart Thermostat.

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Wiring a Smart Thermostat
Electrical Question: My current wiring only has 4 wires but I understand I can use the fan wire and make it a common wire.

I am trying to install a smart thermostat. My current wiring only has 4 wires but I understand I can use the fan wire and make it a common wire. The attached photo is a photo inside the HVAC. ‘Y’ is empty, ‘W’R”G’ are wired then there is a “C’ with a wire attached. On my current thermostat I have a white wire into W, Green into Green, a blue wire into Y, and a red wire into R. I’m just confused why there is a ‘C’ wire on my HVAC and not on my thermostat. Any help would be greatly appreciate. Thanks

This electrical wiring question came from Gregg, a Homeowner in Plainfield, Illinois.

Dave’s Reply:
Thanks for your electrical wiring question Gregg.

How to Wire a Smart Thermostat

Application: Wire a Smart Thermostat – Change the Wiring of a HVAC Thermostat.
Skill Level: Intermediate to Advanced – Best performed by a Licensed Electrical Contractor, a Certified Electrician or an HVAC Technician.
Electrical Tools Required: Basic Electricians Pouch Hand Tools, Voltage Tester, and appropriate Safety Gear.
Estimated Time: Depends on the personal level experience, ability to work with tools, install electrical circuit wiring, and the available access to the project area.
Electrical Safety: Identify the electrical power source to the HVAC Thermostat, turn it OFF and Tag with a Note before working with the electrical wiring.
Electrical Wiring Parts and Materials: Electrical parts and materials for the HVAC Thermostat should be approved for the specific project and compliant with local and national electrical codes.
Electrical Codes and Inspections: Installing or changing home electrical wiring should be done according to local and national electrical codes as adopted in Plainfield, Illinois. A permit and inspections may also be required.

Change the Wiring for a Smart Thermostat

This electrical wiring project is about How to Change the Wiring of a HVAC Thermostat that will be replaced by a Smart Thermostat.

Smart Thermostat Wiring: What You Need to do to Make it Work for You

wiring a smart thermostat to a hvac system

A Smart Thermostat requires a constant power supply, which in this case is 24volts that will be supplied from the power supply at the HVAC system. Normally, a non smart thermostat has only the Power wire of the 24volts to the thermostat, which is used for switching the HVAC unit on and off. Therefore a the Common wire of the 24volts is not available. In order to make a smart thermostat work, we will need the Common wire of the 24volts. If there are no spare wires available withing the thermostat cable, then we will need to create one, but you will have to sacrifice another option, and in this case we will be sacrificing the blower switch function, buy using the blower switch wire for the common wire required by the smart thermostat. IMPORTANT: In order to make this work, the change will need to be made not only at the thermostat location, but also at the HVAC system, otherwise it will not work as needed.

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