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Furnace Thermostat Electrical Wiring

Understanding Furnace Thermostat Electrical Wiring

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Understanding Furnace Thermostat Electrical Wiring
Electrical Repair Question: I started having problems with the thermostat for my furnace.

I recently installed a hanging light fixture in my dining room. Previously, there has not been any fixture upon the ceiling light fixture prior to this. When I removed the cap the the ceiling light mount, I noticed two pairs of power cables (i.e. 2 positive, 2 negative, 2 grounds) tucked in the mount.

There are also 2 power switches for this fixture mount, as I’m assuming there was ever a need to mount a ceiling fan & light unit. When I first attached the new light, I connected only one pair of power cables (1 black, 1 white, 1 ground)cable to the new light. I noticed however, when powering on, the new light worked, but the rest of the devices (another set of ceiling lights, floor lamps, etc) did not power ‘on.’ I had covered with electrical tape, the remaining white/black/ground cable. I then connected those remaining ‘unused’ cables, to the light fixture as well. Now, the light fixture comes ‘on’ with either light switch on the wall, but at least the rest of my ceiling lights, floor lamps, etc are now powering ‘on’ as well. Since this, I’ve noticed now that my thermostat does not power on properly and does not activate my furnace.

The new light fixture and the thermostat/AC/Furnace are on Separate circuits. At times, I’ve had to power off/on my thermostat or trip the circuit in my Power Supply box, to get the furnace to now ‘kick’ on, when the temperature in the house drops. I started having problems with the thermostat powering on the furnace, since I installed this additional light fixture. Any suggestions? Remember, I ended up using the 2 black/white/ground cables to connect the new ceiling light. Thank you so much.

This electrical question came from: John, a Homeowner from Kansas City, MO, Missouri.
See more about Home Wiring for Missouri

Additional Comments: Great site!

Dave’s Reply:
Thanks for your electrical repair question John.

Furnace Thermostat Electrical Wiring

John, the interesting thing is that the thermostat for a typical furnace is low voltage which comes from the circuit at the furnace, which should be a dedicated circuit and should have nothing to do with the lighting circuit.

Not knowing the history of your home or anything further about the electrical wiring I would suggest that the wiring that you have recently worked with should be positively identified for a full understanding of what the wiring truly is.

The Following will assist you with your thermostat wiring:

How to Install Ceiling Fans and Wiring

Step-by-step pictures – Easy wiring diagrams and installation guide – Light and fan switch variations.

Circuit Wiring

Wiring Diagrams

electrical circuit
Electric Circuit Listing
The size of the home electrical service panel is designed by calculating the square footage of the home and factoring in the code requirements for the electrical circuits that are required.

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Complete Guide to Home Electrical Wiring

Perfect for Homeowners, Students,
Handyman, Handy Women, and Electricians
Wiring GFCI Outlets
Wiring Home Electric Circuits
120 Volt and 240 Volt Outlet Circuits
Wiring Light Switches
Wiring 3-Wire and 4-Wire Electric Range
Wiring 3-Wire and 4-Wire Dryer Cord and Dryer Outlet
How to Troubleshoot and Repair Electrical Wiring
Wiring Methods for Upgrading Electrical Wiring
NEC Codes for Home Electrical Wiring
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Be Careful and Be Safe - Never Work on Energized Circuits!
Consult your Local Building Department about Permits and Inspections for all Electric Wiring Projects.

Electrical Tips to Help You Wire it Right

The Safest Way to Test Electrical Devices and Identify Electric Wires!

The Non-Contact Electrical Tester
This is a testing tool that I have had in my personal electrical tool pouch for years, and is the first test tool I grab to help identify electrical wiring. It is a Non-contact tester that I use to easily Detect Voltage in Cables, Cords, Circuit Breakers, Lighting Fixtures, Switches, Outlets and Wires. Simply insert the end of the tester into an outlet, lamp socket, or hold the end of the tester against the wire you wish to test. Very handy and easy to use.

The Quickest Way to Check for Faulty Electrical Wiring!

The Plug-In Outlet Tester
This is the first tool I grab to troubleshoot a problem with outlet circuit wiring. This popular tester is also used by most inspectors to test for power and check the polarity of circuit wiring.
It detects probable improper wiring conditions in standard 110-125 VAC outlets Provides 6 probable wiring conditions that are quick and easy to read for ultimate efficiency Lights indicate if wiring is correct and indicator light chart is included Tests standard 3-wire outlets UL Listed Light indicates if wiring is incorrect Very handy and easy to use.

Strip Off Wire Insulation without Nicking and Damaging the Electric Wire!

The Wire Stripper and Wire Cutter
My absolute favorite wire stripping tool that I have had in my personal electrical tool pouch for years, and this is the tool I use to safely strip electrical wires.
This handy tool has multiple uses:
The wire gauges are shown on the side of the tool so you know which slot to use for stripping insulation.
The end of the tool can be used to grip and bend wire which is handy for attaching wire onto the screw terminals of switches and outlets..

The wire stripper will work on both solid and stranded wire. This tool is Very Handy and Easy to Use.

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