How to Wire a Thermostat
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How to connect the wiring to a new Thermostat Terminals: Typical Thermostat Wiring for HVAC Furnace Heating and Air Conditioning, Thermostat Wire Connections. © By: Dave Rongey |
Thermostat Wire Connections
Thermostat Wiring #1 I am trying to wire a thermostat, how do I know which color wires to connect to the terminals?
- I am purchasing a home which I noticed the thermostat is missing and the wiring is there but the thermostat is missing.
- I have replaced the one in my current home and used the old thermostat wiring set up to do the new one and it works fine.
- The thermostat which I am wanting to install is a Honeywell programmable.
- My question is how do I know which wires go to what terminals since the instructions say do not worry about the colors, use the old one to set up the new one, but there is no old thermostat there.
This electrical question came from: Sid, a Homeowner in St. Lucie, Florida.
Additional Comments: Your website has great articles for the homeowner and the novice.
Thermostat Wiring #2
- I’m attempting to install the Honeywell Lyric T5 thermostat but I’m not sure what setting to use. I live in an apartment building that has geothermal heating and cooling and a heat pump in my unit. However, there is no O or B wire that a typical heat pump wiring should have.
- The wires I have are R, C, G, Y1 and W1 only. When I went to configure the thermostat as Geothermal Heat Pump, it asked for O/B Changeover on Heat or Cool, which neither applies to me.
- When I chose O/B on Heat, the cooling and heating works but the heat did not shut off even once the ambient temperature has reached the set temperature. I’m confused as to what heating/cooling system I actually have.
This electrical wiring question came from Sanny, a Homeowner in Vancouver, Canada.
Thermostat Wiring #3:
- I had a furnace question. I previously switched to a smart thermostat but I’m looking to switch back to the “non-smart” thermostat and I forgot the wiring placement. The system is 1-2 years old and only an AC and heater are connected to it.
- There are two bundles inside the furnace panel, one has two wires and the other has four. The two wire bundle are white and black The four wire bundle are white, black, red, and green.
- The furnace board has 7 connectors. They are R, Y/Y2, W, Com 24V, G, DHUM, and Y1. Can you please tell me which wires connect to where? Also, the furnace is a Carrier 59SP5A.
This electrical wiring question came from Robert in Dallas, Texas.
Dave’s Reply:
Thanks for sharing your thermostat wiring projects.
Yes Sid, that is a bit odd. The wire colors and number of wires are typically the same as shown below in the thermostat wiring table.
Thermostat Wiring Made Easy
Application: Thermostat for Controlling HVAC Heating Ventilation and Air Conditioning.
Skill Level: Intermediate to Advanced. This electrical project is best performed by a Licensed Electrical Contractor or Certified Electrician.
Tools Required: Basic Electrical Hand Tools including a small screw driver and small wire strippers.
Estimated Time: Depends on personal level experience and ability to work with tools and the small thermostat wires.
Precaution: Identify the circuit for the furnace or heat pump, turn it OFF and Tag it with a Note before working with the thermostat wiring.
Troubleshooting and Testing: Most thermostats may be tested using a Volt/Ohm Meter. The thermostat battery should be tested and replaced as needed.
Notice: Make sure to get the exact replacement thermostat with the correct voltage rating.
Thermostat Wiring
Thermostat Wiring Table and Wiring Diagram
Thermostat Wiring: Where do the red and white wires attach on a new thermostat?
- I have a thermostat now that has only red and white wires and I am wondering where I put them on my new thermostat?
This electrical wiring question came from Sylvester, a Homeowner in Cumming, Georgia.
Example: Typical Thermostat Wiring for HVAC, Furnace Heating and Air Conditioning
- Most Common Thermostat Applications:
- Air Conditioning Thermostat
- Heating Thermostat
- Heat Pump Thermostat
Typical Thermostat Wire Colors and Function
| Terminal Label |
Wire Color | Function /Description |
| Rh | Red wire |
24 Volts for Heating |
| Rc | Red wire |
24 Volts for Cooling |
| Y1 | Yellow wire |
First Stage Cooling |
| Y2 | Blue wire |
Second Stage Cooling |
| G | Green wire Gray wire * |
Indoor Fan Blower |
| W1 | White wire |
First Stage Heat |
| W2 | Black wire |
Second Stage Heat |
| O | Orange wire |
Heat Pump Reversing Valve Cooling Mode |
| B | Brown wire |
Heat Pump Reversing Valve Heating Node |
| C | Brown wire ** | 24 Volt Neutral (Common) |
* Found in some Carrier brand furnaces
**The brown is often the first wire used for the 24 volt common leg however white, black and blue are most frequently used.
Typical Thermostat Wiring for Furnace Heating and Air Conditioning
Most Common Thermostat Applications:
- Air Conditioning Thermostat
- Furnace Heating Thermostat
| Terminal Label |
Wire Color | Function /Description |
| R | Red | 24 Volts |
| B | Blue | Cooling |
| G | Green Grey * |
Fan Blower |
| W | White | Heating |
NOTE: Although many home heating and air conditioning systems have a standard type thermostat cable with color coded wires, the wires should be identified to make sure of the exact purpose for each wire.
Heater Manual and Thermostat Wiring
Example: Thermostat Wiring Diagram
See More about Wiring Heating and Air Conditioning
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Heat Pump Wiring
- Heat pumps are used for heating and cooling. Wall mounted units are great for offices, sun rooms, room additions, garages and are used in hotel and motel rooms.
The following may also be helpful for you:
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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.
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