In-depth Thermostat Wiring Guide for Homeowners 2021

This complete 24-volt thermostat wiring guide covers all thermostat wiring issues from the simple to the complex. We address them in order from most common to least common. We do not cover 110/240-volt thermostat wiring and millivolt thermostat wiring issues.

Feel free to jump to the section that covers your specific topic:

Is My HVAC System a 24-volt System?

Success starts with knowing what type of thermostat wiring you have or need. Your options include 24V, 110/240V and millivolt thermostat. This guide addresses 24-volt systems, the majority of HVAC systems.

3 types voltage thermostats

24V or low-voltage heating and cooling systems include one or more of these components:

  • Heating: Gas furnace, either natural gas (NG) or propane (LP), oil furnace, heat pump with or without auxiliary heat strips
  • Cooling: Air conditioner, PTAC (packaged terminal air conditioner), heat pump
  • Accessories: Humidifier, dehumidifier, ERV/HRV ventilator, air purifier and other air quality equipment

These systems can be traditional split systems, with one component outside and others inside, ductless split systems with outside and inside components and packaged systems with all components located in a single large case, usually installed outdoors. They can be single-stage, two-stage or variable-capacity systems. Dual fuel systems with a furnace and heat pump are 24-volt / low-voltage systems.

More than 90% of all HVAC (Heating, Ventilation, Air Conditioning) systems are low voltage 24V systems. The following systems ARE NOT 24-volt systems:

  • Boilers
  • Electric furnaces
  • Baseboard electric heat
  • Solid fuel stoves (wood, pellets, corn, anthracite, coal and other biomass materials)
  • Millivolt furnaces that do not require electricity

3 Method to Wire a Thermostat

Let’s start with the simplest approach. If the old thermostat is still installed, then this will work. If the old thermostat has been removed, then the approach is a bit more complex but can still be a DIY project.

#1 Replace the thermostat wire for wire:

If you haven’t changed your HVAC system components but want a new thermostat, take this approach.

  • Turn off the electrical circuit to the furnace or air handler.
  • Take a picture of the old wiring connected to the various terminals. This will serve as a reference in the event you lose track of the terminal any wire was connected to. You can also use the old-school method of putting tape labels on the thermostat wires and marking them with the letter of the terminal each was attached to.
  • Detach a wire from the old thermostat, and connect it to the terminal with the same letter(s) on the new thermostat. Remove and attach them one at a time, until you’ve changed them all.
  • Note: Your system might not use all the wires in the bundle. This is common and not a problem. Unused wires are usually twisted together and wrapped around the other wires to prevent bare wiring from contacting any bare section of wire being used or a terminal.

  • Turn the circuit back on, and adjust the thermostat to call for heating or cooling.
  • If it works, and it should, you’re done. If it doesn’t, we recommend turning the circuit off again and calling a local HVAC company to complete the job.

#2 Locate the wiring connections in the furnace or air handler:

If the thermostat has been removed, your job is a bit more complex. One of two approaches might work. Here’s the most certain approach.

  • Turn off the circuit used by the furnace or air handler.
  • Double-check that the system is off by adjusting the thermostat to call for heating or cooling. If the system doesn’t turn on, proceed.
  • Remove the cover of the furnace or air handler.
  • Locate where the wire bundle from the thermostat enters the furnace. It will consist of three to eight wires of different colors including red, white, green, yellow, orange, blue, black, etc. The entire bundle might be wrapped with a housing.
  • It might be necessary to remove a cover over the control board to expose the wiring connections.
  • Once you locate the wiring connection terminals on the furnace or air handler, take a picture of them and/or write down what color wire goes to each lettered terminal.
  • Leave the cover(s) off the furnace until you get the thermostat wired.
  • Use your photo and/or the notes you’ve taken to connect the properly colored wire to the correct terminal on the thermostat.
  • Note: As we said above, your system might not use all the wires in the bundle. This is common and not a problem.

  • Turn the circuit back on, and adjust the thermostat to call for heating or cooling.
  • If it works, you’re done. You can put the cover(s) back on. If it doesn’t work, we recommend turning the circuit off again and calling a local HVAC company to wire the thermostat.

#3 Use standard wiring colors to connect the thermostat:

thermostat wiring colors to labels

Wire Colors
Wiring Functions
Y2Light Blue/Other Colors
2 Stage Cooling
W2Brown/Other Colors2 Stage Heating
GGreenIndoor Fan/Air Handler
CBlue/BlackCommon Wire
RRed24V Power
RCRed24V Power
ENo Universal ColorEmergency Heating
X/AUXNo Universal ColorAuxiliary Heating
BDark BlueReverse Valve For Heating
OOrangeReverse Valve For Cooling
S1/S2No Universal ColorFor Outdoor Unit

If the thermostat has been removed, so you don’t know which wires were connected to which terminals, and you can’t or don’t feel comfortable getting into the furnace, this approach might work. It depends upon the original installer having used the traditional wiring color code to when installing the thermostat. Note that wiring colors have no inherent meaning. The wires are all the same: solid copper wire covered with colored PVC insulation. The color code was started to make jobs like you’re tackling easier. Here’s how to wire a thermostat using the color code and, FYI, each terminal’s purpose.

  • Attach the red wire to the R terminal (Call for heating and/or cooling).
  • Attach the green wire to the G terminal (Fan).
  • Attach the white wire to the W terminal (Heat).
  • Attach the yellow wire to the Y terminal (AC).
  • Attach the blue wire to the C terminal (Common wire – see below for details).
  • Wrap any additional wires around the bundle to prevent them from contacting bare wire or terminals.

Note: If any of the components are capable of staged heating or cooling, this configuration either won’t work or won’t allow for the full performance of the system. If it doesn’t work or you aren’t sure you’re getting full performance such as staged heating or cooling, call an HVAC pro. The technician will check the connections in the furnace or air handler and complete the wiring of the thermostat.

Adding a Thermostat C Wire

We’ll cover the basics here that are fully detailed in our exclusive guide, C-Wire Issue: What If I Don’t Have a C-Wire. The C wire delivers constant power to the thermostat, so that it constantly monitors the indoor temperature and other climate features. The purpose of this is to create the most precise indoor comfort.

Many thermostats function without a C wire by periodically pulling power from one of the other wires, usually the red wire. However, if your thermostat information says a C wire is required, then this information is for you. The wire is needed for most brands and models including ecobee and Honeywell Lyric.

Note: The makers of the popular Nest Learning Thermostat say, “Don’t worry if you don’t have a C wire. The Nest thermostat doesn’t require this wire for most installations.” Our pro response is that you’ll get the best performance from any thermostat, including all Nest models, if you install a C wire or an accessory that takes its place.

If you’re removing the old thermostat and no wire is connected to the C terminal, then you have several options. Full details are in the C-Wire Issue guide referred to two paragraphs above. Here’s an overview of them:

  • Exchange your thermostat for one that doesn’t require a C wire (Nest, Lux Geo are two), and live with imperfect results
  • Use an unused wire in the bundle as the C wire. Connect it to the C terminal in the furnace/air handler and the C terminal in the thermostat. Remember to turn off power to the furnace before opening its cover(s).
  • Use the G wire (usually green) as the C wire. You’ll lose independent fan control. Attach it to the C terminal in the furnace and on the thermostat.
  • Install a splitter or jumper approved for use with your thermostat brand. The purpose is to borrow power from another terminal. See our C Wire guide for complete information, branded products for Honeywell, Sensi, and ecobee and a helpful video.
  • Call an HVAC pro for the installation.

Note on Thermostat Wiring for Communicating HVAC Systems

If you have a communicating HVAC system, then you’ll need a communicating thermostat and proper wiring. Details are available in our Communicating vs. Non-Communicating HVAC guide. Communicating systems use only 4 wires. However, the basic wiring principle is the same: Each wire must connect to the furnace or air handler terminal and the corresponding thermostat terminal.

How to Replace Thermostat Wire

You will need to replace the thermostat wiring if your new HVAC system is of a different type (24V replacing a 110V system, for example) or the wiring bundle doesn’t have enough wires to support the new system’s upgraded performance capacity (two-stage replacing a single-stage system, for example).

The important first step is to get the proper replacement wire type. It comes in bundles of 2, 3, 5, 6 and 8 wires. As you shop for it, these will be labeled 18/2, 18/3, 18/5, and so forth. Most systems today require a minimum of 5 wires when both heating and air conditioning equipment is included in the system.

18/8 wire-bundlesOur pro recommendation is to use 18/8 wire. The price difference is pennies per foot, and having 8 wires gives you the most flexibility for upgrading your system in the future. Home improvement stores and online retailers sell thermostat wire in spools of 50 to 250 feet and by the foot. Average cost is 30 cents to 80 cents per foot when sold by the foot. It’s cheaper in spools.

Fishing thermostat wiring is easy when done with care and caution:

  • Detach the old thermostat, but leave the wall mount in place to protect the drywall as you pull new wire through the opening.
  • Gently pull about 6” of the old thermostat wire out of the wall, and put a clamp on it such as locking pliers that are large enough to prevent the wire from falling down into the wall when disconnected.
  • Remove the wiring from the old thermostat base.
  • Overlap at least 4” of the old wiring with new wiring, and wrap electrical tape tightly around the overlap.
  • From the furnace location, gently pull the wiring bundle toward the furnace. Having a second person feeding wire from the spool or coil into the wall will help prevent snags.
  • Once the new wire appears, pull through as much as needed to reach the wire terminals on the furnace or air handler plus an extra foot of wire. Un-tape or cut it. Don’t cut the wire at the thermostat end yet.
  • Remove the old wiring from the furnace or air handler, if it still attached.
  • Strip ½” of insulation off each wire you will use. Wire strippers are available at home improvement stores for less than $15. Add wire connectors, if necessary, to each wire you’ll use. They are available in packs for less than $5.
  • Attach the wires to the terminals on the furnace using the color code and diagram provided with the thermostat and/or the furnace or air handler.
  • At the thermostat, connect the clamp to the new wire bundle about 8” from the wall, cut the wiring bundle, and remove the old thermostat mounting base.
  • Carefully slip the old base plate over the clamp, slip the new plate on, and mount it to the wall. If you have to remove the clamp for this process, hang onto the wiring and replace the clamp when done.
  • Strip the wires you will connect to the new thermostat.
  • Use the wiring diagram and code to attach the wires to the terminals on the thermostat that correspond to the connections on the furnace or air handler. Some thermostat models require connectors to be installed on the wires first.
  • Wrap unused wires around the bundle, remove the clamp and push the bundle gently into the wall before installing the thermostat onto the base plate.

Note: If you’re unclear on the tools and supplies needed, use this guide to compile a list. Take it to the electrical department of your local home improvement or to an electrical supply store. An expert there will assist you in gathering everything you need such as: Thermostat wire, Connectors, A wire stripper and Electrical tape etc.

Pro Tips for Installing Thermostat Wiring

If you are changing the location of the thermostat or installing one in new construction, choose the location carefully. It should be on an interior wall, since exterior walls can be cool in winter and warm in summer. This will lead to incorrect readings, your HVAC system working too hard, wasted energy and your home being too warm in winter and too cool in summer. The best thermostat location is on an interior wall near the middle of your home that isn’t affected by:

  • Direct sunlight
  • Drafty doors or windows
  • Heat or steam from a bathroom
  • Heat from an appliance or from cooking
  • A nearby heating/cooling grate or register

If installing a new thermostat and wiring isn’t something you have time or inclination to do yourself, there are pre-qualified local HVAC contractors available to do the work at a competitive price. Free quotes with no obligation to accept any of them are available using our Free Local Quote service.

3 thoughts on “In-depth Thermostat Wiring Guide for Homeowners 2021”

  1. Yes, this is excellent. Illustrations are great.
    My problem is a 2 stage compressor that has been ADDED to my Trane Furnace/AC unit that does NOT have a Y2 terminal. I see a RELAY has been added years ago, and the C terminal is used to run the wire to the Y2 of my existing thermostat. I want to install a Honeywell Wifi Thermostat, so still trying to figure out the wiring.

  2. I have a situation where I tried installing a NEST Learning thermostat but it went dead after about 3 days. I have a 5 wires coming from the air handler on my system to the thermostat. Two red wires, a green, yellow and white. I had connected the red wires to the R and Rh terminals on the NEST. After it went dead I noticed that the red wire which comes from my boiler is connected to the C terminal on the air handler. Does this suggest that this red wire should be connected to the C terminal on the NEST thermostat?


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