In-depth Thermostat Wiring Guide for 2, 3, 4, 5, 6, 7 & 8 Wires 2022

This complete 24-volt thermostat wiring guide covers all thermostat wiring issues from the simple to the complex – from basic 2 wire thermostat wiring common to gas furnaces all the way to 8 wire thermostat wiring for two stage heat pumps and similar HVAC systems.

Note that only 24-volt wiring is covered: We do not cover 110/240-volt thermostat wiring and millivolt thermostat wiring issues. 

As the Pick HVAC Thermostat Buying Guide says about these thermostats of alternate voltage:

  • 110V or 240V power: These are called high-voltage or line-voltage systems and include electric furnaces and electric baseboard systems.
  • Millivolt power: These are [used with] gas-fired heaters that don’t use electricity because they don’t have a blower fan. Most are used for supplemental heat or space heating.

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
LabelsWire ColorsWiring Functions
YYellow Cooling
Y2Light Blue/Other Colors 2 Stage Cooling
WWhite Heating
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.

Common Thermostat Wiring Options – 2 Wire to 8 Wire Thermostats

Here are full details on wiring for all common HVAC systems from simple gas furnaces to multi-component systems with accessories.

Note – Rather than list the approach for every setup, which would be redundant, follow the same strategy addressed above. After turning off power to the thermostat:

  • Remove the thermostat cover
  • Take a picture of the wiring
  • Detach wires and connect them to the same terminal on the new thermostat one at a time. Wire 1: Detach from the old, attach to the new. Wire 2: Detach/attach.

Installation tip: Make sure all the terminal screws are snug – you don’t have to overtighten them.

Follow this procedure until you have attached all necessary wires to the new thermostat.

Tip for extra wires: Bundle the additional wires, and leave them where they can be reached for potential future use. In other words, don’t shove them so far into the wall that they are “lost.”

Trial and error: There are a few instances in which you might have to use a trial and error methodology on advanced heat pump systems. This is most common when you don’t know what components and features your system has.

For example, if you don’t know whether it has Auxiliary heating strips or aren’t sure if it is two stage or single stage heating and cooling, then you might need to try a couple different wiring schemes to determine which one makes everything work as it should.

But fear not – we give you alternate wiring options for those systems. They are found below

2 Wire Thermostat Wiring

All thermostats have a 2 wire option, though for most there will be additional unused wires.

What uses 2 wire thermostat wiring? Furnaces use 2 wires. If that’s the only component in the system, then two wires is all you need.

How to wire a 2 wire thermostat:

1). Attach the Red wire to the R terminal for 24V Power – or to the RC terminal if that is where it was on the old thermostat.

2). Attach the White wire to the W terminal for Heating.

What about a C wire? Furnace-only setups do not need a Common wire.

3 Wire Thermostat Wiring

This is one level of greater complexity and includes a C wire or Common wire connection. In fact, from our experience and input from readers, 3-wire thermostats might be the trickiest to get wired up correctly. We’ll tell you upfront – without a trip to your home, it can be difficult/impossible to sort out how exactly your 3-wire system should be wired to the thermostat. If our advice doesn’t work for you, a smart, qualified local HVAC technician should be located and contacted. But here goes…

What uses 3 wire thermostat wiring? Boilers and many types of heaters including hot water heaters can have 3-wire connections. Additionally, some AC-only standard split systems use a 3-wire setup, and they seem to present the most challenges. Most online wiring guides neglect to mention AC as a possible 3-wire scenario. If you have a 3-wire connection for AC and you’re having issues, we have advice on how to solve it in this section under Alternate Step 3).

Here is the standard procedure for wiring a 3 wire thermostat.

How to wire a 3 wire thermostat:

1). Attach the Red wire to the R terminal for 24V Power – or to the RC terminal if that is where it was on the old thermostat.

2). Attach the White wire to the W terminal for Heating.

3). Use one of the remaining wires for a C wire. Many technicians choose the Green wire for this purpose – the green wire normally powers a fan, FYI – but your original thermostat might use the Black wire for this purpose. Connect the G wire to the G terminal.

Alternate Step 3). In many systems with AC, including AC-only systems, the Y wire from the air handler is the cooling input. If you have tried the 3-wire steps suggested above and your AC doesn’t work, connect the Y wire to the Y terminal to see if that solves the issue. You might have to go to the air handler (could be a furnace) to locate the color of the wire on the Y terminal there. Note: If the Y wire is running outside to the condensing unit, this is a clear indication that it is the wire you need. Connect it to the Y terminal in your thermostat, and there’s a good chance your problem will be solved.

Still doesn’t work?

One additional thing to try is to use a 5 wire connection to your thermostat. Some DIY homeowners have found this to be a solution when the AC won’t work with a 3 wire connection. Follow the guidance in the section below “How to wire a 5 wire thermostat:” One homeowner who replaced a 3-wire with a 5-wire connection said the new wiring scheme, “allows me to successfully run the AC and also the fan independently.” That’s the kind of results we like to hear!   

Info tip on C Wires: See our practical guide called C-wire Issue: What If I Don’t Have a C-wire? It explains how to use or repurpose one of the extra wires as a C-wire.

4 Wire Thermostat Wiring

Four wire thermostat wiring is becoming more common because of the popular HVAC systems that require it.

What uses 4 wire thermostat wiring? The most common 4 wire thermostat wiring is needed for heat pump systems that include a condensing unit outside and an air handler indoors. Unique to this system are the need for a fan wire and a wire to power AC cooling.

Smart thermostats: ecobee, nest and a few other smart/learning/WiFi thermostats also require 4 wire thermostat wiring.

How to wire a 4 wire thermostat:

1). Attach the Red wire to the R terminal for 24V Power – or to the RC terminal if that is where it was on the old thermostat.

2). Attach the White wire to the W terminal for the heat pump’s Heating mode.

3). Attach the Yellow wire to the Y terminal for AC/cooling mode.

4). Attach the Green wire to the G terminal for the Fan.

What about two stage heat pumps? What about aux heat?

This 4 wire thermostat wiring is for simple, single stage heat pumps. See 5-wire and other options for more complex heat pump systems.

5 Wire Thermostat Wiring

There are many HVAC systems that require 5 wire thermostat wiring to connect all their components. Following these directions will lead to successful thermostat replacement.

What uses 5 wire thermostat wiring? HVAC systems with ACs, heat pumps, furnaces and more.

When 4 wire thermostat wiring plus a C-wire is required, then you need 5 wire thermostat wiring. It’s pretty simple. The key is to use a wire not used for another purpose as the common wire, or C-wire.

How to wire a 5 wire thermostat:

1). Attach the Red wire to the R terminal for 24V Power.

2). Attach the White wire to the W (or W1) terminal for the heat pump’s Heating mode.

3). Attach the Yellow wire to the Y terminal for AC/cooling mode.

4). Attach the Green wire to the G terminal for the Fan.

5). Attach a “spare” wire to the C terminal – such as the Black wire for C or Common.

6 Wire Thermostat Wiring

Your wiring bundle should have 8 wires – so you’re not out of wires yet.

What uses 6 wire thermostat wiring? Heat pump systems with a C-wire and advanced heating and AC require 6 or more wires. For example, a two stage heat pump, one with Aux heat and/or one that has a reversing valve that needs power all require 6 wire thermostat wiring or more (7 and 8 wiring follow).

How to wire a 6 wire thermostat:

1). Attach the Red wire to the R terminal for 24V Power.

2). Attach the White wire to the W (or W1) terminal for the heat pump’s Heating mode.

3). Attach the Yellow wire to the Y terminal for AC/cooling mode.

4). Attach the Green wire to the G terminal for the Fan.

5). Attach a “spare” wire to the C terminal – such as the Black wire for C or Common.

There are a few options for step 6:

6). If it is a single stage heat pump but has Aux heat strips in the air handler, attach an unused wire, such as the Light Blue wire, to the X/Aux terminal for Auxiliary heat.

0r:

6). If the heat pump has two stage cooling, attach the Light Blue wire to the Y2 terminal for 2 Stage Cooling.

Or:

6). If the heat pump has two stage heating, attach the Brown wire to the Y2 terminal for 2 Stage Heating.

Pro tip: Read the thermostat installation manual and check the HVAC system manuals to know specifically what your system includes – Aux heat, reversing valve, 2 stages of heating and cooling, for example, to know which terminals to use.

Note: If it has both two stage heating and cooling, you’ll need 7 or 8 wire wiring.

7 Wire Thermostat Wiring

Now we’re talking about an HVAC system with a lot going on.

What uses 7 wire thermostat wiring? Many types of systems can. A heat pump with 2 stages of heating and cooling, a system with Aux heating or one that has a reversing valve that needs to be energized.

How to wire a 7 wire thermostat:

1). Attach the Red wire to the R terminal for 24V Power.

2). Attach the White wire to the W (or W1) terminal for the heat pump’s Heating mode.

3). Attach the Yellow wire to the Y terminal for AC/cooling mode.

4). Attach the Green wire to the G terminal for the Fan.

5). Attach a “spare” wire to the C terminal – such as the Black wire for C or Common.

6). Attach the Light Blue wire to the X/Aux terminal for Auxiliary heat.

7). Use an unused wire for the accessory.

Alternate options: If you don’t have Aux heat or an accessory light or a powered air cleaner but the system’s reversing valve needs to be energized, then here are steps 6 and 7.

6). Attach the Dark Blue wire to the B terminal for Reversing Valve for Heating.

7). Attach the Orange wire to the O terminal for Reversing Valve for Cooling.

8 Wire Thermostat Wiring

Here is the most common use of 8 wires.

What uses 8 wire thermostat wiring? These are almost always heat pump HVAC systems Aux heat.

How to wire an 8 wire thermostat:

1). Attach the Red wire to the R terminal for 24V Power.

2). Attach the White wire to the W (or W1) terminal for the heat pump’s Heating mode.

3). Attach the Yellow wire to the Y terminal for AC/cooling mode.

4). Attach the Green wire to the G terminal for the Fan.

5). Attach a “spare” wire to the C terminal – such as the Black wire for C or Common.

6). Attach the Light Blue wire to the X/Aux terminal for Auxiliary heat.

7). Attach the Dark Blue wire to the B terminal for Reversing Valve for Heating.

8). Attach the Orange wire to the O terminal for Reversing Valve for Cooling.

Thermostat Wiring Troubleshooting

As we said, on advanced systems that you are unfamiliar with and there is no existing thermostat with wiring to replicate, you might have to do some trial and error.

If you’re not able to wire your thermostat and get all components of your HVAC system to work as they should, then call an HVAC technician for help. Most likely, the technician will be able to properly wire the system in about an hour – and you’ll only be liable for the service call/first hour fee of $75 to $150.

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-bundles

Our 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.

Written by

Rene has worked 10 years in the HVAC field and now is the Senior Comfort Specialist for PICKHVAC. He holds an HVAC associate degree from Lone Star College and EPA & R-410A Certifications.

6 thoughts on “In-depth Thermostat Wiring Guide for 2, 3, 4, 5, 6, 7 & 8 Wires 2022”

  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.

    Reply
  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?

    Reply
  3. This was great and in depth but the only major problem is you did not cover anything that does not have an air conditioner I’ve got a three wire air conditioner that I’m trying to wire up to this stupid thermostat and I can cross all three wires and the whole damn thing works I wire it up to my thermostat and nothing works and you covered nothing in this.

    Reply

Leave a Comment

DMCA.com Protection Status