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Thermostat wiring reference chart - simplest case two-wire thermostat (C) Daniel Friedman How Wire a Thermostat
Thermostat Wiring Connection Tables
Hook-up Procedures for Heating, Heat Pump, or Air Conditioning Thermostats

Room thermostat installation & wiring guide: this article series explains the basics of wiring connections at the thermostat for heating, heat pump, or air conditioning systems.

We provide Honeywell, White Rodgers & other thermostat wiring diagrams and explanation showing how to wire a room thermostat, including just what connections to make and how wires and connectors are color coded to make things easy.

Our page top sketch, courtesy of Honeywell Controls, illustrates the wiring diagram for a traditional Honeywell T87F thermostat used for 2-wire single pole single throw control of heating only in a typical gas-fired heating system.

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

Guide to Room Thermostat Installation, Location, & Wiring

Where NOT to locate the room thermostat  (C) Carson Dunlop AssociatesArticle Series Contents

Where to Locate & Mount a Room Thermostat & what spots to avoid

Locate the room thermostat at about chest height on an interior wall, in a location where the thermostat won't be affected by drafts or other unusual temperature conditions.

[Click to enlarge any image]

Good thermostat locations are

As Carson Dunlop Associates ' sketch (left) explains, there are a number of places where you should not locate the room thermostat.

Bad Room Thermostat Locations

Don't locate the room thermostat in these locations:

Room Installation Thermostat Installation Tips

Does the Room Thermostat Need to be Level on the Wall?

Yes for Older Mercury Bulb Switches

Thermostat wire connections (C) Daniel Friedman

It's important to mount mercury-bulb thermostats as level as possible since otherwise you're putting the thermostat out of accurate temperature calibration.

That's because the coiled bimetallic spring has to move that mercury bulb to a tipped position to turn the heating or air conditioning system on or off in response to room temperature.

When we found a thermostat that did not heat a room accurately to the called-for temperature, we'd take a look to see if it was installed out of level before looking further.

Because the "set" range on these thermostats may have had a lower-end of 55 deg .F., when we wanted the thermostat to maintain a building at a temperature lower than the minimum that the thermostat dial provided, we just tilted the whole thermostat backing plate on the wall in the proper direction to shift the operating range of the switch.

No for Newer Solid State, or Digital Room Thermostats

Thermostat wire connections (C) Daniel Friedman

Newer thermostats that rely on other sensor and switch designs do not have this sensitivity to being out of level and some (such as the 3M-22 thermostat) note in their installation instructions that the thermostat does not have to be level.

Our photo (above left) illustrates the red (R) and white (W) wire connections in a simple two wire heating control thermostat hookup using the Honeywell RTH2300/RTH221 series programmable wall thermostat.

Here is a copy of

the Honeywell RTH2300/RTH221 Series Programmable Thermostat Owners Manual [PDF]

Because some controls are used in common on hot water heat, hot air heat, and steam boilers, readers should see these other articles:

see BOILER CONTROLS & SWITCHES,

If your building uses warm air heat,

see FURNACE CONTROLS & SWITCHES.

If your building uses steam heat

see STEAM HEATING SYSTEMS.

Thermostat Wire Cable Types and Gauge Sizes

Thermostat wire used for heating and cooling systems is usually 18 gauge solid copper conductor, unshielded cable. Some installers may use 20 AWG copper thermostat wire.

Thermostat Replacement Types: What Kind of Thermostat Do I Have?

Traditional round Honeywell wall thermostat (C) Daniel Friedman

A quick scan of heating and cooling thermostats in the catalogs of HVAC suppliers such as Grainger can quickly become overwhelming as there are literally hundreds of thermostat models and quite a few different types and applications.

If you are replacing a heating or cooling room thermostat or "wall thermostat" a shortcut is to examine what is already installed.

Internal parts of a thermostat and how they work

At THERMOSTATS, HEATING / COOLING we include photographs of just about every type of heating and cooling thermostat. Here we describe further details that can help you choose a replacement thermostat that will accomodate the wires used to control you rair conditioning, heat pump, or heating system.

Nest thermostat

Above: A Nest Learning Thermostat. See NEST LEARNING THERMOSTATS - topic home, or

see NEST THERMOSTAT INSTALLATION & WIRING for details about wiring Nest thermostats.

Pay particular attention to the following to help sort out what you've got or what type of thermostat you need:

Hooking up a 2-Wire Thermostat: How Do I Install & Wire Up a Room Thermostat like the Honeywell Round Wall T87F Series Thermostat?

Honeywell thermostat backing plate showing wiring connections

Basic 2-wire Room Thermostat Wiring Instructions

Wiring connections for a room thermostat such as the Honeywell 24-volt T87F, the Honeywell series 10 (out of production), or Penn "Rimset" low-voltage wall thermostat models are pretty simple as are the wiring instructions for White Rogers, Mercoid, General Controls, and similar thermostats.

Our photo shows the backing plate that is mounted first when installing a round Honeywell Series 20 type room thermostat. Notice that the plate shows a "level" line.

We left off the actual wires so that you could see the "R" and "W" by the two screw terminals at the lower left 7 and 8 o'clock positions on the thermostat backing plate.

Note: The thermostat in this HVAC/R set-up is simply acting as an "on-off" switch to turn the heater or air conditioner on or off in response to room temperature.

The electrician or HVAC installer may have run a multi-wire set of low voltage wires through the walls of the building between the low voltage transformer and the thermostat (etc), but in this simple installation the other wires at the thermostat are not being used.

In a two-wire installation, the thermostat backing plate is mounted level on an interior wall in the room which we want to be the master temperature control for the area served by the heating or in a cooling-only climate, the cooling system.

The red wire from

the heater (or air conditioner control) or more-likely from the low voltage transformer is connected to the "R" terminal on the backing plate.

The white wire

 from the heater or air conditioner control is mounted to the "W" terminal on the thermostat mounting plate. Typically the white wire is taking power from the thermostat to the operating control circuit board in the heater or air conditioner.

Typically the red wire is originating at the heating low voltage transformer and brings power to the thermostat. The transformer may be mounted on a wall near your heater, inside the heater or furnace, and on heat pump systems, on occasion the low voltage transformer may be outdoors in the compressor/condenser unit.

At the low voltage transformer you will see two wires labeled "C" for common and "R" for red. We discuss wiring the low voltage transformer itself

If you need to confirm that the low voltage transformer is working and putting out the right voltage,

see LOW VOLTAGE TRANSFORMER TEST

Summarizing, a typical 2-wire room thermostat where just three thermstat wires are all in use, the connections are as follows:

Table of 2-Wire Thermostat Connections

Thermostat Terminal ID Function 2-Wire TT
Wire Color
Typical
Wire
Colors
Comments
R Control Power (24VAC) Red Red

From heater, control relay, or board

originating From 24VAC from transformer to heater

Common or C

W Heating White White To heater control relay or board

Watch out: a few installers may not have followed the convention of Red = power.

Always use a DMM/VOM to confirm the power source. Always follow wires to the controlled device to confirm where they are connected and thus their function.

Also see COMMON HEATING AND COOLING THER MOSTAT WIRING CONNECTIONS for added details for specfic thermostat brands.

Watch Out do not short any wires together. Turn off power & confirm it is off.  

Don't forget to turn off all electrical power involved with your heating system before working on thermostat wiring, and confirm that power is off where you are working by using an appropriate test instrument such as a VOM. Failure to respect this advice risks equipment damage, and in some cases electrical shock or even a building fire.

Watch out: typically an older 2-wire roomthermostat hookup is not using the C wire or COMMON wire but if you replace such a thermostat with a new WiFi or Smart room thermostat you'll need the C-wire.

See solutions at COMMON WIRE at THERMOSTATS

Where do the Thermostat Wires Start and Where do they End-Up?

As we explain in more detail

at THERMOSTAT WIRING DIAGRAMS,

Honeywell T87F Thermostat wiring diagram for 2-wire, spst control of heating only in a typical gas fired heating system - details from Honeywell Controls

At left the thermostat wiring diagram illustrates use of a Honeywell T87F thermostat in a 2-wire application controlling a gas fired heating appliance.

In the Honeywell T87F thermostat series the single pole double throw switch makes (closes) one set of contacts when the temperature falls - to turn on the heating appliance.

In thermostats that also control a cooling system, a second set of contacts will make or close on temperature rise. This second set of contacts may also be used to operate other controls or valves in some heating systems.

[Click any image or thermostat wiring schematic to see an enlarged, detailed version]

Low voltage transformer wires (C) Daniel Friedman

For completeness in understanding where the red and white thermostat wires originate and end up, at below left we illustrate the red and white thermostat wires originating at the low voltage transformer, and at below right we illustrate a typical thermostat wiring connection in an R8182 Aquastat primary controller of a heating boiler, using the T and T terminals.

Low voltage transformer wires (C) Daniel Friedman

3-Wire Thermostats: Where Do We Connect the Red, White, & Blue (if present) Wires to a Typical Three-Wire Wall Thermostat?

Honeywell thermostat backing plate showing wiring connections

Three-wire thermostat wiring instructions, also called "series 20 installations" we have three wires rather than two to connect.

Then the thermostat body is screwed in place. The screws that secure a round Honeywell traditional wall thermostat to its backing plate will also connect it properly to the wiring.

A plug connector may be used:

On other, in fact most contemporary room or wall thermostats it may be necessary to plug in a connector between the thermostat and its mounting plate.

On a typical 3-wire room thermostat where just three thermstat wires are all in use, the connections are as follows:

Table of 3-Wire Thermostat Connections

Thermostat Terminal ID Function 3-Wire TT
Wire Color
Typical
Wire
Colors
Comments
R Control Power (24VAC) Red Red Common or C wire from heater to thermostat
Y Heat White White  
W Cooling Blue Blue from cooling to thermostat

Also see COMMON HEATING AND COOLING THERMOSTAT WIRING CONNECTIONS for added details for specfic thermostat brands.

4-Wire Thermostat Wiring Connections

On a typical 3-wire room thermostat where just three thermstat wires are all in use, the connections are as follows:

Table of 4-Wire Thermostat Connections

Thermostat Terminal ID Function 4-Wire TT
Wire Color
Typical
Wire
Colors
Comments
R / RC Control Power (24VAC) Red Red Common or C or 24VAC voltage, power to heating or cooling device
Y Cooling Yellow Yellow Turns on A/C compressor/condenser
G Fan relay Green Green Furnace or air handler blower
W Heating White White Turns on heating furnace or heat pump contactor relay

Notes to the Table Above

When the thermostat connects red to white you're calling for heat

When the thermostat connects red to yellow you're calling for cooling

When the thermostat connects red to green you're turning on the blower fan or air handler fan

 

Also see COMMON HEATING AND COOLING THERMOSTAT WIRING CONNECTIONS for added details for specfic thermostat brands.

...

5-Wire Thermostat Wiring Connections

On a typical 5-wire room thermostat where five wires are all in use, the connections are as follows:

Table of 5-Wire Thermostat Connections

Thermostat Terminal ID Function 5-Wire TT
Wire Color
Typical
Wire
Colors
Comments
C Common C-wire Black Black Blue Brown COMMON WIRE at THERMOSTATS
R, Rh, Rc Control Power (24VAC) Red Red  
W, W1 Heat White White  
Y, Y1 Cooling Yellow Yellow  
G Fan Green Green  

Notes to the Table Above

Also see COMMON HEATING AND COOLING THERMOSTAT WIRING CONNECTIONS for added details for specfic thermostat brands.

6-Wire Thermostats: Where Do We Connect the Red, White, & Blue, Yellow, Green, Orange Wires in a Typical Six-Wire Wall Thermostat?

3M-22 Filtrete thermostat wiring terminal layout (C) Daniel Friedman

Color Codes and Hookups for Thermostat Wires, typical for heat pump installations and thermostats

At left you can see the 11 possible thermostat wire connectors on the 3M-22 FIltrete thermostat. In this installation only two wires, R and W, have been connected.

Typical Thermostat Wire Connections for Heat Pumps

Typical connections when wiring a heat pump are given in the table below.

Again, you will have to check to see how the installer has ran the wires from the thermostat.

On a typical 6-wire room thermostat where eight wires are all in use, the connections are as follows:

Table of 6-Wire Thermostat Connections

Thermostat Terminal ID Function 6-Wire TT
Wire Color
Typical
Wire
Colors
Comments
C Common C wire Black Black Blue Brown Common connection back to transformer
R, Rh, Rc Control Power (24VAC) Red Red Power
W, W1 Heat White White Heating
Y, Y1 Cooling Yellow Yellow Cooling
G Fan Green Green Fan control
O / B Reversing Valve Orange Orange
Dark Blue
Switch the reversing valve

Notes to the Table Above

BLUE - the blue is usually common, dark blue (or orange) is to reversing valve

Also see COMMON HEATING AND COOLING THERMOSTAT WIRING CONNECTIONS for added details for specfic thermostat brands.

Typical thermostat connections when wiring a thermostat to control only a heating system or only an air conditioning system

If your thermostat is controlling only a heating system or only an air conditioning system, you will only have a red wire, and a white wire coming form the inside unit, to the outside unit.

On an air conditioner these two wires will go on the contactor to send 24 volts to the coil to pull the contactor in to start the air conditioner.

On a heating system these two wires will go to two thermostat connections on the primary controller such as an aquastat or air handler control that turns on the heating unit. Depending on the type of heating system, the thermostat, acting simply as an "on-off" switch will

These are the traditional wiring schematics, again, check the thermostat to see if this is the case with yours.

8-Wire Thermostats, Wiring Connections & Wire Colors

On a typical 8-wire room thermostat where eight wires are all in use, the connections are as follows:

Table of 8-Wire Thermostat Connections

Thermostat Terminal ID Function 8-Wire TT
Wire Color
Typical
Wire
Colors
C Common (24VAC) C-wire Black Black Blue Brown
R, Rh, Rc Control Power (24VAC) Red Red
W, W1 Heat White White
Y, Y1 Cooling Yellow Yellow
G Fan Green Green
O / B Reversing Valve Orange Orange
Dark Blue
W2 2nd Stage Heat Brown Brown
Y2 2nd Stage Cooling / Compressor Blue Blue

Notes to the table above

See generic thermostat wire color code conventions in a complete set of tables

at THERMOSTAT WIRING COLOR CODES

Also see COMMON HEATING AND COOLING THERMOSTAT WIRING CONNECTIONS for added details for specfic thermostat brands.

Also see COMMON WIRE at THERMOSTATS for an explanation of this wire.

...

Instructions for Wiring of Line Voltage 120V Wall Thermostats

We moved this topic to LINE VOLTAGE 120V Wall Thermostat Wiring - separate article in order to keep our text on line voltage thermostats together.

Tables of Common Heating and Cooling Thermostat Wiring Connections for Major HVAC & Thermostat Brands

For clarity & speed we have moved wiring instructions for individual types or brand of heating or cooling thermostats to the individual articles listed below at MORE READING

Watch Out: do not short any wires together. Turn off power & confirm it is off.  

As we have warned before, don't forget to turn off all electrical power involved with your heating system before working on thermostat wiring, and confirm that power is off where you are working by using an appropriate test instrument such as a VOM.

Failure to respect this advice risks equipment damage, and in some cases electrical shock or even a building fire.

We provide thermostat wiring connections for just about every type of residential heating or cooling room thermostat as well as a description of thermostat wiring color codes & conventions.

This article series answers most questions about central heating system troubleshooting, inspection, diagnosis, and repairs. We describe how to inspect, troubleshoot and repair heating and air conditioning systems to inform home owners, buyers, and home inspectors of common heating system defects.

Dayton Thermostat Installation & Wiring Guides

Dayton 4PU51 thermostat manual & wiring instructions at InspectApedia.com

Dayton line voltage thermostat - cited & discussed at InspectApedia.com

Pro1 Thermostat Installation & Use Guides

Sunvic Room Thermostat Wiring Guides

Shown below, a Sunvic room thermostat for sale in the U.K., Models TLM2402, TLM2453, TLM2661, with an installation leaflet from 2012.

UK Sunvic room thermostat cited and discussed at InspectApedia.com

 




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Reader Comments, Questions & Answers About The Article Above

Below you will find questions and answers previously posted on this page at its page bottom reader comment box.

Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2020-11-28 - by (mod) -

Deb you'll want to be sure you know what each of your existing thermostat wires actually is - see details at

THERMOSTAT WIRE CONNECTIONS - 2-WIRE like the T87F inspectapedia.com/heat/Thermostat_Wiring_Instructions.php#2w

where you'll see that the standard wire colours are R - red and W - white.

On 2020-11-28 by Deb

Two wire non programmable thermostat do I use w or b connector

On 2020-11-26 - by (mod) -

Gary

There might be a thermostat that will switch on backup heat but not that I've found.

Instead backup heat usually works like this:

In the heating system itself, normally a heat pump (outdoor compressor/condenser capable of running in heating or cooling mode) supplies warm or cool refrigerant to the air coil in the indoor air handler (typically a forced-air heating or cooling system).

When in heating mode if the outdoor compressor/condenser cannot provide enough heat (because of low outdoor temps) then in the air handler a control board (printed circuits, sensors, relays, etc) senses that condition and switches on a backup heat source - such as electric heaters in the air handler or an external hot water heating boiler that feeds hot water to a hot water coil in the air handler.

All of that time the call for heat was originating at the same basic wall thermostat.

On 2020-11-26 by Gary

Is there a WiFi thermostat that has emergency backup heat? I have a gas pack and want to use the heat pump feature for my heat but if I want gas heat on colder days I can set it to that.

All I see are cold and heat, I have a Sensi now and the heat is gas. Would like to set it to heat pump. I have a trane and replaced the trane thermostat with the Sensi.

On 2020-09-11 - by (mod) -

inspectapedia.com/heat/Thermostat_Wiring_Instructions.php#6w

On 2020-09-11 by Jeff


Thanks, I read through that but the Orange and Blue Wires were not used and there is no C on the therm wiring lug. How would I check the Black wire function?
What other lug could the black wire go to?

On 2020-09-11 by Anonymous

what other lug could the black wire go to?

On 2020-09-11 by Anonymous

Thanks, I read through that but the Orange and Blue Wires were not used and there is no C on the therm wiring lug. How would I check the Black wire function?

On 2020-09-11 - by (mod) -

Jeff

My best-organized reply is at

THERMOSTAT WIRE CONNECTIONS - 6-WIRE Red, White, Blue, Yellow, Green, Orange Wires

https://inspectapedia.com/heat/Thermostat_Wiring_Instructions.php#6w

There you'll see that the O or B spot is usually working the reversing valve - usually that's an orange or dark blue wire.

The black wire is attached to C or common and is a 24VAC power source.

Check to see what your Black is doing.

Also see COMMON WIRE at THERMOSTATS

On 2020-09-11 by Jeff

I replaced an older Honeywell therm with a Honeywell RTH2510/RTH2410. I replaced as they were in the old therm but no work. 6 wire--Red, White, Yellow, Green, Brown, and Black. ALL wires are in the corresponding color spots, and I have the Black in the "B" spot and the Brown in the "O" spot. Do I have it wrong?

On 2020-02-10 - by (mod) -

John

What part are you replacing?

On 2020-02-10 by John Lloyd Chipman

I have a replacement part for a Kenmore wall Air conditioner. The original part had a three wire connection and the replacement has a two wire connection. Original part is 2H01127B. Replacement is 2H01109L. For kenmore Model number 580.72187200. Can anyone tell me how to connect the new part?

On 2018-11-18 - by (mod) -

Ken

This may have already been obvious to you but the first thing I would try in the circumstance you describe is to confirm that the batteries are good.

On 2018-11-17 by Ken

I installed a new Thermostat because my heat and A/C stop working at the same time. My old Thermostat would not stop flashing the battery sign even after intalling new batteries.

On 2018-06-01 - by (mod) -

Mikki

THERMOSTAT WIRING TIPS & COLOR CODES - methods for identifying which thermostat wires - at https://inspectapedia.com/heat/Thermostat_Tips.php

will tell you the standard color codes for wires and how they're used.

But you also need to identify the brand and model of your thermostat so that you know which wires connect to which terminals in the thermostat by following the instructions that come with that device.

On 2018-06-01 by mikki

I'm trouble i have red white. yellow but there alot of wires which one do I need for AC

On 2017-01-17 by Bobbie

My friend has a simple Honeywell digital thermostat that he just bought it's only got two buttons to turn the heat up or turn the heat down the three wires that are coming out of the wall are two white one black where do they go

On 2016-06-10 - by (mod) -

Dale, you need the brand and model of your thermostat; that'll get you is installation instructions that will tell you its wire connections and functions.

On 2016-06-10 by dale browning

Hey guys,I would really like to know what two wires on my digital thermostat will let me manually start the fan,so I can eliminate the possibility of it being bad

On 2016-04-06 - by (mod) -

Rich you'll find links to White Rodgers thermostat wiring in the article above beginning at

https://inspectapedia.com/heat/Thermostat_Diagrams.php#HDiags

On 2016-04-06 by Rich

I need help wiring an older white-rodgers thermostat

On 2014-06-05 by Malcolm Richards

I am replacing a 3 wire digital room thermostat with a 2 wire thermostat but need advice please. The model is a Homexpert by Honeywell THR870CUK


...

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THERMOSTAT WIRE CONNECTIONS at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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