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Thermostat wiring reference chart - simplest case two-wire thermostat (C) Daniel Friedman Thermostat Wire Reference Chart

Function of each wire or terminal letter ID
Function of each wire color

  • POST a QUESTION or COMMENT about what different thermostat wires actually do - what each wire controls is given in this standard thermostat wiring chart

Thermostat wire reference chart:

What is the meaning or use of each thermostat terminal letter or wire color?

Knowing what each thermostat wire does can help in wiring up a new or replacement room thermostat.

This data can also be helpful if thermostat wires have become mixed-up or improperly color-coded by allowing you to trace wires back to their control connection point.

We describe 25 different possible thermostat wires and their functions and connection point.


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Basic Thermostat Wire Reference Chart

What does each thermostat wire do or control and when are the various wires connected, taped-off, or connected to a labeled connector different than the wire's ID label?

Thermostat Wire ID Tags or ID of Connector Terminals in a Room Thermostat
Possible Wire ID Label or Connection Point ID Label on Thermostat Control Function / Connection Point for This Wire Comments
4 RH power for HEAT (RH not connected to RC, jumper, clip removed) or RH
B

B heat pump changeover (cool to heat, powered in heat mode)

In thermostat applications where a blue wire is used, the blue is usually a common circuit wire - e.g. providing the return path for the circuit.

  
B and O together Trane heat pump products. If not Trane, tape off B. See warning in Note 4.
For Trane, see C function below.
See O function below
C C - common 24VAC power used to power the thermostat.
Trane uses B for this connection.

or X

COMMON WIRE at THERMOSTATS
DH DH - external dehumidifier operation  
E Emergency heat control Not used on the 3M-22.
EX EX - external fresh air baffle control  
F F - Fan control or G
Connect to G on the 3M-22
G 

G - Fan control

In most thermostat hookups the green wire is used to control a fan.

 or F
H H - external humidifier control  
L L - System monitor Not used on the 3M-22
O

O Heat pump changeover (heat to cool, powered in cool)

In most thermostat wiring hookups the orange wire is used to control a reversing valve - switching from heating to cooling mode or vice-versa.

 See (B and O) above
R 

RH and RC single power for HEAT and COOL

In most thermostat wiring hookups the red wire is the power source for the system.

 or V or VR
RC RC power for COOL (RH not connected to RC, jumper clip removed)  
RH RH power for HEAT (RH not connected to RC, jumper, clip removed) or 4
T T - outdoor temperature sensor Not used on the 3M-22
V RH and RC single power for HEAT and COOL or R or VR
VR RH and RC single power for HEAT and COOL or R or V
W

W Heat control (usually this is the white wire)

In most thermostat wiring hookups the white wire is used to switch heat on or off from the thermostat.

  
W2 W2 - second stage HEAT or if heat pump, auxiliary / backup heat control  
W3 W3 - third stage HEAT or 2nd stage of 2-stage auxiliary / backup heat  
X  C - common 24VAC power used to power the thermostat.
Trane uses B for this connection.

(or C)

COMMON WIRE at THERMOSTATS
Y

Y - COOL control or 1st stage compression for heat pump control

In most thermostat wiring hookups the yellow wire is used to switch cooling on or off from the thermostat.

  
Y2 Y2 - second stage COOL, 2nd stage compression for heat pump control  

Notes to the table above

1. Reference source: 3M Filtrete™ 7-Day Programmable Thermostat Model 3M-22 installation instructions.

2. Where comments indicate "or ..." most likely this wire or wire ID will be used for just one of the "or" possibilities listed. For example "R or V or VR" means that one of these three wire labels and connection terminals will be present and used for the control function described in the table.

3. For most thermostat hookups you will not be using all of the wires and control functions listed here.

4. Watch out: Where B & O appear together for Trane heat pumps. If not a Trane product O is typically not used. Check your thermostat wiring instructions. For example, for the Thermostat Model 3M-22 this B wire is connected to thermostat terminal C for Trane products and for other brands B is taped off and not used.

5. Tape off wires that are not used and not connected to any terminals in the thermostat you are installing.

To find wiring diagrams for your specific thermostat by brand and model, please see the ARTICLE INDEX or see the articles at the end of this page at CONTINUE READING

 

Summary of he MOST BASIC thermostat wire Identities

Blue or Black – C – Common wire, may be unused, can provide continuous power flow from the red wire as we discussed above.

See details at COMMON WIRE at THERMOSTATS

Red – R – 24VAC power from the furnace’s transformer

Red – Rc – 24VAC (dedicated to heat call)

Red – Rc – 24VAC (dedicated to cooling call)

Green – G – Fan

White – W – Heat

Yellow – Y – Air conditioner

At THERMOSTAT WIRE CONNECTIONS are more detailed thermostat wiring tables identifying more terminals - we should both look over there.

...


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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

What's the Tc wire on my Lennox Heat Pump Thermostat?

I currently have a Lennox heat pump system and the thermostat I have is a touch screen Lennox one.

I've figured out what the To wire would connect to after a bit of research,

but the Tc is throwing me off.

I know my basic wires, thanks to my dad, it was just those two throwing me off. When I get a chance later on, I will upload the wiring and see if that will help.

Hi! I have a heat pump and I am trying to figure out what these two wires are. I am trying to update my thermostat to a Nest 3rd Generation.

I came across two wires that are connected and I don't know what they are. Its: Tc (connected to the black wire) and To (connected to a brown wire). I am trying to figure out what they are for so that I can connect them correctly to the Nest without messing up. Thank you! On 2017-12-26 by Christina

Reply by (mod) -

Christina -

I am guessing (as I don't know exactly what equipment you're hooking up ) that the C wire is used to complete the 24VAC circuit to your wall thermostat - it's not used in all thermostat hookups as often the 24VAC current runs from the transformer through a red wire to the

heating or cooling equipment and thence in to the thermostat and out of the thermostat on another control wire to a specific control relay or device to turn the A/C or heater on and off. .

But somer thermostats need the full circuit to power the thermostat itself, so make use of the Tc wire.

Other possibilities:

Tc can refer to hookup for the cooling control from the thermostat - puts your heat pump in cooling mode

Watch out: for some thermostats the wire labelled TC should not be connected to any terminals on some thermostat models. You'll want to double-check with Nest.

I'm not sure what the Tc wire is either in your description though normally I'd guess it's referring to Thermostat-Common wire.

But as the brown wire is typically for emergency heat that might help us out.

If you identify the specirfic brand and model of your heat pump and air handler then you can download the installation and operations manual.

See LENNOX HVAC MANUALS & PARTS GUIDES

and

See THERMOSTAT INSTRUCTIONS & MANUALS BY BRAND & MODEL

A summary of basic thermostat wire connections is above on this page. Take a look and let me know what more you need.

How do I wire my Flair zone valves to my White Rodgers Thermostat?

Thank you I have been all over those pages and they show my wiring for my flair valve 4-5-6 blk wh red but are labeled open common and closed no where on those pages does it give me the comparable terminal for open closed and common thank you for your response

I have the 3 wire thermostat that you show the wiring 4 5 6 open common and closed

I cannot find in any of you charts which terminals to connect 4 5 6 blk wh red wires respectively on the terminals
I have the 70 series programmable 1F72-151 please tell me which terminals to use
Thank you On 2016-06-13 by Johnny

by (mod) - Where to find details about wiring a White Rodgers thermostat

Johnny

For more details about connections on White Rodgers thermostats, searching InspectApedia for WHITE RODGERS THERMOSTAT WIRING

will take you to the right page

Where you will find a link to a free PDF copy of instructions for your thermostat.

Can a single thermostat control Air Conditioning and Heat when I have both a boiler and a heat pump?

Installing AC with heat pump and emergency electric coil. House already has oil fired hot water heating system. Can one thermostat control all components? On 2015-08-06 by Ed

by (mod) - Probably yes, if wired correctly

Ed

Well yes and no.

IF the thermostat wire bundle routed to the thermostat included enough wires to operate all of the needed functions and IF the model thermostat includes both heating and cooling, the answer is probably yes.

Backup heat will typically switch on automagically depending on outdoor temperature.

But in older homes I typically find that the boiler and the heat pump are handling the heating and cooling of the building through completely separate systems, including heating or cooling distribution; in that case each has its own themosetat.

Daniel Friedman

...

Continue reading at THERMOSTAT INSTALLATION STEPS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

Suggested citation for this web page

THERMOSTAT WIRE TERMINAL ID CODES / FUNCTIONS at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

Or see this

INDEX to RELATED ARTICLES: ARTICLE INDEX to HVAC THERMOSTATS

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Citations & Reviewers

In addition to any citations in the article above, a full list is available on request.

  • [1] Honeywell Owner's Manual, CT87N / CT87K Round Thermostat, Honeywell International, Inc., 1985 Douglas Drive North, Golden Valley MN, 55422 or in Canada: 35 Dynamic Drive, Toronto, Ontario M1V 4Z9, Canada, Website: http://yourhome.honeywell.com retrieved 2016/03/06.
  • [2] Thanks to reader S.R. for discussing loss of heat due to a thermostat wiring mistake, October 2010
  • [3] Thank to Mr. Scott Meenen , G&S Mechanical Services , for providing some common thermostat wiring codes also found at Mr. Meenen's web page Malware Deleted 12/9/2014 . Mr. Meenan provides heating, heat pump, and air conditioning repair services in Maryland, Washington D.C., and northern Virginia. He can be contacted at 301-591-1646 or by Email to Malware Deleted 12/9/2014 - 10/2010. Quoting:

    We service American Standard, Amana, Arco, Arco-Air, Bryant, Carrier, Coleman Evcon, Comfortmaker, Day/Night/Payne, Dunham-Bush, Fedders, Fredrich, Goodman, General Electric, Heil, Intertherm, ICP, Janitrol, Lennox (Armstrong, Johnson Air-Ease), Miller, Modine, Nordyne, Rheem/Ruud/Weatherking, Sears, Stewart Warner, Trane, Weather King, Williams, White-Westinghouse, Whirlpool, Weil Mclain, York, (Frasier Johnson/Borg Warner) and others.
  • [4] Azel Technologies Inc., P.O. Box 53138 10 Royal Orchard Blvd. Thornhill, Ontario, Canada L3T 7R9 Ph: 905-223-5567 Fax: 905-223-3778 Email: info@azeltec.com, Website: www.azeltec.com.
  • [5] Honeywell Controls, the company wants you to use their contact form at this web page: http://www51.honeywell.com/honeywell/contact-support/contact-us.html
    Honeywell Consumer Products, 39 Old Ridgebury Road Danbury, CT 06810-5110 - (203) 830-7800
  • [6] White Rodgers Thermostats and HVAC controls,

    Homeowner information: http://www.emersonclimate.com/en-US/brands/white_rodgers/Pages/wr-homeowner-info.aspx

    Contractor information: http://www.emersonclimate.com/en-US/brands/white_rodgers/wr_contractor_info/Pages/white-rodgers-contractor-info.aspx

    White Rodgers Product Catalog (don't misspell the company's name as White Rogers Thermostats) -
    http://www.emersonclimate.com/Documents/thermostats.pdf - Thermostat Catalog
  • [7] White Rodgers 1F90 Low Voltage Digital Comfort-Set thermostat Installation Instructions, PN 37-3654, White-Rodgers Division, Emerson Electric Co., 9797 Reavis Rd., St. Louis MO 63123
  • [8] "Automatic Oil Burner Controls - Thermostats", Domestic and Commercial Oil Burners, 3rd Ed., Charles H. Burkhardt, McGraw Hill, 1969 (and later editions), ASIN B0000EG4Y8
  • [9] Thermostat wiring color codes & conventions, Thanks to reader " Helpful Pointers" Regarding 24V T, 10/7/2012
  • [10] Domestic Central Heating Wiring Systems and Controls, 2d Ed., Raymond Ward, Newnes, ISBN-10: 0750664363, ISBN-13: 978-0750664363, Quoting from Amazon.com:
  • [11] Proliphix Corporate Headquarters [Website: proliphix.com] , 3 LAN Drive Suite #100, Westford, MA 01886 Phone: +1.978.692.3375 Toll Free (U.S.): 866-IP-LIVING (866.475.4846) Fax: +1.978.692.3378 - Sales: sales@proliphix.com Marketing: marketing@proliphix.com Customer support: support@proliphix.com http://www.proliphix.com/ - quoting from the company's website:

    All Proliphix Network Thermostats come with our free Uniphy Remote Management Service. This unique offering lets you monitor and control your HVAC systems by simply pointing your Browser to our secure Proliphix Web Site.

    Enjoy the convenience of programming a thermostat from any location, using a simple graphical interface.

    No computer equipment or software is required. And since Proliphix takes care of the network configuration for you, you’ll be up and running in no time.

    We’ll even proactively monitor your thermostats and send you an immediate email or SMS message when an HVAC problem is detected.
  • [12] "Heating Control Handbook for the Installer and Service Man,Oil Burner, Gas Burner and Stoker Controls", Honeywell Corporation, March 1949 [copy on file as HoneywellControlsHandbookSA1399-2-1949.pdf] . Some of the controls discussed in detail here include the
    • Honeywell T1 and T11A = Series 10
    • Honeywell T21A (T2) = Series 20
    • Honeywell T847A = Series 80
    • Honeywell RA117A (RA1) = Series 10
    • Honeywell LA101A = Series 10,
    • Honeywell LA419A (LA4) = Series 40
    • V155A = Series 10, V435A = Series 40, V575A = Series 50, V835A = Series 80
  • [13] Trane TCONT800 Series Touch Screen Programmable Comfort Control Ownes Guide, American Standard, Inc., Troup Highway, Tyler TX 75711, January 2005, Telephone: Customer Service: 1-877-3381, website: www.trane.com
  • In addition to citations & references found in this article, see the research citations given at the end of the related articles found at our suggested

    CONTINUE READING or RECOMMENDED ARTICLES.

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