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Thermostat wiring reference chart - simplest case two-wire thermostat (C) Daniel Friedman Thermostat Wire Reference Chart
List of the control function of each ID label or color of room thermostat wires

  • 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:

This article explains just what wire at a room thermostat actually controls. 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

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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 2017-12-26 by Christina


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.

On 2017-12-25 - 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.

Summarizing the 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

I'm not sure what the To wire is either. But as the brown wire is typically for emergency heat that might help us out.

Also it'd help if we knew the brand and model of heat pump and air handler installed.

At inspectapedia.com/heat/Thermostat_Wiring_Instructions.php are more detailed thermostat wiring tables identifying more terminals - we should both look over there.

On 2017-12-24 by Christina

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 2017-10-22 by JCarlos

I have a White Rodgers 1f72-301 thermostat and a heat only system (Weil-McLain EG).

The thermostat has 6 terminals. W, RC, RH, YG, T, TC. There are wires connected to W, T, TC and RH. Im trying to install an Ecobee which requires a ground G wire. What are YG, T and TC used for?

If I have to run a G wire, do I have to run it all the way to the boiler which looks like it just uses the cable sheath for ground which comes from the transformer.

On 2016-06-13 - by (mod) -

Are you asking about a normally open vs normally closed zone valve wiring?

On 2016-06-13 - by (mod) -

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.

On 2016-06-13 by Johnny

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 2015-08-06 - by (mod) -

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.

On 2015-08-06 by Ed

Installing AC with heat pump and emergency electric coil. House already has oil fired hot water heating system. Can one thermostat control all components?


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

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


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INDEX to RELATED ARTICLES: ARTICLE INDEX to HVAC THERMOSTATS

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