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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

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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.


We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

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
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)
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:

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

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Continue reading at THERMOSTAT INSTALLATION STEPS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see THERMOSTAT WIRE CONNECTIONS detailed room thermostat installation & wiring guide for each heating or cooling system type and each thermostat wiring specification

THERMOSTAT WIRING DIAGRAMS

THERMOSTAT WIRING TIPS & COLOR CODES & methods for identifying which thermostat wire is which if yours have lost their labels or have unclear color codes.

Suggested citation for this web page

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

INDEX to RELATED ARTICLES: ARTICLE INDEX to HVAC THERMOSTATS

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