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Thermostat wiring reference chart - simplest case two-wire thermostat (C) Daniel Friedman How Wire a Generic Room Thermostat
Standard Thermostat Wiring Connection Tables

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Thermostat wiring details & connections for the _. This article gives a table showing the proper wire connections for the _ room thermostat used to control heating or air conditioning equipment.

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. 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.



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

The following thermostat wiring suggestions were derived from a thermostat wiring connection code list provided courtesy of J. Scott Meenan.

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.

Table of Common HVAC Thermostat Wiring Connections - Generic HVAC Thermostat Control Wiring Points

Table of Standard or Most-Common Thermostat Wire Colors Matched to Thermostat Terminal Names

[see other thermostat brand specific wiring notes below]

Thermostat
Terminal Name
Nominal Wire "Color"1 Function
(B) Blue, Orange Energize to heat. Used on some systems including Rheem/Ruud HVAC systems. See (O) below.
(B) Blue, brown , or black

Blue, brown , or black wire, common side of the transformer. Needed on some electronic thermostats or if the system uses indicator lamps.

Watch out: do not confuse this terminal with the (B) discussed above. Check you brand, model, and installation manual/wiring diagram.

(Also see (X) below)

(C)   Common side of the transformer (see "B")
(E) Blue, Pink, Gray, or Tan Blue, pink, gray, or tan wire, emergency heat relay on a heat pump. [Active all the time when selected, usually not used -??]
(G) Green

Furnace blower fan (used in air conditioners, heat pumps, some electric furnaces.)

On most thermostats the (G) and (Y) terminals are connected together at all times when the fan switch is in the "Auto" mode.

(O) Orange Energize to cool, used for reversing valve on heat pump systems.
(R) Red Electrically live side ("hot" side) of the transformer wiring. Note 1.
(T) Tan, Gray Outdoor heat anticipator reset
(W) (W1) (W2) White

"Heat", such as for a gas burner, oil burner, electric heat, or auxiliary heat on a heat pump, including the defrost output from an outdoor (condenser) that is used to activate electric heat at the compressor and to turn on the AUX heat lamp indicating that backup electric heat is in use or required.

Some heat pumps require a jumper from (W) to (Y) to operate the heat pump.;

(X)   Also see (B) above.
(Y) Yellow Compressor activity: cooling, or cooling and heating if on a heat pump

Notes:

1. The [Honeywell 5-2 Day Programmable Thermostat - RTH2300B1012] thermostat cannot be used if your old thermostat had and used any two of the following wires: R, RC, RH, 4 and V.[5]


How to Wire a Generic 2-Wire 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.

Two wire thermostat wiring instructions: 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 cooling system.

The red wire from the heater or air conditioner control is mounted to the "R" terminal on the backing plate. Typically the red wire is originating at the heating or air conditioning low voltage transformer and brings power to the thermostat. (See LOW VOLTAGE TRANSFORMER TEST)

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.

Our photo (above left) 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 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.

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 at LOW VOLTAGE TRANSFORMER TEST.

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.

How to Wire Up a 3-Wire 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.

The Red wire coming to the thermostat from the heater or air conditioner is connected to "R".

The white wire is connected to the "Y" terminal, and

The blue wire is connected to the "W" terminal on the backing plate.

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.

How to Hook Up a 6-Wire Thermostat

3M-22 Filtrete thermostat wiring terminal layout (C) Daniel FriedmanColor 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 as follows:

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

Also see Table of Common Heating and Cooling Thermostat Wiring Connections for added details.

How to Wire 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

- Hot water heat In the U.S. the TT wires will turn on a hot water circulator (or open a zone valve) on a hydronic or hot water heating system; falling boiler temperature will cause the aquastat to turn on the oil or gas burner (this is the U.S. typical installation).

- Hot water heat In Canada the TT wires will activate an aquastat control that will turn on and off the boiler based on its temperature; circulators are already running.

- Hot air heat: the TT wires will turn on the furnace heating equipment (oil or gas or electric, say); when the air temperature in the furnace heating plenum is hot enough the fan limit switch will turn on the blower fan.

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

Wiring 120V 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.

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.

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Or see THERMOSTAT WIRING TIPS & COLOR CODES

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