Thermostat Wire and Thermostats Wiring For HVAC

Thermostat wiring colors help designate which wire goes where on a thermostat

For many, wiring a thermostat can be a very big hurdle especially if they do not understand basic HVAC electrical control circuits, thermostat wire color designations, or thermostat terminal designations. No matter what type of thermostat you have, Honeywell, White Rogers, Robertshaw, Maple Chase, or whatever brand you have, High Performance HVAC Thermostat Wiring page will help you better understand this often confusing subject of thermostat wiring.

Thermostat is typically 18 gauge solid wire and is usually designated by different colors of many wires wrapped in an outer insulating jacket which hold the multi-color wires together. Reading the label on different wire reels one will notice 18-2, 18-3, 18-5, or even 18-10. The first numbers represents the gauge of the wire inside the outer jacket. The second number represents the number of wires or conductors inside the outer jacket. A reel labeled 18-2 will have two wires or conductors which are 18 gauge wire. A reel labeled 18-10 will have 10 wires or conductors which are 18 gauge.

Depending on the type of equipment you have will determine the number of wires needed to run from the thermostat to the equipment. The HVAC Technician who installed the equipment made the decision on the number of conductors needed for the installation of the equipment. It is common for the installer to pull more conductors than necessary therefore there may be some unused wires or conductors which can be used as spares in case the conductors which are in service are damaged in some way. In some cases, if you look behind the thermostat you will find these spare wires wrapped around the outer jacket or stuffed back into the hole or chase where the thermostat wire runs through the wall.

Thermostat Terminal Designations

It is important to note that before working on any type of wiring that the power for all the circuits be disabled. Typically, the air handler contains the control voltage for the thermostat (but not always). It is advisable that all HVAC equipment be turned off from the main power disconnect or breaker panel before proceeding with any wiring. Additionally, read the site disclaimer page for more information.

R - The R terminal on many thermostats typically is the 24 volt control power which feeds power to the other terminals (except the common terminal) when the thermostat is manually or automatically switched to an on position.

RC - The RC terminal is not on all thermostats. It is usually on thermostats which allow for separate equipment control transformers where the two different control circuits need to be isolated. The RC terminal is usually fed from the cooling or air conditioning equipment and is used to energize all the components necessary for cooling or air conditioning.

RH - Just as the RC terminal, the RH terminal is not on all thermostats. This will usually accompany the RC terminal where the thermostat will isolated two different control transformers. The RH terminal will feed all the heating components necessary to run the heat according to thermostat manual and automatic settings.

Typically, the 24 volts control power is fed directly from a control transformer located in the HVAC equipment. For best results with modern state of the art solid-state controls and circuitry always use National Electrical Manufacturers Association (NEMA) rated transformers.

The commonly used wire colors for the R, RC, and RH terminal is red. It is possible, when the circuits are isolated, to have more than one red wire in the thermostat. It is also possible that only one control transformer is being used to control the equipment and the RC and RH terminals have a jumper wire running between so that they may share the control power. If you are unsure about this always consult an HVAC professional as miswiring can damage components and cost more to repair than the cost of paying an HVAC professional technician to install a thermostat correctly.

R or RH thermostat terminal - For heating power (24 Volts a.c.) if an RC terminal is present. For heating and cooling if no other R terminals.	R or RH thermostat terminal - For heating power (24 Volts a.c.) if an RC terminal is present. For heating and cooling if no other R terminals.
RC thermostat terminal - for cooling power (24 volts a.c.)	RC thermostat terminal - for cooling power (24 volts a.c.)
W thermostat terminal - for the first stage of heating. Most systems only have one stage of heating so this will be the most commonly used terminal.	W thermostat terminal - for the first stage of heating. Heat pumps systems can have many different stages for back-up heat (unless you have a geothermal system)
W2 thermostat terminal - this will be for the second stage of heating. Some gas furnaces have high fire and low fire and rely on this terminal for high fire staging. It not uncommon for electric furnaces to have two stages for heating.*	W2 thermostat terminal - this will be for the second stage of heating. Some gas furnaces have high fire and low fire and rely on this terminal for high fire staging. It not uncommon for electric furnaces to have two stages for heating.*
Y thermostat terminal - this thermostat terminal will engage the cooling. Usually this is wired directly to the compressor contactor located in the condensing unit. The wiring for this usually passes through the air handling unit on a split system.	Y thermostat terminal - this thermostat terminal will engage the cooling. Usually this is wired directly to the compressor contactor located in the condensing unit. The wiring for this usually passes through the air handling unit on a split system.
Y2 thermostat terminal - this thermostat terminal is for the second stage cooling. Very few residential units have two stages of cooling. There are exceptions. Older two-stage systems rely on a secondary control other than the thermostat to engage the second stage. Consult manufacturer's paperwork or a professional HVAC technician if you have a two-stage compressor or two compressors.	Y2 thermostat terminal - this thermostat terminal is for the second stage cooling. Very few residential units have two stages of cooling. There are exceptions. Older two-stage systems rely on a secondary control other than the thermostat to engage the second stage. Consult manufacturer's paperwork or a professional HVAC technician if you have a two-stage compressor or two compressors.
G thermostat terminal - this thermostat terminal is for indoor blower fan control.	G thermostat terminal - this thermostat terminal is for indoor blower fan control.
C thermostat terminal - this is the common side of the 24 volts step down transformer. If the thermostat pulls power from the transformer (usually located in the air handler) for power then this thermostat terminal is necessary for the thermostat to function properly. Older mercury type thermostats usually have indicator lights and this terminal is necessary so that these indicator lights will function.	C thermostat terminal - this is the common side of the 24 volts step down transformer. If the thermostat pulls power from the transformer (usually located in the air handler) for power then this thermostat terminal is necessary for the thermostat to function properly. Older mercury type thermostats usually have indicator lights and this terminal is necessary so that these indicator lights will function.
S1 and S2 thermostat terminals - these terminals are in newer thermostats and are a bell and whistle for a high tech digital thermostat. This terminal works in conjunction with the S2 thermostat terminal (noted below) for reading outside air temperatures for display purposes only in (conventional) non-heat pump thermostats.	S1 and S2 thermostat terminals - these terminals are in newer thermostats and are a bell and whistle for a high tech digital thermostat. This terminal works in conjunction with the S2 thermostat terminal (noted below) for reading outside air temperatures for display purposes only in (conventional) non-heat pump thermostats. For heat pump thermostats this terminal can be an energy saver by cutting out the outdoor unit off when the outside air temperature drops below a temperature where the outside unit becomes inefficient. This temperature varies depending on geographical location.

O or B Thermostat Terminal - this thermostat terminal controls the reversing valve in the heat pump condensing unit. This switches over the unit from heating to cooling. The O terminal in the thermostat will control the reversing to activate in cooling mode. The O terminal is default heating. Using the B terminal the reversing valve will activate in the heating mode. The default is cooling in this type of unit. Typically, the B terminal is only used in Rheem and Ruud Heat Pump Systems.

If you are unsure of the terminals when wiring a thermostat please consult your local HVAC contractor.

Comments (8) Add Comment

maintenance supervisor
written by jimmy jones, July 02, 2008

i am going to install a new system , air handle and condensing unit is it hard to wire them up? whjat is the precedure
this was a gift and i have no paper instruction

Technical Support Specialist
written by Cory, October 22, 2008

Um? Am I missing something here? You just have the line or "hot" terminals listed.
You do not have the common function terminals like:
G is for the Fan (Green)
W is for Heating (White)
Y is for Cooling (Yellow)

And then the terminals on more specialized HVAC systems (wire colours vary)
W2 for second stage heating
Y2 for second compressor (cooling) or second stage cooling
O and B for Heat pump change over (O is energize to cool and B is energize to heat)
E for emergency heating
C, B, X are usually the common or "neutral" side of the 24VAC transformer.

Changing old thermostat from Lennox to an all new electronic Honeywell
written by Robert10, November 18, 2008

Hi, every diagram on the install instruction from Honeywell that I look have less wires than my old thermostat from Lennox, can you help me please to figure it out. The letter from my old thermostat are R,Y,X,F,V jumper to Vr,M,A,L, I have 8 wires coming from the furnace and the letter don't match with the new one.

Thank you
Robert Caron
This e-mail address is being protected from spambots. You need JavaScript enabled to view it

You are Correct Technical Support Specialist
written by Webmaster, November 18, 2008

The site crashed recently and I had others pull the info from the old database for the rebuilt site. I just read this article and will add to it the original article I wrote which had a table with it explaining all the different STANDARD colors for most applications.

Owner
written by Tom DeWitt, November 27, 2008

I have a 10000sq/ft commercial building with (4) four 175,000 btu unit heaters and they are all on separate thermostats. What is the best way to wire these all together to operate on one stat?

ruud thermostat wiring
written by Tara, January 18, 2009

I disconnected the thermostat to install a digital honeywell thermostat and when I rewired the ruud thermostat it no longer works. I unfortunately did not write down the way it was wired originally. I have 7 wires and O,Y,G,E,R,B,X,L,O. It is cold now and I have no heat on a holiday weekend. Can you give me some suggestions?

For Boiler Installation
written by sohan (boiler installer), May 14, 2009

This article is really nice for sharing. and we have also

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homeowner
written by trotter, May 31, 2009

I have an old bryant heat n Air unit. I need to connect the wire
from themo to board. it read C Y GC GH R W which wire do I hook to I have red white blue green yellow that run from themo