Tip & How-To about Heating & Cooling

Thermostat wiring terminals and colors

R - The R terminal is the power for the thermostat. This comes from the transformer usually located in the blower section for split systems but you may find the transformer in the condensing unit. For this reason, it is a good idea to kill the power at the condensing unit and the blower section before changing or working on the wiring at the thermostat. If you have a package unit then the transformer is in the package unit.
Red for the R terminal.

RC - The RC terminal is designated for the power for cooling. Some HVAC systems use two transformers. A transformer for cooling and a transformer for heating. In this case the power from the transformer in the blower section would go to the thermostat terminal. It should be noted that a jumper can be installed between RC and RH for a heating and cooling system equipped with a single transformer.
Red for RC terminal.

RH - The RH terminal is designated for the power for heating. See RC above for an explanation. It should be noted that a jumper can be installed between RC and RH for a heating and cooling system equipped with a single transformer.

Y - This is the terminal for cooling or air conditioning and goes to the compressor relay. Typically a thermostat wire pull is made to the air handler on split systems and then this wire is spliced for the separate wire pull which is made to the condenser. Some manufacturers put a terminal board strip near the control board in the air handler so a splice is not needed.
Yellow for Y Terminal.

Y2 - This is the thermostat terminal for cooling second stage if your system is so equipped. Many systems only have a single compressor but if you have two compressors which should only operate off of one thermostat then you need the Y2 thermostat terminal for second stage cooling.
*The most common color I've seen used for this terminal and wire designation is light blue but this varies and is completely up to the installer what color to use. Most installers use the color coding as noted but be aware that some do not use the thermostat color coding.

W - This is the thermostat terminal for heating. This wire should go directly to the heating source whether it be a gas or oil furnace, electric furnace, or boiler,
White for W Terminal.

W2 - This is the thermostat terminal used for second stage heat. There are gas furnaces with low fire and high fire and some depend on control from a two-stage heating thermostat with a W2 terminal. Heat Pumps use staging for auxiliary heat and need a W2 terminal.
*The most common color I've seen used for this terminal and wire designation is brown but this varies and is completely up to the installer what color to use.

G - This is the thermostat terminal used for the fan relay to energize the indoor blower fan. On a split system the blower fan is in the blower section while with a package unit the blower fan is in the outdoor package unit.

Green for G Terminal.

C - This is the thermostat terminal which originates from the transformer and is necessary to complete the 24 volts power circuit in the thermostat but only if the thermostat consumes electricity for power. Many digital thermostats require 24 volts for power so the common wire is necessary.
C stands for common and there is no universal color used for this terminal although black is the most common color I've seen.

O or B - These thermostat terminals are for heat pumps and the B thermostat terminal is used on for Rheem or Ruud and any manufacturer that energizes the reversing valve in heating mode for the heat pump. Most other manufacturers of heat pumps will utilize the reversing valve for cooling and the O thermostat terminal will be utilized for this purpose. This wire goes to outside heat pump condenser where the reversing valve is located.

Orange for O and Dark Blue for B depending on the installer of the heat pump and the manufacturer. If you have a Trane, Carrier, Goodman, Lennox, Ducane, Heil, Fedders, Amana, Janitrol, or any other manufacturer other than Rheem or Ruud you will be utilizing the orange wire for reversing valve. Rheem and Ruud will usually utilize the blue wire for reversing valve.

E - This thermostat terminal is for heat pumps and stands for Emergency Heating. If for whatever reason the heat pump condenser fails and it is necessary to run the heat there is an option on heat pump thermostats for emergency heating. Basically this simply utilizes the back-up heat source many heat pumps have to heat the home without sending a signal to the condenser to run for heat.

E - There is no universal color used for this thermostat terminal designation but this should be wired directly to the heating relay or the E terminal on a terminal strip board in the air handler or package unit if you have a heat pump package unit.

X or Aux - This thermostat terminal is for back-up on a heat pump and allows for auxiliary heating from the back-up heat source usually located in the air handler.

X or Aux - There is no universal color used for this thermostat terminal designation but this should be wired directly to the heating relay or the Aux terminal on a terminal strip board in the air handler or package unit if you have a heat pump package unit.

S1 & S2 or Outdoor 1 and Outdoor 2 - Some thermostats have this terminal and it used for an outdoor temperature sensor. The wire uses for this should be special shielded wire and completely separate form the other thermostat wires.







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I am wiring a Honeywell wifi unit and I used a separate 24v transformer and connected to the c and rc terminal. My system is ac and heat and uses both and rc and rh wire. I wired the rc to the r ter


A thermostat is a switch. It connects 24vac from one terminal to another depending on the request. The 24vac comes from a transformer, usually located in the furnace and connects to the R terminal. Since you are using RH and RC without a jumper between it means there are two transformers, one in the furnace for heating (RH) and one in the air conditioner condenser used for cooling (RC).
Your old thermostat was probably mechanical and did not have a C (24vac common) terminal. Your new thermostat requires power to operate therefore needs a wire connected to a C terminal. I would get rid of the transformer you added and look for an unused wire in the bunch at the thermostat. If it's there connect it to C on the thermostat and to C on the furnace. Remove any jumper from R to RC or from RH to RC depending on your thermostat and connect wires same as the old thermostat.

Jul 04, 2014 | Heating & Cooling

2 Answers

i am trying to wire this unit to a honeywell t stat model T87N1026


Wiring a new sub-panel to an isolation transformer is one method of wiring for balanced power. With a balanced power system, the power sent to each "hot" bus of the power panel is 50 percent per bus. This means that the total power of the system is split evenly between each side of the panel. Balanced power systems have the advantage of reduced noise generation. This characteristic makes this type of system great for audio.
1.Shut off the power feeding the isolation mains transformer at the main power panel. Use the documentation supplied with the isolation transformer to locate the center-tap wire on the secondary side of the transformer. Connect that wire to the sub-panel's ground bus bar.
2.Route the remaining two wires from the secondary side of the isolation transformer into the sub-panel. Loosen the two "hot" wire main bus screws in the top center section of the sub-panel. Strip 1/2-inch of insulation off of the two wires.
3.Insert each wire separately into each of the wire screw terminals.Tighten the screws to hold the wire in place. Do not over-tighten these screws. If there is a neutral wire, connect it to the neutral common bar. Once these screws are tight, close the sub-panel.8_8_2012_9_21_38_am.jpg

Jan 18, 2011 | Heating & Cooling

2 Answers

Installing a Honeywell TB8220U programmable thermostat. All of the wires are connected but AC will not come on. I believe I have a single-transformer system. I have the red wire connected to ''C'', since there is no ''R'' on the unit.


On the old thermostat how many and what terminals did you have?
Usually
R = Red One Side of 24 volt Transformer
RH = Usually jumpered to R or RC
C = Common the other side of the 24 volt transformer opposite the R
G = Green Blower
Y = Yellow Cool
W = White Heat

May 30, 2010 | Honeywell Electronic Programmable...

1 Answer

Trane ac, inside blower does not stop. The heater stops, the Ac compressor stops, but the inside blower does not, this the thermostat set at auto. Regards, Faylbp


Hi, I want you to try a couple of things for me if you will. Go to the thermostat and remove the cover. Now if you have a small bladed screw driver, loosen all the screw on this next section and remove it. Turn off power to the unit, if it is a split system, turn off the indoor unit. Once you remove this section which is the thermostat you will see the subbase and wiring. Take the red wire loose and off of the terminal. You may have a red jumper wire going from RH to RC. Remove the red wire coming into the stat at either point. Give it a minute and see if the fan stops now. If it does, you will need to replace the stat. If not, depending on the age of your unit, if your high limit/fan control tripped, that will be the problem. Some units have a fan control combination high limit that trips causing the fan to run all the time. You need to take a look at your heater section at the front of it to see what kind you have. If it is a combination fan/limit. it will be inserted into the fire box. It is square looking box with a cover on it some with a auto/ manual button on them Sometimes the contats will stick causing the fan to run. They can be removed with power off and cleaned up. They will have a dial with a temperature reading on them which tells the fan to come on when it reaches this set-point and off when it cools down. It may have stuck closed. Check what I have told you with the stat, and this and let me know the age of heater and if it is a split system Trane with a remote condenser and furnace located somewhere else. Get back to me on this.
Shastalaker7

Jan 23, 2010 | Heating & Cooling

1 Answer

trying to figure out thermostat wiring diagham on a split system


Typical Thermostat Wiring for 4 or 5 wire Heat/Cool Thermostat (not heat pumps)
4 Wire Termostat
R = Red One Side of 24 volt Transformer
G = Green Blower
Y = Yellow Cool
W = White Heat
5 Wire Thermostat
with one transformer for both units
Rc One Side of 24 volt Transformer
Rh Jumper Rc to Rh
G = Green Blower
Y = Yellow Cool
W = White Heat
5 Wire Thermostat
With 2 transformers one for indoor unit
And one for outdoor unit
Rc = Red One Side of (outside) 24 volt (outside transformer)
G = Green Blower (outside transformer)
Y = Yellow Cool (outside transformer)
Rh = Blue One side of (inside) 24 volt (inside transformer)
W = White Heat (inside transformer)
COOLING ONLY Thermostat
R = Red One Side of 24 volt Transformer
G = Green Blower
Y = Yellow Cool
HEATING ONLY
R or Rh One Side of 24 volt Transformer
W = White Heat

Jan 05, 2010 | Heating & Cooling

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