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Thermostat Wiring Explained

In this article, I am going to explain the function and wiring of the most common home climate control thermostats. This information is designed to help you understand the function of the thermostat to assist you when installing a new one, or replacing or up-grading an old one. We will focus mainly on the basics of home heating / cooling thermostats, and first of all, I will explain the function of them.

The thermostat is the control device that provides a simple user interface with the internal workings of your homes climate control system. By the use of an adjustable set-point, the job of the thermostat is to turn on either the heating or cooling system to maintain the desired room temperature in the home, and to turn off the system when the desired temperature is achieved.

The most basic of systems (such as an older ‘heat only’ forced air / gas furnace with a standing pilot light) only need two wires for control. They connect to a two-wire thermostat (generally a mechanical thermostat with a mercury filled ball connected to a coiled bi-metal strip).

A basic two-wire thermostat can be compared to a simple single-pole switch that you will find throughout your home, only instead of you turning the switch on and off as required, a mechanical or electronic temperature controlled mechanism is the operator of the switch.

The terminals are usually marked ‘R’ and ‘W’. They usually operate at 24V ac power, and the source of this control power comes from a control transformer mounted either just outside, or inside the furnace body. The line voltage feeding the furnace (to operate the fan blower motor) is transformed down to a safer level of 24 volts (the gas control valve needs 24 volts to open), and after making a series loop through at least one safety device (the most basic and mandatory one is an over-temperature shut down), the power goes up to the thermostat, and when the room temperature falls below the set-point, the contacts close completing the circuit to the gas valve allowing it to open, the main burner to ignite, and begin the heating cycle.

In this the most basic of heating systems, as the temperature of the heat exchanger rises, another contact is closed on the line voltage side of the equation, and the fan blower motor starts moving air through the heat exchanger and out through the ducting in the home. If the fan fails to run for any reason, the heat exchanger will get too hot and the hi-limit temperature device will open the circuit to the gas valve, closing the valve and stopping the heat cycle.

If your home’s system of this vintage has provisions for air-conditioning (cooling), then the thermostat wiring will have at least three wires (some will require a separate ‘R’ terminal for heating and cooling and will be labeled ‘Rh’ and ‘Rc’ now needing a minimum of 4 wires), ‘R’, ‘W’, and a ‘Y’ terminal.

When the mode is selected for cooling (basic heat / cool thermostats have a mode selector for either ‘heat’, ‘cool’, or ‘auto’) when the room temperature rises above the set-point, the thermostat will close the connection between the ‘R’ and the ‘Y’ terminals completing the circuit to the compressor and condenser unit that provides the cooling for the evaporator coils mounted in the discharge ducting of the system.

The next step up from the older home systems was the introduction of the ‘fan on / auto’ switch. This function allows you to circulate the air in the house by using the fan in the furnace without heating or cooling the air moving through the system.

In the most basic system, this functionality is provided by use of a fan center relay, and the low voltage wiring to the thermostat now will require a minimum of three wires (for heat only units) and four wires (for heat / cool / fan) for control. This additional terminal is labeled ‘G’ in the thermostat.

When the ‘fan on’ setting is selected, the contacts between ‘R’ and ‘G’ are closed turning on the relay that powers the blower motor directly regardless of a call for heating or cooling.

If you are replacing an old thermostat with a new digital thermostat, the electronics in these units may need yet another wire for a terminal labeled ‘C’. This terminal is for a common connection from the control transformer that will provide steady 24 volt power between terminal ‘R’ and ‘C’ to power the thermostat itself.

If adding wires to your thermostat location is not possible, or very difficult, look for a thermostat that is battery powered which won’t require the terminal ‘C’ for operation. The obvious down side of this is that if the battery goes dead the thermostat will no longer function.

Climate control systems have evolved a great deal in the last several years with the evolution of electronic control. This has made the units more efficient, with added safety features, and with this comes increased difficulty for the average home-owner to repair or replace any parts of the system that may break down (and they will, but only when you really need it!)

However, the terminals of the thermostats of today are still labeled the same, and provide the same functions as what we have discussed thus far. They just get a little more sophisticated so if you are pre-wiring a new home, check with the contractor providing the HVAC system as to what thermostat wiring is required. A new system may need as many as ten wires (like a two-stage heat, two-stage cooling system, heat pump reversing valves, fan control, etc.)

If the system had two-stage heating, and/or cooling, the 2nd stage termainals would be labelled W1 and W2 for heating, Y1 and Y2 for cooling.  These additional terminals are not shown in this diagram.

Now that you are armed with a basic understanding of the temperature control system in your home, you should now be able to identify what kind of system you have, and what type of functions you will need if replacing or up-grading your existing thermostat.

I hope this article helped with your question about thermostat wiring.  Thanks for taking the time to read it!  If you have any questions or comments on your furnace situation, feel free to add a comment below.

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  • John Albo

    tv aneena

  • Anonymous

    Thanks for the comments!

    • kenny

      I installed a honeywell programable thermostat. When i turn on the heat the ac kicks on and vice versa. Any suggestions?

      • Internetelectrician

        Try switching the wire connected to the ‘w’ terminal with the wire connected to the ‘y’ terminal

  • Mike

    I have installed a new thermostat and my fan is running all of the time in heat mode.
    Old – TotallineP474-2150
    New – Honeywell Vision pro 8000

  • Tracy

    I have installed a new thermostat and my heat will come on like its supposed to but will not go off when it reaches temprature, any suggestions ?

  • Jon

    I’ve got a 3-wire furnace (R, W, G) and am upgrading my 2-wire t-stat to a new Wifi T-stat. I’m replacing the wiring with 5-conductor cable. Where do I terminate the t-stat C-wire to? Does that go back to the 24V side of the control xfmr? Would it become double-tapped with both the R and C wires? Thanks!!

    • The C terminal is only used as a common return for accessories(humidifier, electrostatic filter, etc) that use power from the 24VAC circuit of the appliance, and is usually terminated within the appliance itself. If you don’t have a C terminal on the appliance just leave it disconnected at both ends. If you were to connect the power source at R to a C terminal without a “load”, it will short the transformer and either cause the 3 Amp fuse to blow or you will burn out the transformer. Source: is an HVAC tech.

      • Jon

        Ok, understood. So would I use the G-wire to provide necessary power to the thermostat for its features, and forfeit the use of fan-only in non-heating mode? I understand that I’ll need to preserve the R-wire and W-wire for their functions, but need power for the new stat. Is that the g-wire??

        • Jon

          Update: The 24V xfmr I was confused by is for my doorbell. Oops. Looks like my furnace derives its own 24V internally, so I need to locate a position on its controller for a new C-wire, as I’d like to keep the G-wire for fan-only use. Need to track down a wiring diagram for my unit.

  • Gary EvenMe Billings

    I live in an apartment that has an old ac/heat system which is water blown. About a month ago the thermostat stopped working properly (would no longer automatically turn on shut off the unit according to the desired temperature setting). Maintenance came in and replaced the t-stat with a new, different kind of t-stat that has a 3 screw (1-heat, 2-ac, 3-not marked) and a (apparently) factory installed wire plug in (green wire). When they wired the new t-stat they used (from the wall) the two black wires, one to the ac screw and the second to the unmarked screw. The two remaining wires, red and black are not in use. Now the unit will only turn on when I turn the temp control knob past 68 degrees ans will only shut off when the knob is turned past 74 degrees. Any suggestions to correct this will be greatly appreciated. Thank you. God bless.

  • ivailosavoff

    Hello ,
    I have an old electric furnace with AT72H Honeywell control center and 2 wire thermostat. Now I have replaced the old thermostat with a nest one.I replaced the wire with 18/8 just in case and my question is can and should I plug in the G therminal on both thermostat and on the control fan center!
    Now with 2 wires when I tunr on the furnace the fan kick in and works non stop!
    Thank you in advance!

  • david5300

    I have a Rheem 90+ gas furnace, the control board blinked twice with a pause indicating a faulty pressure switch. I tested the 2 pressure switches what seen to be fine. I some how crossed terminals C & W and now I can not get the control board to power up. I tested the transformer and got a ” zero ” reading from the secondary side. I am now in the process of testing the 2 amp fuse and installing a transformer that is showing a secondary reading.

  • Tom

    My thermostat display is out and my blower is on continuously(cold air no flame). i checked voltage at the thermostat and it was 13v. same at the output of the control board. the voltage at the sec windings of the transformer is 24v. does this mean my circuit control board is bad?

  • stephen

    i installed a new honeywell thermostat. it had places for red white green yellow and left it set to use orange wire. i have no place for blue. everything works but i have no heat

  • Malky

    I want to replace an old Honeywell analog thermostat with a new programmable one. When I unscrewed the old one from the wall, there are only 2 WHITE wires coming out of the wall and into the back of the thermostat!! There are no markings on the rear of the back plate to indicate which one is red and which is truly white!! Any ideas onhow to choose which of the white wires should go to the W terminal and which one to the Rh one?

    • supershwa

      Sounds like an older house if you have white wires. But, I don’t think it matters since the voltage is A/C – there is no “positive” or “negative” in A/C.

  • Terry

    getting AC, I have thermostat with 2 wires, red and white, want to run new wiring to upgrade for the AC. Got into the crawl space looked at the thermostat wiring, I don’t see a transformer hooked up to these wires, there is a transformer in the furnace, is this all I need to reduce the power to 24v

  • Terry

    the house was built in 1956 I’ve replaced the furnace, put in a digital thermostat and used the 2 wires r and w and now I’m installing AC. I need to tie all this wiring together to get it going. Then I’ll I a heating and cooling guy to finish adding the freon. DIY with the help of the computer!!!!

  • Pingback: How I Wired My Humidifier Into My Nest Thermostat | Something to Read on the Train()


The Basics of Household Wiring DVD

"If you're looking for a good reference to help you understand simple home wiring, I personally recommend 'The Basics of Household Wiring' DVD".

Terry Peterman, the Internet Electrician
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