- this article includes a comprehensive list showing thermostat wire terminations for an air conditioner and a source of heat that includes five wires.
- a detailed description of where each wire goes in the air handler and condenser and what it controls.
- a detailed diagram illustrating where the wires go for 5 wire air conditioner and heating system control.
- resource and related links to help you with wiring and installing a thermostat.
How to Wire an Air Conditioner for Control - 5 Wires
How to Wire an Air Conditioner for Control - 5 Wires – The diagram below includes the typical control wiring for a conventional central air conditioning system. It includes a thermostat, a condenser, and an air handler with a heat source. The heat source for a basic AC system can include heat strips for electric heat or even a hot water coil inside the air handler that is fed from a water heater. Every conventional residential AC system uses 24-volts for the control of the system. All the relays in the system have 24-volt coils.
Circuit boards or printed circuit boards found in modern systems utilize 24-volt control which originates from the control transformer. Typical systems that do not use 24-volt control are window unit air conditioners, ductless mini split systems, and baseboard electric heating systems.
If you are unsure about the control voltage of your system consult an HVAC professional as some appliances use high voltage and this can be dangerous.
Related Link: Standard 18 gauge thermostat wire is used for all control wiring.(opens in a new window)
How to Wire an Air Conditioner for Control - 5 Wires - Thermostat Wiring Diagram
How to Wire an Air Conditioner for Control - 5 Wires - Breakdown of colors and terminals
- The R terminal is the 24-volt hot feed from the control step down transformer that will power the relay, contactor, or complete the circuit in the circuit board feeding it with 24 volts when the thermostat calls for whatever it is set for.
- The W terminal is for heating including the heating for gas furnaces, electric furnaces, boiler systems and the backup heat for heat pumps.
- The G terminal is the blower fan located in the air handler. It is the fan that circulates air throughout the ductwork system.
- The Y terminal is the terminal that will turn on the air conditioner and typically is terminated at the compressor contactor in the condensing unit located outside. It will be one of the few control wires (thermostat wires) that will terminate at the condenser in a split system air conditioner and heating system. Typically, the yellow wire will be ran to the air handler where at the air handler this wire is usually connected to another wire (typically by wire nut but could be connected via a terminal strip in the air handler) and then ran outside to the condensing unit along with at least one other wire in an air conditioner split system set up. The other control wire will be the common wire need at the compressor contactor where the coil in the compressor contactor needs a hot and a common from the control transformer to work.
- Finally, the C terminal is the common terminal and this terminal feeds the common side of the load from the control transformer.
Temperature Control, HVAC Thermostats, and HVAC Systems
Heating and cooling systems always require some kind of control. When you combine a home heating system with cooling equipment such as an air conditioning unit you will typically need a 5 wire thermostat to control the entire system with a single thermostat. This will not be the case with a heat pump system as they will require more wires for thermostat control. Therefore it is important that you know the type of system you have before replacing your thermostat. Furthermore, you can lower energy bills and energy cost by installing a smart thermostat or a programmable thermostat. Many digital thermostats available today are very forgiving in thermostat selection. Kind of like a one size fits all so a heat pump thermostat will be okay with an air conditioner and gas furnace combination. You will simply follow the instructions for wiring including color codes (the wire color code for thermostat control) and then the setup instructions.
Many manufacturers produce smart thermostats that will typically require a WiFi connection. Honeywell, Nest, Ecobee, Sensi and many others are available and can save in home energy costs. Use High Performance HVAC to install the new thermostat along with the manufacturer’s instructions and you will save energy. Getting rid of an old mechanical thermostat will definitely save you in the long run.
Terminal Strip where wire terminations are made during installation. Typically inside the air handler.
Additional resource links for wiring a thermostat:
1) Thermostat Wiring Colors
2) How to wire a thermostat
3) Heat Pump Thermostat Wiring Chart & Diagram
4) Select the Proper Thermostat for Your HVAC System
How to Wire an Air Conditioner for Control - 5 Wires
To learn more about heating systems and HVAC use this resource.
If you are purchasing new HVAC equipment don't forget to check out our HVAC Consumers Buyers Guide Category for lots of help.
See the About the Author page for information about the author of this article.
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I have a Nordyne system and the “super charge” went out it is the part that hooks into the compressor to get the unit running however it burned up and I got another part that the company say’s works for this unit only it does not have a hookup for two of my “main” pieces in the unit so how would I go about getting this part hooked up for my unit to work?
Sorry there is not enough information and I do not understand the terminology used. Super Charge? You are dealing with high voltage you know and it is potentially dangerous. Probably best to call a professional because with seeing it myself I do not know how to help you. I can assume but when dealing with technology we never want to assume. If it hooks to the compressor and it burned up you could have other problems also that caused the part to burn up.
Wanted to say thank you. I recently graduated from Manatee Technical College here in Florida. With one year to learn EPA, R-410A,ESOC-HVAC ELECTRICAL,AIR CONDITIONING,OSHA,FORKLIFT,FIRST AID,CPA. Going over the basics threw your videos has helped me. Having three awesome instructors with three different ways of doing things. One might get confused. LOL. Of course I bought the service guide by Michael Prokup and 25th edition air conditioning books. I want to be prepared when that new job comes my way. I have just about all my tools and ready to work. Thank you again sir.
*The person you mentioned in the initial comment is no longer a part of the website. We will pass your comments on to him though. Thanks for visiting.
Techs are thieves that’s why they refuse to answer any questions on the internet concerning your issues unless you want to pay a service call, otherwise they’d rather rob you like they did the American Indians while kicking them off they’re own land so they could live on it. Thank You. Your information was helpful.
Editors Note: Sorry, I disagree……………there are so many different scenarios for refrigeration, electrical, and other related things concerning HVAC…………it is so much to answer anyone over the internet or even the telephone especially in the sue happy society we have here in the USA. So I recommend you either go to school and learn about it yourself and fix it yourself or pay a contractor to work on it for you. Not all techs are dishonest and not all HVAC contractors are dishonest. The same with car mechanics and any other technical related trade or service. Not everyone is dishonest but there are some.
Hi,
Just bought a house that was wired with aluminum wiring. Decided to change it all over to copper. Got everything cut over, but didn’t change out any of the high voltage wiring for stove, oven and A/C. I have an A/c w/gas heat. After I cut out all the old aluminum wiring and everything else in the house is working fine, I noticed that my thermostat doesn’t have any power. I had assumed that the high voltage wires handled all the air conditioning systems.
I went into the attic and I notice that there is a 12 or 14 gauge aluminum wire going into the air handler and it’s now dead because I cut all of the 12 and 14 gauge aluminum wiring.
Question: Is this why my thermostat is off?
What are the power requirements for replacing that cable? In other words, do I need to run a dedicated cable from the panel or is it fairly low amperage that I can just feed a cable from another source such as a nearby outlet. There is also a high amperage cable going to the air handler and I’m again assuming that it supplies all the high amperage needs of the air handler.
Thanks for any help
The power for your thermostat typically originates from the air handler from a step-down transformer. In a few rare cases, the transformer is installed in the condensing unit but that is not the norm. If you cut either the thermostat wire (usually 18 gauge multiple conductors in one jacket) or the line voltage going to the air handler then that would definitely kill the power to the thermostat. The wire size for the line voltage depends on the ampacity requirements of the unit itself. And aluminum wire is sized differently than copper wire according to the NEC tables so you first need to figure out the ampacity requirements for the unit and then follow the NEC tables for wire size.
This is not advice but the typical straight air conditioner or gas furnace (as opposed to a heat pump with electric heat strips) could possibly use 12 or 14 gauge copper for the air handler with controls and a fractional horsepower blower. I can’t stress the importance of making sure you do this the right way and calling in an electrician or HVAC company to do this as using undersized wire would create a fire hazard. Using the proper wire size and breakers is very important to prevent a possible fire hazard. Sizing requirements are in the NEC for HVAC equipment.
Thanks Richard,
I didn’t cut the thermostat wire. I cut the aluminum wiring at the breaker panel in the basement. I pulled all new copper wiring throughout the house, but apparently missed that one of the cables went to the air handler in the attic. As I said, I ‘assumed’ that all the high amperage cables, which I didn’t changeover were all that supplied the A/C system. I was wrong! LOL I guess my main question at this point is whether or not the 12/14 gauge cable that I cut has to be run as a dedicated circut as the built in microwave, dishwasher now has to be by code or can I just pull a cable from a nearby outlet? A dedicated cable, of course, has to go all the way back to the circuit panel.
it needs to be on a dedicated circuit
ok thanks. I appreciate the info. It’s a 5,000sf home and it’s taken me three months to pull all the new wire and I was feeling pretty good when everything was working, but then it started getting warm in the house and found the problem. I’ll pull a 12/2 up there tomorrow.
God bless you,
Ted
I just recently purchased a Honeywell RTH9585 smart thermostat. I installed with all 5 colored wires in their proper locations. After installation and setup, I noticed that the display screen would intermittently go blank. I took a voltage reading between the red wire and the blue common wire while the screen was working and it measured 27 volts. I also measured the voltage of the same wires when the display was not working and it measured 11 volts. I know that this difference in voltage is what is causing the display to go blank. However, I don’t know what is causing the voltage to drop from 27v to 11v. I am under the impression that the voltage between the red and blue common should be continuous at 24 volts or more. Need help. Thanks
I would go to the transformer and disconnect everything on the control side and test the voltage. This will be difficult if the problem is intermittent and only does it at mysterious times. Then you will need a meter that has logging capabilities. If it is the mysterious intermittent problem then you need to check all the electrical connections and the continuity of the thermostat wire. Sometimes old copper wire becomes brittle and can break. The break may still be making some contact so it will give you something like you are experiencing. Make sure you check the electrical connections at the line side (high voltage) of the transformer also.
If you have a spare wire or two use those in place of red and blue (hot and common). If you only have one spare wire use the single spare wire in place of the red wire and see if the problem disappears. If it does not then use the spare wire for the blue wire and restore the red. If you have no spare wires then see below.
Lastly, pulling out all stops, I would replace the transformer with a new one (transformers are cheap just make sure you get the right one) and pull brand new thermostat wire from the air handler to the thermostat location – sometimes easier said than done. Since you have 5 wires I would go with an 18-7 conductor wire to give you extra wires in the future.
If you are uncomfortable working around electricity then call a professional HVAC service to do this for you.
a nest thermostat was installed in June and by December it lost power. At some point I had turned off the pilot valve which I was told may have something to do with the nest loosing power. Then the nest was replaced and there was some rewiring and the nest has been working. The problem is that now, 6 months later I try to turn the a/c and it does not turn on. When I look at the y terminal in the Furnace board, there is no wire in the y terminal. The yellow wire which is connected in the nest thermostat y terminal is connected to the C terminal on the furnace board along with the brown wire. The brown wire is connected to the c terminal of the nest thermostat. So if the nest thermostat has a wire in the y terminal (yellow) which is supposed to be the one that activates the a/c but that yellow wire is connected to the c terminal in the furnace board (and not the y terminal of the furnace) and the c in the furnace board is the common (right?), then how is the a/c supposed to receive the signal to turn on?
Sounds like it is not wired correctly. Best advice, call an HVAC company and get them to wire it correctly. As far as the Y wire, it does not have to connect to the furnace control board for the AC to work. Many times, the Y will go straight to the condensing unit many times via the furnace or air handler where it is coupled with the C or common for a complete circuit for condenser control.