- This article includes a comprehensive table 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
- Lastly, resource and related links to help you with wiring and installing a thermostat
How to Wire an Air Conditioner for Control - 5 Wires
The question is: 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. Furthermore, it includes a thermostat, a condenser, and an air handler with a heat source. Moreover, 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 gets hot water from a water heater or boiler.
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. Finally, typical systems that do not use 24-volt control are window unit air conditioners, ductless mini-split systems, and baseboard electric heating systems.
Honeywell Thermostats Available Here
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 | Thermostat Wiring Diagram for AC Unit
- R Terminal for the Red Wire
- W Terminal to the White Wire
- G Terminal to the Green Wire
- Y Terminal to the Yellow Wire
- C Terminal to the Blue Wire
Red Wire for Air Conditioner Control Power (Hot)
1The 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. Finally, it will feed it with 24 volts, provide power to the thermostat, and when the thermostat calls for heating or cooling, provide everything with control power.
White Wire for Heating System (If so equipped)
2The W terminal is for heating, including the heating for gas furnaces, electric furnaces, boiler systems. Additionally, the back-up heat for heat pumps.
G Wire for Fan Control | How to Wire an Air Conditioning Thermostat
3The G terminal is for the blower fan located in the air handler. Moreover, it is the fan that circulates air throughout the ductwork system.
Y Terminal for Yellow Wire - Air Conditioning Thermostat Wiring
4The Y terminal is the terminal that will turn on the air conditioner. It typically is terminated at the compressor contactor in the condensing unit located outside. Furthermore, 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 run to the air handler where at the air handler, this wire is usually connected to another wire (typically by a wire nut. However, it could be connected via a terminal strip in the air handler).
Additionally, it then runs outside to the condensing unit along with at least one other wire in an air conditioner split system set up. Furthermore, the other control wire will be the common wire. That is needed at the compressor contactor. Finally, that is where the coil in the compressor contactor needs a hot and a common from the control transformer to work.
C Terminal and C-Wire for the 24-Volt Common side of the Transformer
5Finally, the C terminal is the common terminal, and this terminal feeds the common side of the load from the control transformer.
Honeywell Thermostats Available Here
Terminal Strips - Wiring Thermostat for AC
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
COVID 19 Home Protection
UVC Light to Kill Viruses and Other Airborne Harmful Things that Affect Health
For additional protection you can also use a UV Light that will kill harmful viruses and bacteria inside the airflow of the air handler. In labs researchers used a spectrum of ultraviolet light called UVC to kill viruses. This product requires professional installation so it will turn on and off with the blower fan in your air handler but it will offer the protection you need for you and your family from any viruses.
Click the image to the right for purchase options of the UV light to protect your family.
The article should state whether the 24 volts is AC (like a 16 VAC doorbell that is simply stepped down from 120 VAC through a transformer) or DC (which is stepped down but also rectified with a circuit).
Not difficult to tell with a meter or looking at the transformer.
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.
I have 18 gauge wire running from the compessor to the condencer inside of the house people are saying it sould be 12 gauge but i checked it could go either way my concern is it safe i have two fujitsu pumps both 12 years old no problems running the communication wire with 18 what would be your recomendation all info much apperciated wayne
I can’t be 100% sure about it. It could be a control wire. A control wire is typically 18-gauge which is rated for 16 amps. 12-gauge wire is rated for 20 amps. If you look at the condenser there should be an ampacity rating on the data plate. That number will tell you the max amps that unit draws when it is under heavy load. If it has worked for 12 years I wouldn’t worry about it. Just make sure the wire is protected from damage by weed eaters et al things or a trip hazard. Communication wire usually runs 18 gauge also. And communication wires are usually very low voltage/amperage. The only thing with comm wire is: do you want shielded or unshielded wire.
Follow the manufacturer’s recommendation for everything and you will be safe. Use the wrong wire and you can have problems and create a dangerous situation such as a fire. Good luck.
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.
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
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.
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.
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.