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Controlling a Room/Window Air Conditioner with an External Thermostat
(also usable for heating control with slight changes)

Here's another project that I've put off writing up for a really long time. Although I may or may not end up finishing it in one sitting, starting out on writing it up seems like a good thing to do on the back end of a long weekend. Please be aware that I assume a residential 120 volt, 60 Hz North American perspective on electrical wiring because that's where I am. Readers in other countries and situations may need to adjust certain things or even seek a permit to do this sort of project.

Warning, Danger, Caution: this is yet another project involving the AC powerline, high currents and by implication, electricity. As useful as electricity can be, things can quickly turn dangerous if you don't know what you're doing or things happen to go wrong. Don't overestimate your skill level. If you're not sure, stop and seek qualified assistance. The time to find out that you didn't understand something or made a mistake is NOT after you've started a fire in your house, nor after you've hurt or killed yourself or someone else. I am not responsible for your actions or those of others, nor am I responsible for the results of your following or inability to follow the information on this page. If you can't accept that, please move on and don't attempt anything explained here.

Ready made products for this same purpose exist, in case you don't feel that you're up to the challenge.

Also, here's an example of how you shouldn't do things, taken from a video I made a long time ago:

Picture of some of the components involved in controlling an air conditioner with an externally located thermostat.
It's OK to do stuff like this for testing. Just don't leave things this way. And don't use a really cheap multimeter (like the yellow DT-830 seen in the picture) to do work with line voltage. You don't have to spend a lot more to get something much better and safer.

What You'll Need:
How To Hook It Up
(a picture's worth a thousand words, so that's where I'll start)

A/C Thermostat Power/Control Wiring Diagram
Prepare all of your wiring materials and make sure your tools are ready. You don't need to break the bank on tools, but cheap tools are not the bargain they may seem, even if you're only ever planning on using them once. You'll be cursing a subpar tool all through the build process and long after you're done.

Bring the "live" (or "hot") line voltage to your contactor or relay's input (often labeled as "L1" or similar) connector and also to your transformer's primary (if you are using one). Most transformers come with short leads attached, usually black for the "hot" and white for the "neutral" primary connections. The neutral side of the line is never switched. (Yes, you can, but doing so gives rise to potentially dangerous conditions.) From the output (often labeled as "T1" or similar) of your contactor, go to the "hot" side of your outlet. The correct screws are usually gold in color.

Connect the neutral (again, unswitched!) to the silver colored terminals on your outlet and to the transformer's primary. Again, most transformers come with short wires preattached for the primary (and very often for the secondary as well). The last thing to do here is to connect the safety ground -- yes, really. The same as being plugged into a properly grounded outlet, connecting safety ground to your outlet and anywhere else it's provided for is very important.

If you have a "blinkenlight" (BL1 in the diagram above), now's the time to hook it up. Whatever kind of indicator you use, it needs to be designed for line voltage. Neon lamps or small incandescent indicator lights are ideal. A LED can be used, but will require a substantial (both in terms of value and power dissipation!) resistor. Otherwise, your LED will soon turn into a DED (dark emitting diode), possibly after catching fire, blowing a fuse or tripping a circuit breaker. Should you want to use an LED, I'd recommend powering from the low voltage part of the circuit.

Warning: if you are using a relay or contactor whose coil is designed for use at line voltage, you also need a thermostat rated for line voltage use! A regular thermostat is not suitable!

Warning: don't connect any heating or cooling appliance directly to a thermostat unless you are SURE the thermostat is designed for that application.

I'd strongly recommend using a conventional low voltage thermostat, control transformer and a contactor or relay having a low voltage coil. It's far safer than trying to bring full line voltage to everything.

Now it's time for the secondary, or low voltage wiring. This is where you'll be connecting your thermostat. I'd recommend inserting a fuse "first thing". You can buy a fuse holder or use some of your female spade lugs (along with some heat shrink tubing) to make a fuse holder. The fuse mainly serves to protect your transformer, (hopefully) by blowing before the transformer overheats or burns itself out. Depending upon the contactor or relay you have selected, a one to three amp fast-blow fuse will suffice. Connect one side of the transformer secondary to your fuse. Leave the other one unconnected for now.

Go on from your fuse to the "R" terminal on your thermostat. Power goes in to the thermostat here to be switched when the thermostat calls for cooling. If your thermostat has "Rh" and "Rc" terminals, use the one labeled "Rc". Should there be a metal jumper connecting these two terminals (as there often is on a brand new thermostat having both of them) you can connect to either. Just make sure that the little metal jumper stays in place.

Connect a wire to the "Y" terminal on your thermostat and take this to one of the coil connections (not the line or load connections!) on your relay or contactor.

Heating variation: if you are controlling a HEATER, connect to the "W" terminal on your thermostat instead of "Y".

Take another wire and connect it to the other set of coil connection(s) on your relay or contactor. Go from there to the other (as yet unused) wire from your transformer. This is the common wire (not to be confused with, nor connected to, the "neutral" wire on the high voltage side of things).

You're just about ready to test for basic operation. Prepare the thermostat as needed, and connect the power cord (or turn on the dedicated circuit if you used one). Set the thermostat for COOL and adjust the temperature so it's less than your room temperature. Within seconds, the thermostat should call for cooling and energize your contactor or relay. Most contactors will produce a pretty good thump when they turn on, while a relay will usually emit a fairly quiet click.

If things don't work, go back over the steps and diagram above carefully. Look for stray bits of wire poking out from around connectors, wires that may not have gotten stripped, incorrect connections or other problems. When you've found the trouble, correct it and replace the fuse if needed. Test again and see if things work. (Be aware that most thermostats have a delay built in to protect a controlled air conditioner from short cycling, which can damage things. Be patient and wait for the delay to pass if you shut the thermostat off and turned it back on right away.)

Turn the thermostat off. Within seconds, your contactor or relay should open and disconnect power. You can verify this is happening with a meter set to measure voltage or something like a lamp.

Once you're sure everything is operating and switching correctly, build it into a suitable enclosure, test it again, and then try hooking your air conditioner or heater as appropriate.

If possible, place your thermostat away from drafts, outside walls, supply registers or the supply of conditioned (cooled or heated) air from the controlled air conditioner or heater. The better your thermostat's placement, the more economically your equipment will operate.

Possible Results

These are still images from videos about the two units I built. Please accept my apologies for the poor image quality. These were recorded roughly a decade ago relative to this page's creation, using old digital cameras operating in their video recording mode. I still have both boxes, though neither one is fully wired up any longer.

A/C Thermostat Control Wiring Box

A/C Thermostat Control Wiring Box #2

It is bad practice (and definitely against electrical codes) to run unprotected Romex® wire on walls and whatnot as shown in both pictures. Both of these controllers were temporary installations in a house I didn't care about, and they have both since been taken out of service.


These are left to the reader as an exercise. (I just love it when other written works say this. Don't you?)
This method works best with older and simpler air conditioners having mechanical temperature controls. Still, you may find that a more modern unit may operate more efficiently when paired up with a temperature sensor that is located further away from the unit and its airflow. I realized a considerable savings on my utility bill by not only building these, but also programming the thermostat to raise the temperature in the conditioned spaces when I was away. (Turning the units off entirely would save even more energy, but I had a DVR system that I didn't want to bake, at least not too much.)

Hopefully by this point, you have created your own working example of an external thermostatic control for your air conditioner or heater. As always, I'm interested in your thoughts and comments.

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Copyright © 2021 by William R. Walsh. Some rights reserved. Your rights to this material are governed by the terms and conditions available from the top level page of this server. Although this article is presented in good faith, it is provided AS IS and without warranty of ANY KIND. You are responsible for the results of your actions, whether good or bad. Last updated: 04/11/2021. Revised to add SW1 to the schematic above. Created 04/04/2021.