Many electric utilities are looking for ways to reduce their load during peak hours, especially in the high-use days of summer when everyone has their A/C cranking. A few years ago ConEd in NYC started giving away WiFi-enabled smart switches meant for controlling older window A/C units which have no thermostat or internet connectivity of their own. Plug their modlet, pictured here, into the wall, plug the dumb A/C unit into the modlet, set up the Smartphone app, and you're done. On top of adding a handy thermostat to a device that badly needs it, you can now turn it on or off from your phone. Even better you can opt into a program that allows ConEd to raise the thermostat a few degrees during the heaviest demand days, in exchange for about $2.50 per thermostat each time they do that. You can always opt out before the demand event or during it.
If you're in ConEd's service area here's their information page, and here's where to sign up. There's no referral code embedded in those links but if you do sign up, please leave a comment so I'll know I helped. (If you're feeling generous you can use my email address, firstname.lastname@example.org, as a referral, which gets me a few dollars, but don't feel obligated!)
Note that you could use any smart switch intended for window A/C units here in place of the ConEd unit as long as it connects the same way.
My apartment doesn't have actual A/C units in the window. Instead, the building circulates hot or cold water (depending on the season, of course) which runs through fan-backed radiators in each room. They're wired directly to 120VAC power so there's nothing to unplug, which seems to mean I couldn't participate in ConEd's program. Worse, it means my system would stay dumb and waste energy by cooling or heating my apartment with reckless abandon while I'm not there since the only control is a high/medium/low rotary switch. Fortunately, the fan motors are 120VAC so all I had to do was splice in a standard household plug and outlet and I'd be in business. It turned out to be even easier than I expected.
The only parts you may have to buy would be some wire nuts and short extension cables, like these, which are $9 for a pair and are pictured above.
Step 1: Turn Off the Breaker
First and foremost: you're working with 120VAC mains power, which can kill you if you're not careful. Please do not try this unless you're comfortable in that environment and you know how to take all necessary precautions. Seriously.
If you're reading this site and you're unsure, it's likely you know someone who can help and teach you best practices as you go. Or, in the worst case, it shouldn't cost much to have an electrician do this for you.
Be smart and be safe.
Step 2: Prepare Your Splice
Months earlier I'd bought a bunch of short extension cords, meant to give wall warts more room in an outlet or power strip. Since I needed one plug and one outlet for this project, they were the perfect sacrificial lambs. 3' cords give you more flexibility so I strongly suggest using those. Pictured here is the 1' that I used.
Cut it carefully and expose about an inch of wire. Unfortunately, there was no ground in my radiator so I clipped the green ground wire right against the insulation jacket. Yes, I tested whether the radiator body was grounded. It wasn't. This is an old building.
Step 3: Open Up the Radiator
Turn off the breaker and use a multimeter to verify there's no power. I've done a lot of household wiring and I've seen more cases than I can count where badly-done work leaves current where it's not expected and doesn't belong. Always err on the side of caution.
To start, I turned on the fan switch then turned off the breaker at which point the fan stopped spinning and I confirmed, on the fan's terminals, there was no voltage present. I removed the switch, as you can see in the picture and, after making sure nothing was exposed, turned the power back on and used the multimeter to verify that the wiring was done correctly. It was. Then I turned it off again, of course.
The photos show the red/yellow hot and white/white neutral wires that power the unit. If I were installing a simple switch or battery-powered thermostat, I would only need to deal with the hot lead, but the modlet needs its own power so I had to deal with both.
Step 4: Install the New Plug and Outlet
Punch out one of the knockout ports in the junction box so your new wires have somewhere to go.
The solid red and white wires are the feed from the wall and the stranded yellow and white wires go to the rotary speed switch. Take the plug half of the splice, feed it up through the knockout port so the plug is in the main body of the radiator, facing down, and the exposed wire is in the junction box. Connect it to the stranded wires: white to white (neutral) and black to yellow (hot) and secure it with wire nuts. In the photos, that's the red pair of wire nuts.
Since the modlet needs to be plugged into a standard outlet, that's what we're going to create. Like with the plug, take the outlet half of the splice, feed it up through the knockout port so the outlet is in the main body of the radiator, facing down, and the exposed wire is in the junction box. Connect it to the solid wires: white to white (neutral) and black to red (hot) and secure it with wire nuts. Those are the black wire nuts in the fourth picture. It looks like the stranded yellow wire is going into the lower of the black wire nuts, but it isn't - it's just behind it. Look carefully between the red and white wires near the top and you'll see the black wire, coming from the outlet splice, which is connected to the red wire. I've highlighted that in the picture for clarity.
The last photo shows the view from under the junction box with both ends of the splice cables hanging down into the body of the radiator, ready for the modlet.
Step 5: After Testing, Install the Modlet
These shots are from the first of my four radiators. I used a 1' extension cable here, but for the others, I used a 3', which made testing easier because the two ends could be plugged directly into each other. Here I had to use another of the extensions to connect them, as you can see in the second picture. It's not a big deal with the modlet installed, but if you ever want to remove that it'll be easier if your splice ends connect without assistance.
Plug the splice plug into the outlet, make sure all of your wiring is tidy and everything is where it belongs, then turn the power on to test whether the radiator works as it used to. Since you didn't change anything electrically it should. If there's a problem here turn off the power and double check everything.
Once it's is working, reinstall the rotary switch and reassemble the radiator housing. Then install the modlet as you can see in the last picture and set it up as per its instructions.
Our radiator housings are metal but the WiFi is still able to penetrate well enough that the modlets never disconnected. (I'm also a network engineer and I've spent a lot of effort to make sure every square inch of the apartment has solid coverage despite the mesh-backed walls in our 60-year old building acting as a very good Faraday cage.)
Total time was about an hour for the first radiator and 30 minutes for the others.
For added functionality, connect it to the home automation package of your choice, like Home Assistant.
Enjoy bringing your 20th-century cooling system into the 21st century!