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I currently have a thermostat with two wires: a C wire that powers the thermostat and an R wire that controls both the heater and central air. It's my understanding that smart thermostats won't work with this type of 2 wire setup. I've been trying to research this and not coming up with much. I did see an article about plugging the R wire into the RH terminal and connecting a jumper wire to the RC terminal. Could this work?

Thermostat

Thermostat2

Sending Unit

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  • Are you sure there are only 2 wires? Commented Dec 5, 2022 at 1:14
  • For smart thermostat to work you would have at least 4 wires, two in and two out. If you have only 2 wires coming the smart thermostat will not work. In that case you only need dumb turn dial thermosat
    – Traveler
    Commented Dec 5, 2022 at 1:19
  • I just don't see how two wires can control two devices (except with lots of smarts, but that is not the case normally). I would expect a minimum of 3 wires - some sort of common + heat + cool. Commented Dec 5, 2022 at 1:25
  • I added a picture if that helps any.
    – Adriane S
    Commented Dec 5, 2022 at 1:29
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    On the thermostat I switch between heating and cooling by the the touchscreen. It is a Pro1: Model#955WHhttps://www.pro1iaq.com/images/900Manuals/PM-955WH-IM-Installation-Manual-Alitho-1845.pdf
    – Adriane S
    Commented Dec 5, 2022 at 1:57

2 Answers 2

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This is not a typical thermostat. It is, arguably, already a smart thermostat. Based on the manual link added in comments, this is a Pro1 955WH.

All the traditional connections (RC, RH, C, Y, G, etc.) are in a base unit which is hardwired within several feet of the HVAC equipment. The thermostat is actually a wireless remote, but it can (and generally should) be connected with a pair of wires to provide power instead of relying on batteries.

Replacing this with a Nest or other advanced smart thermostat (e.g., internet monitoring and control) would either require running a new multi-conductor thermostat cable or replicating this setup by placing a base unit near the HVAC equipment and a thermostat remote ("zone") in the preferred thermostat location.

If you replaced the existing thermostat with a 2-wire (presumably heat or a/c only) thermostat, it would not operate the HVAC equipment in any way.

Update based on picture of base unit and comments:

You should be able to either put a new thermostat in place of the base, or splice in 18/5 (or even more wires if you want to future-proof) cable from the base location to the desired thermostat location. If you are going to do that, I would recommend first doing a trial install of the new thermostat in the base location. Assuming it works - heat, cool, fan all turn on/off as expected - then use wire nuts to attach the new cable to the old wires and run the new cable to the desired location and install the thermostat there. While thermostat wires do not need to be in a junction box, putting the wires and connections inside a small junction box may be a good idea just to keep the connections out of harm's way.

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    Okay now that makes some sense to me! I've seen the unit in the basement but didn't realize that's what it was. Are you aware of any base units that would be compatible with the Amazon smart thermostat: amazon.com/dp/B08J4C8871/… Lastly, my thermostat doesn't require batteries to work, I just tested this by pulling the batteries out and putting it back on the wall and it worked.
    – Adriane S
    Commented Dec 5, 2022 at 2:30
  • Sorry, I don't know anything about the Amazon thermostat. Correct that your thermostat does not need batteries - the pair of wires connecting the base and the thermostat provides power. Commented Dec 5, 2022 at 3:36
  • Ahhhh. So the approach I "just invented" is, in fact, already in use. Figured it ought to be possible, had started wondering why the smart thermostat I'd seen didn't use it -- probably they just didn't want to license the patent. MANY thanks for the education!
    – keshlam
    Commented Dec 5, 2022 at 5:53
  • Patent license costs are definitely an issue with some of these things. But there are often installation issues that make wired better (don't have to worry about steel doors or thick walls in between) or wireless better (don't have to deal with wires, except power). No one-size-fits-all solution. Commented Dec 5, 2022 at 13:35
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Ruskes is slightly off -- a smart thermostat doesn't REQUIRE Y, W, and G any more than a dumb one does. However, like other modern thermostats, it is designed to be able to take advantage of these more precise controls if the HVAC system exposes them and the thermostat cable has the wires to make them available. You can use a smart thermostat in an older system that has only heat/no-heat, it just won't be as versatile. And you'll have an issue getting power to it.

Unfortunately most of the "smart" thermostats draw too much power to run from batteries, and generally they try to get their power from the system they're controlling. That, obviously, requires a few more wires, which means you need to think about using the existing wire to pull a replacement cable through the building. If you're lucky that can happen fairly smoothly, but don't bet on it being a simple straight-line pull. (I replaced the 4-wire runs in my house with 7-wire a year or two ago, but to do so I had to access the upstairs thermostat's wire from the attic since it went up there and then back down into the wall.)

Some smart thermostats have a way to run off a special plug-in power supply without having separate 24V power in the thermostat wires. But I'm not sure whether any of those will work all the way down to a two-wire configuration. (I can see how they might, using the same pair of wires for power and digital signalling, but I don't know the details of how they're implemented and I haven't tried using them.)

All I can suggest is contacting the manufacturers, telling them what you have to work with, and asking them if there's an adapter that will let you use their product with this system without replacing the wiring.

Or just go for an older-style clock-thermostat, which will run on batteries. That won't give you remote control of heating or the fanciest controls, but it will let you set separate temperatures for weekday sleep, weekday waking, weekday working hours, weekday evenings, weekend sleep and weekend days, and let you set the times of the day at which each of these is selected. That's enough to let you achieve some nontrivial savings on heating costs. And clock-thermostats are CHEAP these days.

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  • Depends on how they set this up. My impression is that this is a "phantom power" kind of situation, where digital signalling is imposed onto the same wires carrying the power and at the power supply those signals are converted back into opening and closing a relay. That's certainly doable, and if I was designing this retrofit adaptation that's exactly what I would have done. As to whether it's how the manufacturers actually addressed this use case -- as I say, I haven't worked with these, only seen some installation docs; ask the manufacturer, or get one and reverse-engineer it yourself.
    – keshlam
    Commented Dec 5, 2022 at 2:13
  • @Ruskes Not "just invented". Power and signal on same line is quite common for phones and many other things. As far as I can tell, this particular thermostat actually uses a wireless (not "WiFi") signal but the concept is quite valid. Commented Dec 5, 2022 at 3:49
  • @manassehkatz-Moving2Codidact maybe before digital/wireless age
    – Traveler
    Commented Dec 5, 2022 at 4:10
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    Phantom power -- sorry, sound technician term. Actually a slightly different concept, too. In this case I'm actually speculating that it's something closer to X10 signalling, or coupling a higher-frequency signal in, or perhaps just pulsing the current draw -- with modern electronics there are multiple solutions to pushing a signal on top of the power pair, as long as the devices at both ends agree on how it's going to be done. No, I didn't "just invent" any of this.
    – keshlam
    Commented Dec 5, 2022 at 5:47
  • (Actually, if you're gong to go that route, it might even be possible to push the full 7-wire signal and power through the single pair. That one I am inventing on the fly. Except that we've just been told it's already in use by this very thermostat; see other answers. Digital is actually easier to piggyback on power than analog.)
    – keshlam
    Commented Dec 5, 2022 at 5:56

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