Thermostat wiring diagram, voltages?

Thanks, Craig, for this info.

From what you’re saying, I assume that to convert my old-school 2-wire heat-only thermostat with a Wyse thermostat would require running a 3rd wire and interconnecting the furnace, xformer, and Wyse thermostat as follows. Would you agree with this?

24 VAC xformer lead 1 of 2: to Wyse “C” and to furnace lead 1 of 2
24 VAC xformer lead 2 of 2: to Wysie “RH”
Wyze W1: to furnace lead 2 of 2

I will give you my two cents. First, it is easier if your furnace has a common(C) terminal on the controller. My 22 year old system does.

Mechanical thermostats, what we had prior to modern electronics, were simple temperature sensing switches, those systems could function with two leads, one powered conductor up to the thermostat, one back. Connect the two and the furnace comes on.

Old people may remember the mercury blob thermostat, it was a coil of bi-metal foil that expanded, rotating as the temperature changed, the rotation moved liquid mercury in a glass vial to bridge two pins, connecting the two leads. In my 1960’s youth my dad, a frugal engineer who grew up in the Depression, would turn ours down at night before he went to bed, then back up in the morning.

I installed a “programmable” on my system maybe 20 years ago, it was powered by two AA batteries that lasted five years between changes. I bought a Wyze so I could control the furnace from my iPad, retiring that programmable.

24 VAC seems to be the standard voltage in heating systems, that’s a legacy issue. I don’t know why they used 24 VAC. It may have been it was easy to make from 110V, also under 50V is less likely to do damage, easier to insulate, less likely to start fires. Old doorbells used a similar approach.

Why not 5 volts, or DC? It may be way back then, before solid state devices were inexpensive, that converting to DC was cost prohibitive. And voltage drop, a problem on longer runs of wire, is less of an issue the higher the starting voltage is, 24 volts may have been the sweet spot. Now we use WiFi instead of running low voltage wires, but you still need power. A friend, working at Amazon, told me years ago that Bezos implored him not to use batteries on the secret project he was running, it was Echo/Alexa.

A thinking, WiFi connected thermostat needs a return or earth to power the electronics. Wyze gets around this on old systems with the small hockey puck thing. I have not yet taken mine apart to see what is in it, but it must somehow supply a ground/earth. I am a Mech E, not an EE, but have worked with electronics a lot, I have not seen this done before.

If your furnace has a C terminal run a wire from it to the Wyze thermostat and you are done. C is common, or earth. You could get an earth somewhere else, but do that with caution, only using a VOM, and only if you know what you are doing. All earths are not always the same.

Voltage should come up on the red wire, hook it to Rc, (not Rh) white is the return, that is all you need to switch the furnace on and off, provided you can power the electronics, to do that you need a common. Use a VOM to be safe and sure. If you don’t know what that is, own and know how to use one, this may not be the project to learn on.

I personally would not use another 24V source, that’s beyond my pay grade. Before I did that I’d try to get past the AC/DC converter on the board and just use a DC supply to run the control electronics, or ask an EE who owes you a favor what the issues are.

The Wyze use of the green wire (fan) is, in my heating system, superfluous. My furnace controller delays the fan until the heat exchanger is up to temperature. I hooked up the green wire and now my fan comes on early, I plan to disconnect it.

The Wyze UI is very beta; it is poorly written, does things I hate and Alexa can’t see it. Hopefully they will get savaged in forums and improve it. Wyze lightbulbs are not much better, so who knows.

Electricity starts fires, so don’t mess with stuff you don’t understand.

Did I say electricity starts fires? It does. Really. All the time. Yes, heating systems have safety fail-safes built in, but they get very hot, use combustible gases, fire and other things that kill people and burn down buildings. Be careful. To learn play with 5V DC electronics, like Arduinos, not heating systems.

The Rh terminal won’t power the Wyze thermostat, use Rc instead.

are you using the transformer in the furnace?
Currently your thermostat is an automatic switch that closes a between RH and W1 to turn on the heat.
RH is 24vac
For wyze power you also need a common (from the same 24v souce).
If you can add a wire from the thermostat to the furnace. That would be ideal.

If you use an external transformer:
you would connect 24v on “RC” and “C”
You would put your existing heating wires on “RH” and “W1”

If you use the furnace transformer:
you would connect your existing “RH” to “RC” and common from furnace to “C”
You would put your existing heating wire “W1” on “W1”

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I took my wyze adapter apart, it does something like this and uses the relay option for full sine wave output:


Just a simple diode bridge

White appears to be passively passed through without any modification. Red is used to power the relays (not pictured) The circled inputs (left) are the labels wyze puts on the terminals.

I opened the (unused) Wyze C wire adapter and created a circuit diagram. (Apologies for the reversed labels on the screw terminals, first time using the schematic progam and couldn’t find the fix).

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If your furnace has a fan delay you don’t need G, and in fact Wyze turns on the fan early. I disconnected mine. You simply need an earth (C), then Rc and W are the switched pins.

Then it takes time to figure out the UI, which seems to have been written in Reverse Polish. I am not thrilled about that, but it should be decipherable, however easier it would be with a manual written by someone who understands English grammar. A hire coming soon I hope.

My old thermostat was doing fine after twenty years. I am starting to realize the risk with these are once the company is gone what happens to the app. Its a “Little Shop of Horrors” business model. I won’t toss my battery operated programmable just yet.

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One of the main reasons I bought the Wyze thermostat was to use the Fan Cycle function, so I need the green hooked up. We use a wood pellet stove for the majority of our heat and being able to cycle the fan 10 minutes per hour works great to circulate and even out the heat from the wood stove in our house.

First of all, what’s wrong with reverse polish? Those were the best calculators!

Second … you’re going to label the ownership of a $50 thermostat as a “risk” … because “once the company is gone” there will be no app support? I’m sorry but not only are you being mellow dramatic about this, but you’re also investing all of this negative thought and energy into a scenario that has no guarantee of ever happening, and statistically speaking, it has almost zero chance of happening at all.

Wyze is probably the most advanced and cost-effective technology company I have ever seen in the United States. They have the engineering standards of Apple, but they sell their products at Dell prices … these guys aren’t going anywhere because they put out the best quality electronics this side of the planet at insanely low prices and their marketing is working … no … if a company like this is ever at risk of going away, it would be because they have been so successful, that a larger fish would try and buy them out at which point the new company would be obligated to support everything that Wyze put onto the market.

When was the last time you got burned by an electronics manufacturer in the United States? Maybe you’re all pent up because you blew a tube in that Fisher stereo system you still have in the den … Maybe you can’t play Exodus anymore on your Commodore 64 because they had the audacity to succumb to an emerging industry that their products couldn’t compete with … and now that $300 you spent in 1982 was just a total waste and had you known better you wouldn’t have taken on that “risk” in the first place…

Since when has consumer electronics EVER been an investment? Don’t tell me you haven’t dropped $50 on going out to dinner and a movie … were you taken advantage of then too?

Tell you what … go to Amazon and find me one smart thermostat that can do what the Wyze can do for less than $100 to $150 … then come back and tell us how much of a financial risk you took in purchasing this product.

Heck, I almost want to buy it from you just to keep you from irresponsible posting in the forums … what is it about speaking negatively about people in public that works so well for you?

There is NOTHING wrong with this thermostat … and it’s not a crime for an electronics manufacturer to keep their documentation simple … because it will work for the majority of their customers and it won’t freak them out by bombarding them with engineer speak that only confuses the majority of people. The intimidation factor of technobabble is one of the main reasons why people don’t try to better their lives in the first place with cutting edge technology like this … I think Wyze deserves some credit for the insanely thoughtful, thorough, and accurate engineering that they invested in this product (and all of their products). It looks elegant, it has functionality in the areas that make a difference … it has features that a smart thermostat should have yet sadly none of them in this price range do have …

Wyze stepped it up and raised the bar for their competitors … they are winning the battle because their products work at an amazing price and they are built like mac trucks! … their competition will get eaten alive once this catches on… mark my words! I’ve been a technology buff for over 30 years and companies like Wyze are diamonds in the rough … you really should give credit where it’s due instead of shouting that the sky is falling … cause it’s not!

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Making an educated guess here, but the choice to use 24 volts AC for HVAC control - given the era of when that decision was made … which Im thinking was probably just before the transistor was invented … would have been the right choice since 24 VAC has been a common voltage for driving relays and even starting motors… it’s much more resilient to wire oxidation and when it comes to moving electricity over long distances, AC is the optimal choice by far since I2R losses are significantly lower in AC than with DC and those control wires - depending on the size of the house or the industrial building could be a substantially long run of wire … obviously with todays electronics, even a low 3 volt DC line over twisted pair would be ideal … but were still dealing with technology that started taking a foot hold back in the 40s and 50s and theres still a ton of it out there…

I’m not sure I agree with you on the whole chopping into the device to find its 5 volt pickup on the circuit board … power supplies tend to be a balance point in a circuit for overall current flow and in the elimination of transient current and voltage spikes which are normally handled in the power supply portion of the circuit. It would be much safer to use an external 24V AC source to power the unit if it cannot be powered from the existing wire … its always best practice to keep a device running as it was designed to run rather than hacking inside it and bypassing what the engineer did… thats more of a spit and duct tape approach and has a much higher risk in terms of causing damage (or fires as you pointed out). If you can learn which two pins need the 24 VAC to power … then using a simple 110 to 24 VAC wall adapter would be the safest way … assuming it cannot be powered by the means that the manufacturer is wanting it to be powered.

Also wanted to comment on what you said about a ground wire … with AC … ground is not your return … your return as I’m sure you know … is the other lead on the transformer that is providing the voltage in the first place. Alternating Current taps off of a transformer and earth ground is only used in AC circuits with appliances to keep them electrically protected so that if one of the AC wires inside the appliance should happen to get loose and touch the chassis … when the chassis is grounded to the earth then if a human touches it, there will already be a path for the electrons to travel to the earth through the chassis ground where if it were not grounded to the earth and a human touched the chassis with the busted wire in it, the electricity would seek the path of least resistance to the earth and it would find it through the person who touched the appliance … so for AC, earth ground is only used for safety, not for the circuit path.

This is not the same for DC … as we can actually use Earth ground in DC as part of a circuits path if we chose to do so … but AC current only flows back and forth from one side of a transformer to the other. In Home AC, one side of the transformer is the black and the “return line” is usually white with green being earth ground but playing no part in the path of electron flow except when something goes wrong.

Thanks for the info re RH not being used by Wyze in spite of standards that define it is related to heating circuits and RC being related to cooling circuits. I wired as depicted and it works fine.

For clarification, I have a very old furnace without a circuit board. It has a standalone transformer that connects directly to the gas valve with one lead passing through a two-wire thermostat.

Adding a 3rd wire into the mix and interconnected everything as depicted in this (revised) sketch makes this thermostat work with my arrangement.

Clipboard01w

Hi,

It is possible to wire it into an RV without an adapter. I just wrote a write up on it if you want to see how I have mine setup. The one thing that I haven’t added yet is that you need to select boiler, radiator, or other, otherwise the AC fan will come on with the furnace. Check it out and let me know if it works for you too.
Wyze Thermostat in an RV | Cole Family Adventures

Thank you. I hope it continues to work for many years. My concern is two fold. First is the switching requirements of of DC over AC. Second is the current draw of different AC and Furnace units. The first may be of no concern as I appears the switching in the T-stat is solid state but that make the second concern more important. In my search for reasonably priced parts to convert to 24vac input to 12dvc output it is difficult to find relays rated for dc switching where the coil is powered by 24vac. I was going to try using one only rated for AC but just haven’t found the time. Did you happen to check the current draw of the AC and Furnace trigger wires? With a fractured fibula I find myself with some down time and will do some testing here also. Thank you again for being willing to test it out using 12vdc.

Great questions. I haven’t measured the current draw, but that actually sounds like fun so I might report back with some stats. That said the signal relays used are ac/dc relays that look appropriate for switching the DC furnace. Here is the datasheet for the relays that they use if you are interested. All that said take this as an opinion and not me saying that nothing could possibly go wrong.

Thank you. Is that data sheet for the relays on the wyze Tstat or the AC/Furnace? I didn’t see them on the tstat but only looked on one side of the circuit board. Didn’t want to break anything pulling off the dial.

https://fccid.io/2AUIU-WTHERM/Internal-Photos/Int-Photos-4932586
That datasheet is for the relays in the wyze.

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Awesome, thanks. I just did some testing with an inexpensive Craftsman clamp on amp meter. These are my findings, your mileage may very.
Yellow 0.1 amps
Green 0,08 amps
White 0.15 amps
I have a coleman Tstat with unkown brand of roof mounted AC (only has one speed) and a propane furnace (identifying stickers and marking are either warn or missing). We only recently bought the trailer.

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When you mention *** Do not connect the Blue Wire to O/B *** what is O/B?
Sorry for my ignorance. In your link I think you say to connect the blue wire from the RV to the C terminal on the Tstat.

Wiring Color | Function | Thermostat Label
Green – Fan High –> G
Gray – Fan Low –> Not Connected
Yellow – AC Compressor –> Y1
White – Furnace –> W1
Red – 12 VDC + –> RC
Blue – 12 VCD – –> C

Warning!!!

*** Do not connect the Blue Wire to O/B *** It will blow fuses if you do.
Your devices all share a common ground on the trailer and do not need this to be connected. The thermostat is setup for AC power and does not handle the DC ground properly.

Duh, Sorry, I now see an O/B terminal on the Tstat.

Yeah, You can learn from my mistakes on that one.

Pile of shame. :laughing: