Thermostat wiring diagram, voltages?

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:

I just installed mine and got this error message. I clicked through and ran through the test procedure in the app and everything seemed to work. Do you know if there is a way to constantly display the ambient temperature?

You can click skip. The RV system works a little differently so it is not being sensed.

Thanks to the feedback provided here, I’ve updated my article to include the new details and the screenshots for setting up the thermostat in the app. Please let me know if there is anything missing. I hope it helps everyone.

OK, had some issues last night. The first time the heater was called to run I realized the AC was running also. I updated the firmware with no luck resolving the issue. So next I deleted the tstat from the app and tried to re pair this time omitting the yellow wire from the set up process but when it went to find the device on the network it would not pair/find. Prior to this I could always access the tstat from my phone. This morning I found that the green wire goes to the AC fan not the furnace fan so anytime it calls for heat the AC fan comes on also. Remedy for now was to remove both the yellow and green wires so now only have heat. Also can only control it manually from the tstat. Still can’t get it to pair. Pic from original installation, “Yellow” wire edited.

Did you select “other” or “radiator” in the setup? I kept running into that issue as well. On a home furnace/AC system it runs fans for both systems so it thinks it needs to do that on our setup as well. Telling it to use it as a radiator lets it know that there are no fans involved. The RV furnace controls its fans on its own as soon it comes on so it doesn’t need an additional signal line.

As for re-pairing it if you press and hold the tstat button it will eventually ask you if you want to factory reset. You need to do this before it will let you pair it again. It is also under settings if you can still get to that screen on the tstat.

Think we have success. Thank you so much for all your help. One more question I don’t believe anyone answered when I asked before.

Is there a way to get the Tstat to constantly display the ambient temp?

Awesome, I glad it worked out. I haven’t found any way to have it always on. It only shows up when I walk by. You might be able to do a feature request. I think that would be a nice option so I can see it across the room.

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So I am trying to replace my old nest thermostat with a Wyze. Here is how my nest was wired and worked fine. I put the wires back in the exact same spots and nothing. I swapped the RC to RH and it turned on but the furnace itself started getting warm.

Do I have something hooked up wrong?

No, wyze currently has an issue with heat pumps in heating mode. If you check in settings>advanced>fan delay, make sure it is set to 0 second delay. If you do not have this option, then you will want to go back to your nest until wyze figures out their software/firmware bug.