Two-wire thermostat

I live in a area that - like Seattle - seldom needs cooling, but - unlike Seattle - doesn’t always need heat, either. My existing thermostat is two-wire. It connects to the furnace and just controls on/off. There is 48vac across the two wires, which is what makes the connection for the thermostat. I would think that 48vac could be used to power a smart thermostat, particularly since the only decision the thermostat would need to make is “on” or “off.” My XXXX doorbell uses the 48vac circuit from the chimes to not only ring the chimes, but to charge the battery in the doorbell. I should think a similar arrangement could be used for the thermostat.

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48VAC is non-standard, Industry standard in North America is 24VAC. That being said, the current wyze thermostat could do what you are asking, with a C adapter transformer to power the wyze. You would need to attach a 24VAC relay to the furnace to deal with the odd voltage though.

This i s how I would do it:
Turn off the power to the furnace.
At the furnace side, cut the wires, leaving enough slack on both sides to strip the wires back and connect the 24VAC relay. Attach the NO and C terminals of the relay to the wires that come from the furnace. Attach the 2 wires that come from the thermostat to the coil side of the relay. Then move to the thermostat area.
Attach one side of the 24VAC transformer to C, the other side needs to be spliced with a pigtail to one of the wires that used to go to your furnace. The pigtail needs to go into the Rc terminal on the wyze. Attach the G connector the other wire in your wall that used to go to the furnace.
Lie to the thermostat during setup and tell it you have the following wires: W1,G,Y1,Rc,C. It will likely complain that it can’t find the W1 and Y1 wires, but that’s not a problem, because they don’t exist, so you would just keep pressing ignore the error until it allows you to skip detection. You’d want to select “Electric” as your heat source when it asks, and tell it you have a furnace, so that it tries to run the fan.
Now, whenever the wyze tries to run the fan, it will run your furnace, and as you have told it that both your heat and cooling source require a fan, it should run whichever you ask it to. I don’t know how you select whether heat or cooling is run on your current system, with no third wire, but it would still work the same way with the wyze. Auto heat/cool would of course not work, because the wyze has no way to signal to the furnace to run anything but the fan, but as long as you’re there to do the actual switching between equipment, it would work the same way as your current thermostat.

BIG BOO-BOO! The circuit to the thermostat is 24, not 48! Don’t know how I got 48 stuck in my head…
(Oh, yes I do… Used to be a telephone installer back in the days of only landline phones. That’s still 48vac.) What does that do to your instructions? In the first place, I’d love to use the Wyze thermostat, since almost all of my smart home devices are Wyze. I got the XXXX doorbell before my wife started me on a couple of the Wyze plugs, so there we are.

Shoulda read your whole message: There is no cooling. Don’t need it, Just open the windows to cool, since high 80’s is the worst it gets. Strictly a furnace with a fan.

For 2 wire, heating only, all you need is a C adapter transformer. plug the C adapter transformer into an available outlet and attach the wires that come out of it to Rc and C. Attach the 2 wires that run to your furnace to Rh and W1. Tell the wyze that your “old thermostat” has those wires (C, Rc, Rh, and W1) During setup, it will ask you what is attached to W1, I’d assume you have a natural gas or oil furnace, if that’s the case tell it that, no need to trick it to make it do what you want.

Example C adapter transformer:

Does the C adapter transformer also works with a 2-wire millivolt gas-fired heating wall furnace like Williams 2509622? Or, do you also need a NO Isolation Relay? If relay is needed what is a good relay to buy? Thanks

The wyze should isolate the Rh and W terminals from the Rc and C 24V terminals when you tell it that you have both Rh and Rc wires. Otherwise any 24V AC relay could be used.

So, you are saying I should first try to use a 24 V AC transformer connected to C and Rc and my furnace wiring connected to to W1 and Rh terminal of Wyze terminal block…If it didn’t work then I would need a NO Isolation Relay?

Yes. if you wanted to make sure there was no 24V leakage onto the milivolt connections before hookup, you could take a multimeter and connect it to the C terminal and the RH terminal and verify that there is not 24V present with Rc and C hooked up to the wall adapter.

Thank You and Happy Holidays…!

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Now that it’s (almost, but the cleanup) done, thanks for the assistance. However… the thermometer on the Wyze seems to be W.A.Y off compared to another (that I can see) only a couple of feet away. This is disconcerting, since trying to build a schedule is made a lot less simple. For instance, where the Wyze will say “68”, the one I can see (and seems to be more inline with how a perceive the temp) shows “65.8.” Trying to build a schedule is a little confusing - particularly since I’m retired and don’t really have an “Away” segment to my day.

Got any hints how to reconcile this problem?

You can calibrate the wyze’s thermostat reading in advanced settings. The setting you are looking for is Temperature correction.

If you don’t have any general scheduled away time, it doesn’t matter, you might still want to set the thermostat lower when you leave the house for some reason, and you will be able to put it into away mode from your phone. You don’t need any schedule really, it’s just a nice feature to have if you have a schedule you follow - most people like to sleep at a lower temperature than when they are awake, for instance.

One last question, If I do need a N.O. isolation relay to make the furnace work, the wiring from 24 V AC transformer connects to C and Rc and wiring from the coil side of relay connects to C and W1 terminal of Wyze thermostat. So, no wiring connects to Rh of Wyze…Is that correct?


OK, I got the Wyze T-Stat and connect my 24 v AC transformer wiring to the Rc and C terminal and it came on with “Wyze” on display. Then I did the following tests on it with a multimeter:

  1. Voltage across the Rh & W1 terminals with Rh at - and W1 at + of multimeter is 0-0.25 volts
  2. Voltage across the Rh & W1 terminals with Rh at + and W1 at - of multimeter is 15 volts
  3. Voltage across the Rh & C terminals with Rh at - or + and C at - or + of multimeter is 26 volts

Do I still need a NO Isolation Transformer for my millivolt Furnace to work properly?

When you setup your wyze, did you make sure to tell it that your “old thermostat” had the following wires: Rc, Rh, C, W1`? If yes, you will need an isolation transformer. If no, reset it and tell it you have those wires and retest it. You will need to go through the setup procedure before it maps the thermostat terminals properly, it defaults to connecting Rc and Rh internally unless you tell it you have 2 different electrical sources (Rc and Rh)

You can skip connecting it to your actual equipment during the setup until you are sure it’s not going to damage the mililvolt system, it will detect that the wires are not hooked up properly, but you should be able to try again and ignore, just so that you can test the system properly.

Thanks for the tip. I haven’t gone thru the set up yet but what you’re suggesting make sense. Tell Wyze that I have Rc, Rh, C, W1 wires and ignore the “wires not hooked up” messages… After set up is complete, do I also need to adjust the thermostat to start a heating cycle…so current start flowing in W1 and Rh before testing with multimeter? Will I get an error on the display because furnace is not yet connected to thermostat?

You would want the wyze to be trying to “heat” if you were testing between W1 and C, but if Rh is properly isolated from Rc, it shouldn’t matter if it’s trying to heat or not when you test between Rh and C.

It also shouldn’t care if the furnace is actually hooked up after the setup procedure. Eventually, it might throw some sort of error, because it will take an infinite time for the temperature to rise, but I’m not sure if they programmed a test like that into the thermostat or not.

I completed the setup on Wyze T-stat with 4-wire (Rc, Rh, C, W1) setting the Home temperature at 70 F. My apartment temperature was 64 at the time of setup. It said your room temperature is 32 F (?) and then it went to heating mode to reach 70 in 50 minutes. No error… I tested Wyze terminal wirings after the setup was complete and T-stat in heating mode and I got exactly the same results in my post above a couple of days ego. After 5 minutes T-Stat turned it self off for Delay Safety reason shown on the screen. Additionally I got the following:

  1. Voltage across the W1 & C terminals with W1 at - and C at + of multimeter is 15 volts
  2. Voltage across the W1 & C terminals with W1 at + and C at - of multimeter is 0.25 volts
  3. Voltage across the Rh & Rc terminals with Rh at - or + and Rc at - or + of multimeter is 0 volts

So what gives…? Since Rh and W1 voltage under scenario (1) is only 0.25 volts, can I just hook them up to Furnace gas valve terminals with W1 connected to the millivolt Hot side and Rh connected to the millivolt Neutral side without any Isolation Relay?

Are you testing this with a digital multimeter or an analog multimeter? you shouldn’t be getting a different voltage when you reverse the leads if the meter is in AC mode.

And if you are still seeing this

Voltage across the Rh & C terminals with Rh at - or + and C at - or + of multimeter is 26 volts

Then I’ve got no idea of what wyze has done with their internal design of the thermostat. This should not be possible if Rh is properly isolated. If I wasn’t using my thermostat to heat my house, I would test it on my system with my fluke meter, because this shouldn’t be possible. If there is indeed 24VAC across Rh and C, then you will have to use an isolation relay.

The multimeter is an analog meter GB GMT-12P Korean made. All voltages are in AC mode with the control knob at 10 or 50 AC volts. I will test again…

After further testing the thermostat is stuck at 32 F which I find odd. Researching shows it is the internal temperature sensor failure issue. Testing passed for heating both for thermostat and the NO relay. However after the completion of setup the actual heat cycle never comes on. The T-stat will be returned soon. The voltage across the Rh & W1 is 25 volts during testing so I decided to use the relay as soon as I get my hands on a replacement thermostat… Thanks for all your help…!