Two-wire thermostat

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?

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…!

I state with confidence, that wiring to a millivolt fireplace/stove/space heater with almost any smart thermostat, including the Wyze, is as simple as;
24v transformer to Rc and C
Heater leads to Rh and W1
Rc and Rh must NOT be connected

This solution has worked for me for years and with several different thermostats.

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Did not work for me. With separate transformer the thermostat could not accurately sense room temp. The reading wandered all over the place.
When i isolated the thermostat from the condensing furnace with a simple relay - everything worked.

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Which type of isolation the thermostat you are using? 24 V AC Relay? or expensive one?

A cheap and simple one. About $15.

Mark Case

In my circumstances, none.

Cheers