Wyze thermostat simplified block diagrams

I made a couple of simplified block diagrams to show why running the wyze thermostat requires a C wire and why if you use an external transformer, you cannot use the Cooling aspects of the wyze.
Not sure if this will help anyone, but it’s worth trying.

Current needs to flow through the R and C terminals of the transformer, through a load in order for that load to get powered properly. Without a complete loop back to the transformer, you run the risk of the current finding its way back through your high voltage wiring in your house, or through a ground loop, which can cause equipment damage and relay chatter.

Here’s a diagram of a thermostat that is being powered by an external transformer.
Note that the only load that has a complete loop is the heat, fed by the W wire.

By contrast, here is a thermostat that is being powered with the furnace transformer, with a C wire (or C wire adapter).

Once again, not sure if this will help anyone, but it’s often easier to see a diagram and understand it than to have it explained to you without a visual aid.


good info!

This is awesome

This is not the place for help with your furnace. I’ve responded to you in your thread.

So does the C wire basically provide voltage to the thermostat when one of heat/cooling/fan is not engaged? You obviously wouldn’t want it, but if the heat/cooling/fan was always running the thermostat would work (well, power on) and not need a separate C wire?

And one more question: how come in the second diagram the Y W G can all go to a common Rc, but they don’t in the first diagram (Rh and Rc)? Couldn’t you do that and also splice to the external transformer R to complete it’s circuit?

The C wire provides a return path (similar to a ground) to complete the circuit providing constant power to the thermostat.


The C wire provides a return path (similar to a ground) to complete the circuit providing constant power to the thermostat.

So it’s basically the Neutral (common) wire, and the current is coming from R, which would be the Hot?

And does that still mean that if the fan was always running, the thermostat would run without a C wire on the other side? (again, knowing it’s not a way you would want to do it)


I’m not following your question exactly but the way I understand it is when the thermostat runs the fan it’s bridging the R and G wires without needing a C wire.

The wyze gets it’s own power from Rc and C, so technically, if you connected one of the normally switched lines to C, there might be enough juice to power the wyze and the other load, although putting them in series like that might drop the voltage to the point where it doesn’t work quite right. Normally the loads are in parallel, to minimize voltage drop.

For your second question, the wyze has internal circuitry that detects if something is connected to Rh and switches the heating loads to it if there is. The cooling loads stay on the transformer with Rc, which is not connected to the other (common) side of those loads, and thus a full circuit cannot be made.

Furthermore, some people have attempted to connect the C from the transformer to the wyze and have then spliced the other wire into Rc, which creates a hazardous situation. Just because 2 transformers have a 24 volt difference between the leads does not mean that they are both the same potential to ground. Often, the transformer in your furnace will have one side directly connected to ground, however the wall adapters that people are using will usually be floating, and may be several hundred volts to ground. They are both still 24V between the leads, but in reference to ground they are very different and cannot be connected together.

If you plan on using a system like this, a normally open isolation relay is required on the furnace end. I’ve added a diagram to this post to show what that would look like in a 2 wire run, with the wyze getting power from a wall adapter and switching an isolation relay. This is commonly used if you have an aquastat with T T connections, as they usually do not provide enough current to drive a smart thermostat, however it could also be used to run cooling on a system when you are using an external transformer, if you had enough relays, because you would be able to use the R wire that normally runs between the furnace and the thermostat as the C wire for all of the relays. The limit then becomes the VA rating on your external transformer.
Heating with 2 wire run and non-24V equipment:

In addition, if you have 3 wires in your run, and wanted to not have a wall adapter running to your thermostat with an aquastat, you could put the external transformer in the same room as the aquastat like this:

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Thanks for the great diagrams. In the one showing the external transformer, shouldn’t the “C” connection between the transformers be made?

If not, I don’t see how the Y,W, & G devices have a complete path as they are now powered by the external transformer, not the HVAC transformer.

That’s the point. When you use an external transformer, only the W relay works correctly. All the other relays in the wyze thermostat appear to use the RC source of power, so they do not operate correctly when you are using an external transformer.

I have been searching the internet for a great solution and you supplied it. TY.
My question concerns diagram where you use an ext 24v transformer for your C and RC connectors on t-stat.

Just wondering what you mean by that. The AC will work properly as in turn-off and turn-on, correct? I am simply trying to get the t-stat to work at that level. My current situation is that the HVAC will supply AC but not heat. I am thinking that because there is no “c” wire the thermostat is erroring out. If I put a standard t-stat in place of smart t-stat, then the heat works.
So, there is a jumper for the RC&RH that I can remove. In this way, I can recreate your diagram. Make sense?

It’s best to start your own thread with pictures of both ends of your setup and I can troubleshoot it there.

:+1: Thanks!

Thank you for you excellent description of powering the Wyze thermostat with an external transformer in the room with the thermostat, and then connecting the thermostat W and C terminals via two existing in-wall thermostat wires to the primary of a normally open isolation relay installed at the boiler.

In your diagram, the secondary of the isolation relay is connected to the TT terminals of an aquastat. Can the relay secondary instead be connected to the R and W terminals of one zone of a Taco 6-zone relay?

You show the external transformer connected to the Rc and C terminals of the Wyze Thermostat. For a heat-only system, should the external transformer be connected to the Rh terminal instead of Rc?

I would like to replace a Honeywell two-wire mercury thermostat in one zone of a hot water baseboard heat only system with a Wyze wifi thermostat. There are two existing wires in the wall connecting the old thermostat to the R and W terminals of one zone of a Taco 6-zone relay. It is not feasible to snake a third wire (C wire) inside the wall from the thermostat room to the boiler room.

Thank you.

Anything that relies on a switched contact and provides its own current can be switched with a relay, so yes, it can be joined to a zone control via the R and W wires.

The wyze gets power via its Rc and C terminals, that’s why the external power supply is connected there. It will automatically send power elsewhere as needed.

Understood. That’s perfect. Thank you.

Good morning. I’ve been doing more reading on the Wyze Thermostat, and I found a February 6, 2021 posting from Wyze Support stating that the Wyze Thermostat is compatible with dual transformers.

“Some systems with heat and cool are configured with a dual transformer design. That means you have Rc and Rh wires being used; Rc = cooling side, Rh = heating side. As long as you have a C terminal on the same board as Rc, your system is compatible with Wyze Thermostat.”

In Case 1, the C-wire is coming from the Rc board, and Wyze Support states that nothing needs to be done at the control board. Does this mean when a heating relay transformer is connected to Rh and W, and a separate power transformer is connected to Rc and C, then the Wyze Thermostat isolates the two transformers?

See, https://support.wyze.com/hc/en-us/articles/360054655811-Are-Dual-Transformers-compatible-

In your January and February 2021 posts, you were concerned that connecting the C from the (external) transformer to the wyze and then splicing the other wire into Rc, creates a hazardous situation, and so a normally open isolation relay is required on the furnace end.

Since the Wyze Thermostat appears to be compatible with dual transformers, is a hazardous situation created in a heating-only system that does not use the cooling functions of the Wyze Thermostat when the R and W terminals of my Taco 6-zone heating relay with an internal transformer are connected to the Rh and W terminals of the Wyze Thermostat, and a separate external 24 VAC power transformer is connected to Rc and C terminals of the Wyze Transformer? In this configuration, is an isolation relay required to avoid a hazardous situation?

Will the Wyze Thermostat fully and correctly operate in this configuration?

Thank you for your help and patience with my questions.

When the wyze is configured to use a separate heating transformer, there is no issue, as the wyze internally isolates (mostly) the Rh from the Rc terminals.

The issue with using 2 different transformers on the Rc side is that those 2 transformers might not have the same reference to C. You could eliminate this problem by grounding both of the transformer outputs on one side. Several people have asked to be able to use Rh as a power source when they have a Rc transformer, and this could be unsafe unless both the Rc and Rh transformers have a common C connection.

As long as your zone control doesn’t mind a small bit of external leakage current from the Rc transformer, it should be fine to use without a relay.
Even when the wyze is configured to use separate transformers (by telling it that the “old thermostat” has both Rc and Rh wires), there is still a small amount of leakage current that flows between Rc and Rh, which can cause some devices to falsely trigger, or could potentially cause damage to millivolt systems.