2-wire thermostat

My existing thermostat is an old Honeywell mercury switch 2-wire. It connects to a zone valve head (see photo). White and red wires are connected at each end. There is a blue wire but it’s not connected at either end. There are no open terminals on the zone valve head to connect a C wire to. Is a separate 24 volt transformer my only option?

Welcome to the Wyze community @mikemartin53!
The thermostat will not work without a third wire©. You have stated that you have a third wire that already goes from the furnace to the thermostat.
I don’t see a C terminal in the picture so you may need a transformer. I’m going to tag in a knowledgeable user, @speadie for a second opinion.

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TR is common ( C ) on these. Do you have a fan limit in line with your transformer? If so, and depending on how it is wired, your thermostat could turn off if the limit is hit and would only turn back on when the furnace cooled down.

If you notice that your thermostat turns off when the furnace runs for a long time, I can help you with fixing that problem, but for now, just connecting the blue wire to TR on the valve and to C on your wyze should get you up and running.
Connect the red thermostat wire to Rc and the white thermostat wire to W1.

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This was happening to me. Thermostat would lose power when the furnace hit a temp limit and cut the power.

Just yesterday I converted the thermostat to a wall plug 24v transformer. It’s perfect.

I made sure it didn’t backfeed to the furnace, ensuring the temp limit still cuts the furnace power.

Many posts on here taught me to move the RC wire to Rh. Transformer the goes to C and RC.

I didn’t bother going through the Wyze set up; worked perfect.

Thanks. I’m far from an expert, but considering that my heating system is hot water baseboard, I assume that there is no fan limit. Right?

There shouldn’t be, but the boiler itself will have a high limit, However it shouldn’t be heating high enough that it trips that. It depends on how the control for the boiler is wired up, and if the boiler shuts off power to the valve when it hits the off temp or the high limit.

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Drawing a schematic of this rat’s nest would take more patience than I have right now. I’m going to go with your original suggestion unless you see something in these pics to change your mind. Thanks again!

It looks like the limit is in line with the gas valve, not with the thermostat, so you should be fine. The only issue I’m seeing is that there appears to be a 1k resistor between the two thermostat terminals on your aquastat, so there might not be enough power there to run the wyze. Might as well try it, but you might need an external adapter if it doesn’t power up completely

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