Thermostat Missing 3 wires

[Richard12]

Wired my first Wyze thermostat, but stuck at the wiring detection step. Yes, all the wires are fully home in the connector and match the mapping from old connections to new. I’ve measured the voltage between Rc and C, it’s about 29vac. However, no voltage from Rc to the other wires (Y1, O/B and G). Obviously the old thermostat works fine, so is the Wyze thermostat not compatible with open circuit furnace wiring (assuming that’s a thing)?

It’s a standard split HVAC A/C, gas furnace system, although one unusual factor is that this thermostat is connected to the furnace via a separate board that manages two thermostats and a pair of duct valves as there are two separate spaces that use the same HVAC system.

Don’t really see why that would impact the Wyze thermostat, but just thinking the intermediate control board may be electrically different from what the Wyze thermostat is expecting…

[Richard12]

Ok, so I was wrong in that it is not zero volts between Rc and those other wires, but actually ~15vac. I’ve checked at the intermediate board as well (a SmartZone 2 (2Z-2DF)) and they are indeed the same at 15vac. Also, notably the voltages at the furnace side are all 24vac+, so it does strongly imply that the Wyze thermostat isn’t liking the lower voltages of these wires, even though the intermediate board and indeed the old thermostat don’t care. Is there a way around this, Wyze Support?

[Richard12]

Used the thermostat to replace a different one (connected directly to the furnace this time) and it works great. I guess it would be possible to temporarily work-around the test for 24vac on these lines, but if the thermostat keeps checking for a ‘good’ voltage, it won’t be a solution. Software update to allow the use to accept a lower voltage on these lines, Wyze Support?

[speadie]

Likely your EIM is using serial data and not 24v relay logic. what brand of thermostat do you have, and can i see a picture of its connections, as well as the connections on the EIM.

[Richard12]

I doubt it’s using serial comms. The wiring is ‘standard’ as is the thermostat, some typical honeywell 7-day thing.

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[Richard12]

Had to post another message due to forum rules for attachments.

[speadie]

If the voltage is getting pulled down to 15v, that means you are severely overloading your 24v transformer or you have a bad wire/high resistance somewhere in the circuit. Healthy “24VAC” usually tests out at around 26-29VAC at unloaded. It shouldn’t drop below 24VAC when loaded.

Does your current thermostat use batteries? And what is the brown wire used for?