What smart thermostats work without a c-wire?

Home Security Systems
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Does anyone know a smart thermostat that can work without a c-wire for a heating/furnace-only system?

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At my previous house, I experienced something similar. I temporarily fixed the issue by moving the G wire in my system to function as the common at the HVAC unit and on my thermostat. My fan continued to function as normal when the system ran unfortunately it could no longer be operated manually. I eventually got help and installed a new wire to function as a dedicated C.

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Comments suggest using the C wire adapter that comes with the Ecobee, but as an HVAC technician, I’ve had many problems with those. I recommend using your G or fan wire as your C wire, as another comment suggested, or running a new wire with 6 or more conductors.

I also don’t recommend using Nest thermostats without a C wire. They claim it can work, but it often leads to issues like getting stuck in heating or cooling mode or failing to turn the system on properly.