I have a Air handling unit that uses a 4 wire thermostat, there is a 5th wire available that is not connected at both ends. If I connect it to the common on the 24v transformer can I then connect the other end at the thermostat to the C terminal and not use the C adapter? There is no control board on this AHU just wire nuts.
Yes you can.
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Yes, I did the same thing to power my last thermostat which was a plain dumb programable thermostat so I didn’t need batteries and now it works for the Wyze stat. It’s a much better alternative than using the C adapter.