Wiring – share “C” wire on two thermostats

I also have a thermostat that requires a C wire hookup for wifi access. The spec calls for a 24 volt DC and I presently have it wired to share a 24 volt termination on my AC air handler. What I discovered was that this termination in the air handler does not provide a constant output of 24 volts. Instead, it cycles as called upon by the air handler and the thermostat would run until the battery failed which took about a month. Plus I could not get the wifi to properly pair with the thermostat due to the cycling of voltage on and off to my C wire termination at the thermostat.

This afternoon I have an electrician coming to give me a dedicated wire from a 24 volt transformer for my C terminal. I expect this to solve my problem.

So based on my experience, be cautious of sharing unless you know the source will provide constant power.

The "C" wire is typically a common, or the other side of the 24 VAC transformer. The wireless Tstat would need the power source to operate. If there are unused wires in the wiring harness at the thermostat, then using one of these wires for the common is acceptable if that wire is clearly marked (as common). This wire would connect to the other side of the transformer that supplies the 24 VAC (follow the red wire back to the transformer and see how the other side/wire is connected. does it go to ground? does it leave the transformer to somewhere else?).