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I'm trying to install a new thermostat that needs a C wire hookup. There are currently 4 wires running from the thermostat to the furnace, none of them are a c wire. When I look at my furnace itself it has a spot to put a c wire but it's currently being used by my whole-home humidifier.

If I run a separate 18-1 wire from my thermostat to the furnace can I just wire both the humidifier and thermostat to the one c-wire hookup?

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up vote 5 down vote accepted

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.

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This is exactly what we found. The C-Wire would only turn on with the furnace was running. We bought a 24vac transformer and hooked up that to the c-wire slot and an r-wire slot and it worked great! –  Paul Lemke Nov 6 '11 at 20:02
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You can have multiple devices utilizing the C terminal at your furnace.

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