furnacethermostatthermostat-c-wirewiring
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?
There should be no problem connecting the C wire from your thermostat to the C terminal in the furnace. Take a look at this diagram, which is a rough approximation of your system.
Notice the cable going to the condensing unit has a red wire connected to the Y terminal in the furnace, and a white wire attached to the C terminal. When the thermostat calls for COOL, it does so by energizing the Y terminal (connecting R to Y). This provides power to the coil of the contactor, which allows it to close the contacts and turn the condensing unit on. The other side of the contactor coil is connected to the C terminal, as to provide a complete circuit.
You'll notice in the first image that two relays in the furnace, the thermostat, and the contactor coil all connect to the same leg of the transformer. This is because the thermostat basically works like a switch, connecting the R wire to the C wire through control circuitry. Anything that needs 24VAC power, will be connected to both legs of the transformer.
You can only share the C wire, if you're also sharing the R wire. If the R wires come from different equipment, the C wires must also come from the different equipment.
Best Answer
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.