Are there any HVAC experts here that could answer a question?
The situation is this.
My thermostat does not have a powered c-wire at the thermostat. The cable coming into it has the wire available but it is not being used.
Down inside the Hvac system there is a terminal labeled "C" on the board. There are two wires hooked up to this. A white one that goes to a whole home humidifier, and a blue one that looks like it goes out to the outdoor unit.
So my question is simply this. Can I hook up the unused c-wire that goes to the thermostat to this terminal without screwing something else up?
Yes. Just make sure the breaker to the air-handler/furnace is turned off when you do this.
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Yep, all this ^^^. Removing power to HVAC stuff first is often missed - you may get away with not doing it, but it's a good way to blow a board fuse or fry your board, so definitely remove power. 
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Thanks for your quick replies. I realize I've changed the thermostat twice in this house and also a few times in other houses without even bothering with turning off any power. I always just assumed the lower voltage made it not an issue. I guess that makes me a lucky idiot.
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Yes. Accidentally shorting the low voltage R & C conductors can have unfortunate consequences.
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Update. I think I have successfully done it.
I first cut the power and tested it with a meter to be sure. (I found out my HVAC system has a convenient switch right next to it to shut it off). I installed the wire on both ends and turned everything back on and all seems well. The system is running and the thermostat is showing mains power in HE.
I've lived here for a couple years and always thought I had no way to get a c-wire to the thermostat, until I actually opened up the thing and looked today.
The house came with a very dumb thermostat, so I see why they didn't bother hooking the wire to the board, but I'm sure glad they did run a cable with more than 4 wires. If it would have required fishing new cables I just wouldn't have bothered.
I am thrilled to have had it been so easy.
Thanks all for your help.
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Nice! Yeah, 18-5 (5-wire cable) is arguably the most common t-stat wiring to find, so glad this worked.
if you ever end up replacing that run for some reason, consider running 18-8 (8-wire cable) for future-proofing multistage HVAC, zoning options, etc. The cost difference between 18-5 and 18-8 is pretty negligible, and future-proofing is always a win.
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That's what I ran. And I think I'm using all 8 conductors. 2-stage heat pump with two Aux heat stages - so R, C, G, O, Y1, Y2, W2, E.
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Closest I could find local was 18-10 sprinkler (irrigation) wire. It is solid wire, and has the same brown jacket, so why not?
Over the years I have used all but one of these. I have R, C, G, O, Y1, Y2, W2, E, plus dehumidify using low blower.
The neat thing is that the Honeywell Prestige IAQ only requires 2 wires between the furnace interface module (EIM module) and the thermostat. So I only needed a couple feet of this wire to go from the EIM to the furnace itself.
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