Connecting a "C" wire at the furnace for smart thermostat
April 12, 2016 9:02 AM   Subscribe

I'm trying to hook up my smart thermostat, but it needs a wire that isn't currently connected at the furnace. Am I doing the right thing?

It's a basic Honeywell Wifi programmable thermostat that requires a C wire to operate. The place I recently moved into has a 4-wire thermostat installed, but there are also two unused wires at the thermostat (one brown, one blue). At the furnace, the unused wires are not connected to anything--shown here at the bottom. There's already a white wire at the C terminal going to the AC unit. My research indicates that (1) I could probably connect both wires directly to the C terminal but (2) it would be better to connect them together and use a pigtail to connect to the C terminal. Is there a reason I shouldn't proceed with (2)? Is there anything I should be wary of?
posted by almostmanda to Home & Garden (5 answers total) 1 user marked this as a favorite
To clarify, by "both wires" I mean one of the unused ones and the white one currently connected to the C terminal.
posted by almostmanda at 9:04 AM on April 12, 2016

Blue is the color code for the C wire. Connecting both the white wire and the blue wire via a pigtail to the furnace's C terminal, then connecting the blue wire to the Honeywell's C terminal should be a perfectly acceptable approach.
posted by I EAT TAPAS at 9:27 AM on April 12, 2016

Yes, what you suggest should be fine.
posted by jgreco at 11:37 AM on April 12, 2016

For what it's worth when we hooked up our thermostat we ended up calling the help line at Honeywell and they have obviously-well-trained people who told us exactly how to deal with our weird-ass wiring. So that's an option if you're not 100 percent sure.
posted by Elementary Penguin at 12:30 PM on April 12, 2016

I finally got a day off to tackle this. It worked on the first try! I connected the white wire to the blue ("c") wire and had a third wire going from those to the C terminal on the furnace board. I installed my thermostat like the previous one, but with the blue wire in the "c" terminal.

Thanks for the help everyone. Electricity is terrifying!
posted by almostmanda at 7:08 AM on May 20, 2016

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