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Author Topic: Running a C Wire from Thermostat to Furnace  (Read 834 times)

Offline 77W

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Running a C Wire from Thermostat to Furnace
« on: October 31, 2016, 01:54:30 PM »
Hey everyone, I have a question about running a common wire from my thermostat to my furnace.  I'll start with some background to my issue and try to keep it short.  My local hydro company in Manitoba, Canada has a rebate on currently that allows a homeowner to purchase a smart  WIFI thermostat and receive a $100 credit.

I went out an purchased a Honeywell Lyric T5, a recently released thermostat that I think will work well with my shift work and odd hours.  The T5 requires a "C" wire and my current thermostat doesn't have such a wire.  My house is 12 years and and has the standard 4 wires from the furnace to the thermostat.  After much research, I've concluded that I would like to add the wire and not use the G wire to C wire workaround that can cause electrical gremlins to pop up.

I've pulled my current white/rodgers thermostat apart and it has the standard wires to the R, W1, Y1, and G terminals.  Those same wires then go into the corresponding terminals in my York natural gas furnace.  There is also another 18/2 gauge wire that comes into the furnace control board, which I believe is for the air conditioner.  Those two wires go to the Y1 and C terminals.  I haven't been able to trace where this wire bundle goes, but i believe it goes to the air conditioning unit outside.

So my question(s):  Is it appropriate for my to buy 18 gauge wire, and run it from my thermostat along the same path as the current 18/4 gauge wire all the way to the furnace and hook it up to the C terminal on my furnace?  I have a suspended ceiling in my basement and the wall is hollow, so I can see exactly where the current wire runs.  Is it acceptable to have two wires on the single "C" contact on my furnace control board?

I also have an HRV with a separate relative humidity sensor/control next to my thermostat, that I don't believe has any wiring going into the furnace.

I would appreciate any help or wisdom!


Offline Porcupinepuffer

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Re: Running a C Wire from Thermostat to Furnace
« Reply #1 on: October 31, 2016, 02:15:22 PM »
Yes, you can do what you describe to get a common up there. You're correct in assuming the other 18/2 is for your air conditioner. Just make sure you do a good job with good connections, because once a common goes up there, any errors in wiring can start popping fuses with dead shorts. It sounds like you have enough knowledge to get it right.

You're fine to have more than 1 wire on the common, just make sure they're equal gauge. Sometimes a smaller sized wire won't pinch down tightly since the thicker one takes the pressure. Give them each a gentle tug to make sure they're attached well.

Offline Admin

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Re: Running a C Wire from Thermostat to Furnace
« Reply #2 on: October 31, 2016, 06:17:51 PM »
The HRV could be interlocked with the furnace so that the furnace fan starts when the HRV starts.  It sounds like you only have the 4 thermostat wires and 2 AC wires connected to the furnace control board though.

How close is the HRV control to the thermostat?  There may be an unused wire at the HRV control that you could fish over to the thermostat C terminal.  Then extend the unused wire at the HRV to the furnace C terminal.

Performing the pull test on all the low voltage wires connected to the furnace is a good tip!

Offline RYOUNVS

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Re: Running a C Wire from Thermostat to Furnace
« Reply #3 on: November 18, 2016, 01:03:31 PM »
If it was me, and I could see the wire in a hollow cavity, I would just pull a new 5 wire down and be done with it. Much neater and more professional IMO.

Offline ainsworthCA

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Re: Running a C Wire from Thermostat to Furnace
« Reply #4 on: December 23, 2016, 05:15:32 AM »
on a call for heating most boards run the fan cycle.
but on a call for cooling G is energized through the stat "most of the time".

by removing G your cooling cycle would be running with no fan, and you no longer have the Fan On setting.


Offline Sergroum

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Re: Running a C Wire from Thermostat to Furnace
« Reply #5 on: December 24, 2016, 01:42:42 AM »
Doesnt the Lyric tstat come with a splicer that allows you to use one of the other wires for common? I never installed a Lyric, but I've serviced some that have it and they always had that thing. It looked the same and seemed pretty standard, I assumed it comes with the t-stat itself. I know Ecobee always comes with one.

Offline Admin

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Re: Running a C Wire from Thermostat to Furnace
« Reply #6 on: December 24, 2016, 10:35:39 AM »
You can use a faststat,

http://www.fast-stat.net