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Old 08-27-2020, 07:12 AM   #1
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C wire for Nest thermostat on a 4 wire system


my current thermostat is powered by 2 AA batteries. it's a 4 wire system and there is no c wire to the thermostat. I was planning to use the fast-stat kit or the ventstar add a wire kit. however the furnace control panel has 2 or 3 wires attached to C already, but those are not wires from the thermostat bundle? maybe it's the AC condensor? can i still use the wire kit to add a c wire to power my nest?

would there be too many wires on the C terminal? if so, do i have other options?

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Old 08-27-2020, 08:33 AM   #2
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Re: C wire for Nest thermostat on a 4 wire system


Those other wires are devices within the furnace.
The red wire on C is likely for the condenser.
That furnace is not to code. Your not permitted to run flexible gas line through the cabinet. It’s a safety hazard.
Do not install a nest. They are junk, lots of problems and issues. Add a wire devices will amplify the issues.
I would start saving for a replacement furnace rather then putting a fancy thermostat on an old standing pilot unit. Has that unit been checked for a cracked heat exchanger?
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Old 08-27-2020, 08:47 AM   #3
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Re: C wire for Nest thermostat on a 4 wire system


Hi, how difficult is it to pull a new cable to the Tstat from the furnace? stay away from these add on devices.
Geo
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Old 08-27-2020, 08:51 AM   #4
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Re: C wire for Nest thermostat on a 4 wire system


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Originally Posted by roughneck View Post
Those other wires are devices within the furnace.
The red wire on C is likely for the condenser.
That furnace is not to code. Your not permitted to run flexible gas line through the cabinet. It’s a safety hazard.
Do not install a nest. They are junk, lots of problems and issues. Add a wire devices will amplify the issues.
I would start saving for a replacement furnace rather then putting a fancy thermostat on an old standing pilot unit. Has that unit been checked for a cracked heat exchanger?
You’re right this is a pretty old unit, the condo was built in 1972 I believe, I am not sure when this furnace was put in.

So far I just have the home inspection guy and my contractor look at it, and both said it looks fine. But changing the unit is a plan for the future. Just want to see if I can get the smart thermostat to work in the mean time haha
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Old 08-27-2020, 08:52 AM   #5
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Re: C wire for Nest thermostat on a 4 wire system


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Originally Posted by Geochurchi View Post
Hi, how difficult is it to pull a new cable to the Tstat from the furnace? stay away from these add on devices.
Geo
Too difficult for a DIYer haha. Are these add On kits dangerous?
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Old 08-27-2020, 08:57 AM   #6
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Re: C wire for Nest thermostat on a 4 wire system


Hi, don’t know about dangerous, I just try avoiding things like that, is the Stat on the first floor? if so not that difficult to fish a new cable, especially if you can see where it goes up through the floor.
Geo
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Old 08-27-2020, 09:12 AM   #7
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Re: C wire for Nest thermostat on a 4 wire system


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Originally Posted by Geochurchi View Post
Hi, donít know about dangerous, I just try avoiding things like that, is the Stat on the first floor? if so not that difficult to fish a new cable, especially if you can see where it goes up through the floor.
Geo
I see that makes sense. Yeah the stat is on the first floor and the wires doesnít just go straight up
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Old 08-27-2020, 09:52 AM   #8
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Re: C wire for Nest thermostat on a 4 wire system


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So far I just have the home inspection guy and my contractor look at it, and both said it looks fine. But changing the unit is a plan for the future. Just want to see if I can get the smart thermostat to work in the mean time haha
I would not trust a home inspector to check a furnace.

If you want to keep it, the heat exchanger needs to be thoroughly inspected - it also needs to be cleaned, and tested*.

*Measuring Gas pressure, exhaust carbon monoxide, temperature rise at a minimum for testing -> not just seeing if the flames look okay.
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I am not in the business of any trade I give advice on. I have non-professional hvac experience + good knowledge of theory. Attempt repairs at your own risk. Never jump out safeties - especially pressure switches - on a furnace for testing with fuel supply on; use a meter. Do not troubleshoot with live line voltage present unless there's no alternative.
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Old 08-27-2020, 09:54 AM   #9
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Re: C wire for Nest thermostat on a 4 wire system


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Originally Posted by ed_423 View Post
You’re right this is a pretty old unit, the condo was built in 1972 I believe, I am not sure when this furnace was put in.

So far I just have the home inspection guy and my contractor look at it, and both said it looks fine. But changing the unit is a plan for the future. Just want to see if I can get the smart thermostat to work in the mean time haha
There are other thermostats that are superior with far less problems.
The Nest is not “smart”. And is a horrible choice for a thermostat. I honestly wouldn’t have one even if you gave it to me for free.
The furnace needs to be checked with a combustion analyzer. Visual is not good enough.
Most standing pilot furnaces are long dead.
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