Help with Furnace/Thermostat Wiring


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Old 08-16-22, 10:40 PM
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Help with Furnace/Thermostat Wiring

Hello, I'm looking to change my HoneyWell MagicStat CT3200 thermostat and get a smart thermostat (Google Nest if possible). I went to check out my wiring to make sure it would be compatible, but something is confusing me.

My thermostat is a HoneyWell MagicStat and has 5 terminals (R, Rc, W, Y, G):
Red wire in R
Black wire in Rc
White wire in W
Orange wire in Y
Green wire in G.




From this set-up I see that I do not have a C wire. When I went to check my furnace to see if there is a C wire connection, I found something odd.

My furnace has 5 terminals:
Red wire in R
Cream wire in C
White wire in W
Orange and Black wire in Y
Green wire in G







I figured out that the cream wire in C and the black wire in Y are joined together and most likely go to my outside A/C unit.

What I can't figure out is what that black wire connected in the thermostat to the Rc terminal is from.

If anyone can help explain that part and also whether or not I can install a c-wire into a new smart thermostat I would appreciate it!

 
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Old 08-17-22, 09:15 AM
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I decided to go back to the furnace and make sense of the wiring connections and I found out what is connected to what.

So, I was right. The cream and black wire are going to my A/C unit outside. That weird mess of wires and electrical tape I figured out was just an extension because the wire wasn't long enough. The A/C wires are blue/white and the person connected the black wire to the blue and the white to white, but used electrical tape instead of marrets so I took the tape off and put marrets on. I hope that's fine.



Now, the answer to my last question of the black wire on the Rc terminal. Once again, the thermostat wire with the 5 wires (B,R,O,W,G) wasn't long enough to reach the furnace so they spliced it to another wire. Here are the connections:

Thermostat -> splice -> furnace
Green wire - G terminal -> green wire on extension -> G terminal
White wire - W terminal -> white wire on extension -> W terminal
Orange wire - Y terminal -> yellow wire on extension -> Y terminal
Red wire - R terminal -> red wire on extension -> R terminal
Black wire - Rc terminal -> red wire on extension -> R terminal

So, as you can see, the black wire on Rc is splice together with the red wire. So the red wire on the extension is connected to the R terminal on the furnace, and then that one red wire is splice together with both the black and red wire that goes to the thermostat. Red wire is then connected to R and black to Rc.



The extension wire has a black wire as well but it is not connected to anything on either side.




So, I found the connections for each wire, so hopefully someone can make sense of this and let me know what I'd need to do for a new smart thermostat.

(Pictures to help clarify).
 
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Old 08-17-22, 09:39 AM
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Welcome to the forums.

You have red and black at thermostat getting connected in a splice and going to furnace R.
That was done by someone that wasn't sure what they were doing.
R and Rc should have been connected with a jumper at the thermostat requiring a single wire.

You can convert the black wire to C. You don't need two R wires to the stat.

Remember to turn the furnace off while changing the wiring.
 
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Old 08-17-22, 09:47 AM
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Thanks for replying! I wonder if the person who installed the thermostat removed the jumper because I found the manual online for my thermostat and it says to remove the jumper from the thermostat.

So what is the ideal connections to make now? Do I keep the red wires connected together and have each end connected to the R terminals on the furnace and stat? Then, do I take the black wire off the Rc, remove it from the red wires at the splice, splice it to the unused black wire and connect the other end of the unused black wire to the C terminal at the furnace? Then, I guess use the black wire at the thermostat to connect to the C terminal on the new smart stat?

So, with my set-up, I don't need a wire connecting to Rc? On the new thermostat, what would the connections be? R-Rc or Rh, O-Y, W-W, G-G, B-C?

Sorry for the questions, I hope it's clear.
 
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Old 08-17-22, 12:25 PM
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Thermostats have two R connections.
R = 24v heat
Rc = 24v cooling.
The jumper is removed if you have two different systems like a split A/C and a boiler.
With two systems you would have two separate 24v power supplies.
You have a furnace that operates for heat and cool. One common R.
So that means an R to Rc jumper is required

No need to have double wires going to R.
Make the connections simpler and easier to diagnose in the future.

Many smart stats don't require an R/Rc jumper. This includes nests and ecobees.
They will specify no jumper required.
Many Honeywells have a slide switch to select R only for one wire.

Yes.... you want to splice the black wire all the way thru and use it on C.
Red to R and Rc. A new stat would come with a jumper or slide switch.
Orange to Y.
White to W.
Green to G.
Black to C.




 
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Old 08-17-22, 12:32 PM
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So, in the manual for my current thermostat, it says to remove the jumper for 5 wire systems. Unfortunately, I wasn't around for the original thermostat or this thermostat's installation so I'm not sure if it's a 5 wire system. From what I've gathered here, though, is that it's a 4-wire system and the jumper should not have been removed? Which means that the black wire should have just been loose behind the wall because this thermostat doesn't have a C terminal? And the jumper should still be attached to R and Rc?

I'm going to leave the current set up alone because it's working, but when I change to the Nest, would this be the correct set-up?


G terminal at furnace -> green wire on extension -> green to green on stat wire -> green wire to G terminal on Nest
W terminal at furnace -> white wire on extension -> white to white on stat wire -> white wire to W (W1) terminal on Nest
Y terminal at furnace -> orange wire on extension -> orange to orange on stat wire -> orange wire to Y (Y1) terminal on Nest
R terminal at furnace -> red wire on extension -> red to red on stat wire -> red wire to Rh terminal on Nest
C terminal at furnace -> black wire on extension -> black to black on stat wire -> black wire to C terminal on Nest

Then, for the A/C, I wouldn't have to change anything, right?
Y terminal at furnace -> black wire on extension (separate from above) -> black to blue on A/C wire -> blue to A/C
C terminal at furnace -> cream wire on extension -> cream to white on A/C wire -> white to A/C

So that would mean, at the furnace, there would be 2 wires on the Y terminal (Orange to stat/Black to A/C) and 2 wires on the C terminal (cream to A/C, Black to stat). Does that sound correct?

Sorry for all the questions again, just want be to very sure. I appreciate the help!
 
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Old 08-17-22, 12:36 PM
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No problem.

I snuck in and changed the Nest designations.
Depending on the model you could have Y or Y1, W or W1 and use Rh.
 
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Old 08-17-22, 12:43 PM
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Ah, ok so the red wire would go to the Rh terminal, ok.

So am I correct in saying that my system is a 4 wire one and the jumper should not have been removed from R-Rc? And the black wire should be on its own?
 
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Old 08-17-22, 12:50 PM
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Comparing a 4 wire system vs a 5 wire system is not defining anything.

A typical 4 wire system is R, W, G, Y.
A typical 5 wire system is R, W, G, Y and C.
With both.... the R and Rc remain combined.

A true split 5 wire system would be W and R or Rh for heat
Y, G and Rc for cooling.
Two wires for heat and three wires for cool.

 
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Old 08-17-22, 12:54 PM
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So, in a 5 wire system, what would the Rc wire be connected to at the furnace?

Since my Rc terminal is connected to the R at the furnace, does that mean the Rc has no use in my set-up?
 
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Old 08-17-22, 01:02 PM
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R = 24v. A thermostat switches 24v to a specific device.
If you only had the R connected....the thermostat would switch R (24v) to W only. No A/C operations.

Rc = also 24v and switches 24v to different specific devices.
Rc could switch 24v to Y or 24v to G. No heat operations.

That's why they need to be combined. You need to common R from the furnace.

R must always be combined unless you have two independent system like a split A/C and a boiler.
 
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Old 08-17-22, 01:08 PM
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So wires on R, W, G, Y means A/C won't work.

Wires on R, W, G, Y, Rc means A/C will work.

In my set-up, they connected the Rc wire to the R terminal in the furnace, but they could have just put a jumper between R-Rc instead and not combined the wires together and saved the black wire as a c-wire?

Is that right?
 
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Old 08-17-22, 03:15 PM
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Yes... that is correct.
 
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Old 08-17-22, 03:41 PM
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Amazing, thank you so much for walking me through this! I feel confident now whenever we change thermostats!
 
 

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