Add C Wire with second transformer

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  #1  
Old 08-30-14, 12:44 AM
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Add C Wire with second transformer

Good Evening I want to put in a wireless thermostat but dont have a c wire. I was reading online that i could use a additional transformer since all 5 wires are in use.

I have a 2 stage cool, 1 stage heat system. With R connected to R/Rh (Jumper)

If i get a 24V Transformer and hook 1 wire to C where does the other one go.

Here is a picture of my setup:
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Last edited by PJmax; 08-30-14 at 09:21 AM. Reason: added pic
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  #2  
Old 08-30-14, 12:56 AM
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Welcome to the forums.

You would remove the metal jumper between R and Rc.
The red wire connects to R and the transformer connects to C and Rc
 
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Old 08-30-14, 01:01 AM
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So the Red (24 from Air Handler) would go to R, and the 24 from Transformer would goto Rc thus removing need for jumper?
 
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Old 08-30-14, 09:18 AM
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That is correct........plus C and Rc for your external transformer.
 
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Old 08-31-14, 09:09 AM
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Why not just run a 2 wire from the air handler transformer and use the C or common terminal from that transformer,no need to remove jumper.
Geo
 
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Old 08-31-14, 04:50 PM
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Would not be easy to run wire up thru wall to Air Handler. House was build in 1960 with almost 0 attic space. Also the Air handler is fairly new and sealed 100% by Metal Duct Tape. I would have to break all the seals to get inside to the wiring block. There is a 120v outlet right behind the t-stat on the opposite side of the wall.
 
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Old 08-31-14, 05:04 PM
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Using External transformer is a safe and effective method correct?
 
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Old 09-01-14, 06:02 PM
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Usually the access cover on the air handlers are not insulated ,that would certainly be the best way,how do you plan on installing this transformer?
Geo
 
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Old 09-01-14, 07:38 PM
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You can do it exactly as PJmax said. Place one of the wires from your external transformer (it doesn't matter which one) onto the "C" terminal on the thermostat. Since you have a jumper between "R" and "Rc" on your thermostat you remove the jumper and place the other wire from the external transformer on the "Rc" terminal on the thermostat. The second diagram I attached shows the wiring.

Plug in your external transformer
Verify your thermostat has power by removing the batteries to see if it powers up.

Hopefully you have an electrical outlet near the thermostat to power your external transformer. Of course you can fish the wires behind the wall to hide them. I usually use the hole in the wall where the thermostat cable comes out to connect to the thermostat. Then I tie off a heavy string with a steel washer or nut tied to the end and drop it straight down into the wall void between the studs. I figure out beforehand where I want the cable from the external transformer power supply to enter the wall and travel up to the hole behind the thermostat. I like to use an outlet in the wall behind the thermostat (if available) to power the external transformer. Then I use a small screwdriver to make a small hole in the wall to feed the transformer wires through. Then I stick a small magnet on a rod into the hole I made and grab the steel nut or washer and pull the string out a couple of feet. You can also use a vacuum cleaner to suck the string toward the hole and grab the string/washer or nut. Then I attach the two wires from the transformer onto the string hanging out of the hole at the bottom and tape securely and pull the wires up through the hole behind the thermostat. Then I wire up the thermostat. If necessary you can extend the wire length using thermostat wire such as 18-22 gauge or whatever size you currently have. Then plug it in and verify operation.

You could also wire it as shown in the first diagram I attached.
 
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Last edited by firedawgsatx; 09-01-14 at 08:08 PM.
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Old 09-01-14, 08:19 PM
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There is an outlet right behind it. Yea for some reason they sealed entire Air Handler.

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Old 09-02-14, 07:07 PM
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Hello everyone-

I've been searching for a while. I have the same problem and was thinking along the same line as the OP. My setup on the stat is very similar except mine is 1H/1C, so no Y2 wire. I would love to follow pjmax's and firedawgsatx's suggestion. Here're my questions though:

If I add the external transformer's hot wire to Rc in addition to the existing one (firedawgsatx's 2nd diagram), would the feed to Rc not becoming 48v?

On the other hand if I add it to R with the jumper removed, would the heat relay be able to activated considering W is no longer in the loop as it goes R and C with external transformer.

What's the impact of leaving the jumper on?

Sorry if the questions are non-sense. I'm not so savvy at this, have to be honest. But having been searching for a while, I can't seem to get the questions answered.

Thanks so much.

Btw, firedawgsatx, very nice tip with the wire fishing tips (with the magnet and vacuum). Will have to try it .
 
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Old 09-02-14, 07:23 PM
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On a normal set up the red wire connected to R on the thermostat comes directly from the transformer. If a person desires the C terminal on the thermostat to be powered the wire for it comes from the other side of the transformer. The R terminal on the thermostat provides 24V to power the other terminals. The jumper wire between R and RC is so that the R terminal can power the RC at all times. When the jumper wire is removed (such as when you install an external transformer) the RC will no longer have constant 24V to it. If you add an external transformer (because you don't have a wire available from the primary transformer) then the wires send 24V to the C terminal and 24V to the RC terminal. So, to answer your question, no you will not have 48V at the RC terminal. When you install the external transformer and wire it to the RC and C terminals you should measure 24VAC across them. The heat is activated at the W terminal. When heat is called for a relay inside the thermostat closes between the R and W. There is no need to leave the jumper on.
 
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Old 09-04-14, 11:39 AM
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I did the wiring today something is wrong. When i use the external transformer nothing runs. If i put fan into On mode it doesnt start. If i go back to using the red with the jumper fan runs fine.

Wiring

Red from Air Handler (24V) --- R
External Transformer: Rc & C.

There is no jumper between R and Rc.

I even tried External Transformer to Rc & C, with Jumper between Rc & R with red hooked up to nothing basically using entire External transformer as the main transformer still nothing.

Any idea what could be causing this?

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Last edited by bweaver1985; 09-04-14 at 12:04 PM.
  #14  
Old 09-04-14, 12:03 PM
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Did you test between the external transformer wires without it being connected to the transformer?
Also, verify you have 120V at the outlet the external transformer is plugged into.
 
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Old 09-04-14, 12:06 PM
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When i had it hooked up i tested voltage to R and C got 27 volts.
 
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Old 09-04-14, 12:09 PM
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I took the batteries out of thermostat and it also powers up when plugged in.
 
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Old 09-04-14, 12:19 PM
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Could it be related to an incomplete circuit?
 
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Old 09-04-14, 12:26 PM
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27V between R and C terminals is what you are looking for. If the thermostat powers up without batteries installed that would appear to mean the external transformer is connected properly.
 
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Old 09-04-14, 12:27 PM
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Yep. Its really weird. Dont know what to do. Any advice? Cause i have a $100 Thermostat coming tomorrow
 
  #20  
Old 09-04-14, 12:37 PM
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When you did the wiring did you shutdown power to the air handler? If not, I would check to see if you blew the automotive type fuse.

I would remove the external transformer wiring you did and restore to the original configuration and see if it works properly.
 
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Old 09-04-14, 12:44 PM
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Works fine if I revert back
 
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Old 09-04-14, 12:46 PM
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Did you test the voltage between the two wires from the external transformer without it connected to anything?
 
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Old 09-04-14, 12:51 PM
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Yep I got 27 volts testing transformer only
 
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Old 09-04-14, 12:55 PM
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Did you try wiring it the way I showed in the two photos I posted in post #9?
 
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Old 09-04-14, 01:00 PM
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Thats what I thought I did. Can you right that up in words. Will try in a little
 
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Old 09-04-14, 01:03 PM
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I have seen some videos saying leave jumper and put r and external together then other wire from external into c. But I was scared putting 2 hots together
 
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Old 09-04-14, 01:11 PM
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If you look at the two photos I posted I show where you leave the wiring exactly as you had it originally. Remove the jumper between Rc and R. Then place one of the white wires from the transformer to R and one to C terminals.
 
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Old 09-04-14, 01:11 PM
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When I get home I can try ext to r instead of rc. Only reason I did it to rc was manual said c must be on cooling transformer
 
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Old 09-04-14, 01:14 PM
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You ever hear that before? It was in honeywell manual. Do you think putting External into R and C will make any difference. I'm kinda worried now. I wont be home for another 8 hours to mess with it.

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Last edited by bweaver1985; 09-04-14 at 01:54 PM.
  #30  
Old 09-04-14, 02:19 PM
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The guide shows Rc is the power supply terminal to the thermostat. It says to remove the jumper for a two transformer system. So apparently Rc and R are not connected internally and requires a jumper for Rc to power R terminal. To me, (I could be wrong), the proper method to wire your external transformer is to leave red wire on Rc terminal. Remove jumper. Connect white wires from external transformer to C and R. This is the way I would do it for myself.
 
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Old 09-04-14, 02:24 PM
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Ok will do. If that doesnt work it looks like im going to have to find a way to pull a 18/6 or 18/7 wire.
 
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Old 09-04-14, 02:34 PM
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Pulling a new cable is overall the best solution depending on how far you have to pull it and the configuration of the walls between the thermostat and the air handler. If the existing thermostat cable is not stapled to the wall studs/structure (I rarely get that lucky). it will actually be a lot simpler. If it is not stapled you would disconnect the wires at the thermostat or air handler and then tape the new cable on to the old and pull on the old cable until the new cable reaches. Just make sure the old and new cables are taped extremely well as you don't want to have them separate mid-run.
 
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Old 09-04-14, 02:36 PM
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Distance between Air Handler and thermostat is like 5 feet horizontally. But the wires go thru the attic house was built in 1950. I might be exploring with that option.
 
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Old 09-04-14, 02:52 PM
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If i cannot pull the wire due to it being stapled. I have a hole in the wall behind thermostat where the external transformer wires were running. I could technically put a 18/2 wire in this hole and use just 1 of the 2 wires to connect to C and run the 18/2 up the wall in bathroom and into ceiling then down into air handler closet, i would then connect that one wire to c. This would work correct?
 
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Old 09-04-14, 03:02 PM
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That is correct. All you need do is get a single conductor between C at the thermostat and C connection at the air handler to make appropriate connection. Having an extra wire would be great in case something happens to another wire in the future.
 
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Old 09-04-14, 03:10 PM
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Yea. I just need to find a way to open air handler without voiding warranty. Since entire service panel is sealed with metal tape around seam.

You ever hear of a scenario where removing all the foil tape to access the service panel will cause warranty issues?
 
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Old 09-04-14, 03:17 PM
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That could be tough. Does the metal tape sealing it have specific markings on it or is it just generic foil tape? I always like to run thermostat cables with several extra conductors so a person will have the required wires if they decide to upgrade to two-stage heating and/or cooling systems.

The C wire should be going outside to the a/c condenser. You could splice into the 18/2 wire there where it comes out of the air handler.
 
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Old 09-04-14, 03:19 PM
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It appears to be Venture Tape. I know it says 181B-FX on it. wont be able to get more details until later. Yea i have 2 stage so thats where my extra pair went.
 
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Old 09-04-14, 03:23 PM
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The C wire should be going outside to the a/c condenser. You could splice into the 18/2 wire there where it comes out of the air handler.
 
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Old 09-04-14, 03:25 PM
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My first method as soon as I get home will be to put Red Main Transformer (Red Wire) on Rc, and external Transformer on R (heat) and C. Hopefully this makes some kind of difference. The only reason I could think this could be happening is incomplete circuit. Because with the external transformer on Rc & Jumped to R, and other wire on C basically replacing internal transformer with external transformer I still had nothing.
 
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