Adding "C" wire to 2-wire thermostat

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  #1  
Old 02-04-18, 06:39 PM
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Adding "C" wire to 2-wire thermostat

My thermostat currently have only 2 wires - R and W (Heat only). I am looking to get a wireless thermostat and will need to add a C wire. I have very limited electrical knowledge and was looking at various workarounds available via wire extenders to provide continues power to the thermostat. What is the right wire extender for my current 2 wire thermostat? Also, is installing the wire extender the easiest way or is there another option available that i should consider?

Also any recommendations for a good wireless thermostat?
 
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  #2  
Old 02-04-18, 07:15 PM
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I prefer to run new wiring when I install wifi thermostats. You can use an item called the Fast Stat Common maker to carry a C connection to the stat.

What is the heating system you have ?
 
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Old 02-04-18, 07:42 PM
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Thanks PJmax. My system is single zone gas fired water boiler.
 
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Old 02-04-18, 07:50 PM
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You can use fast stat common maker suggested by PJmax. I never knew such a device existed.
It basically is a transmitter and receiver that runs on 2 wire, while supply power on the same wire.

Other than that your only option is running new wire with at least 3 wires in them.
R is 24V power
W is heat
C is 24V common (opposite pole of the power)
 
  #5  
Old 02-05-18, 07:16 AM
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I also like running a new cable ,can you follow the old wire from the boiler to where it goes up to the current location?
Geo
 
  #6  
Old 02-05-18, 12:00 PM
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Yes, I can see where the wire pipe is running. The boiler is in the basement whereas the thermostat is right above on the first floor (wire going up to the thermostat inside a pipe). So i think access is not a problem. I just dont know how to run new wiring. Is it as simple as pulling the wires through the existing line? I am pretty careful with electrical stuff so if anyone can layout the steps to run new wiring i think i should be able to handle it. I am at work right now but I can post pictures of the small electrical box that connects the thermostat and the boiler unit once i get home.
 
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Old 02-05-18, 05:13 PM
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wire going up to the thermostat inside a pipe
Can you post a picture of this? It could be in a conduit, but that is very rare in residential house.

I just dont know how to run new wiring. Is it as simple as pulling the wires through the existing line?
Unless it was installed after the drywall or put on, probably not. The cable will be stapled in multiple places and you won't be able to pull the wire.
 
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Old 02-05-18, 05:18 PM
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Yes... post a few pictures for us as low voltage thermostat wiring is not usually in a conduit. How-to-insert-pictures
 
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Old 02-05-18, 07:24 PM
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Please see 3 pictures for your reference. The conduit shown in pic 1 (the one that is going to the thermostat) runs abt 5ft in the basement (touching the ceiling) and then upwards for another 6 feet or so hidden behind the drywall.
 
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Last edited by PJmax; 02-05-18 at 07:53 PM. Reason: added enlarged picture
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Old 02-05-18, 07:50 PM
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Do you have a voltmeter ?
You have a disaster in the making there.

I see either thermostat (24VAC) and 120VAC wiring intermixed in the same box OR you have 120VAC to that thermostat. It looks like they're switching the 120V line. You cannot make any changes until you know 100% what you are working with.
 
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Old 02-05-18, 08:23 PM
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I have a multimeter. I have used it to mainly check for DC current on car batteries etc. Where and what exactly am i checking here?
 
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Old 02-05-18, 08:42 PM
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Looking at the thermostat, I'm sure you don't have line voltage (120V or 240V) thermostat. It is low voltage thermostat.

The wiring is all wrong. Low voltage and line voltage wiring never should be in the same conduit and junction box. It is supposed to run separately.

If the conduit going to the thermostat has only thermostat wiring, you should disconnect that conduit from the junction box.
Try pulling slightly on the wiring at the thermostat. If there is conduit all the way or at least very close to the thermostat, you should be able to pull the cable. If you see the cable moving, twist and tape new cable on to the existing cable and pull new cable.

Run the new cable outside of the conduit between the junction box and the furnace.
 
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Old 02-06-18, 04:58 AM
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Is that Blue wire in the J box connected to anything of just wire nutted, har to tell in the pic. where does that white cable go?
Geo
 
  #14  
Old 02-06-18, 06:58 PM
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My apologies. I was mistaken what I originally thought was the main power line. Turned out that wire was not even connected to anything. I have posted a better picture this time.

There are 3 wires going into the boiler. Thermostat (white), blue/black and white/white (from the main power line). I have also checked, there is 26V coming in at the thermostat.
 
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Old 02-06-18, 07:27 PM
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Good job. Here's what I would do. Since the thermostat wire going upstairs will probably need to stay running in the conduit. When you run your new cable.... when it enters that junction box..... take it out of one of the knockouts so that it is no longer running in the conduit with power. You will need to follow the conduit to the boiler, pull out the red safety switch and remove the thermostat wire from that pipe. You can run your new line taped or tied to that conduit.

Follow that thermostat cable into the boiler and tell us where it connects or post a picture. We'll see if we can locate a C connection for you.
 
  #16  
Old 02-07-18, 11:08 AM
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Thanks PJmax, lambition and Geochurchi.

What kind of wire should i get? As lambition mentioned, I will try to pull on the wire at the thermostat, if it moves I will tape the new wire at the junction box and pull out the old wire at the thermostat which should bring up the new wire thru the conduit.

Stupid question - Is this new wire connected directly from thermostat to the boiler or does it need to be cut at the junction box (IDK, to make any connections) similar to the current set up? In other words, is a single piece of wire (and atleast 3 wire inside) connected from thermostate to the boiler without any connections?

PJmax , I follow your instructions in second half of your message but not clear on what you meant by -- " Since the thermostat wire going upstairs will probably need to stay running in the conduit. When you run your new cable.... when it enters that junction box..... take it out of one of the knockouts so that it is no longer running in the conduit with power"
 
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Old 02-07-18, 03:24 PM
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What kind of wire should i get?
18/3 thermostat wire. It will have 3 conductors.

Looking at your new picture, your existing wire looks like it has 2 more unused conductors. Does it?

If so, try pulling to the wire behind thermostat. There should be unused wire behind.
If this is the case, you can just run new wire between the junction box and the boiler.




Is this new wire connected directly from thermostat to the boiler or does it need to be cut at the junction box
It can and should go directly to the boiler. There is no need for the splice. If there has to be a splice, it doesn't have to be in the junction box because it is low voltage wiring.
As mentioned already, line voltage and low voltage wiring can not be in a same conduit or junction box. You should run new wire outside of the conduit and junction box. Disconnect conduit going to the thermostat from the junction box and run it outside.
You can cut the conduit shorter with a tubing cutter to make it easy to remove. Ream the sharp edges created by cutting and install bushing to so the cable is not damaged by sharp edge of the conduit.

Cover open knock out holes with knock out caps and put a junction box cover to cover this junction box.
 
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Old 02-07-18, 07:15 PM
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lambition, you are spot on. I found 3 additional unused wires where you pointed out. That makes my life so much easier. Thank man.

Guess what, the wire that's going into the boiler from the junction box has one unused wire (green). I had my boiler replaced a few years ago and probably the installer used 18/3 wire to run from the junction box to the boiler. I assume I should use this green wire to connect to one of the unused thermostat wires in the J box (where you pointed out) and use the other end upstairs to connect to "C" in the thermostat. I will also make sure the other end of the green wire is connected to "C" in the boiler and separate out the thermostat wires from line voltage wires.

One question, as there are 3 unused wires of the same color, how can I ensure i have narrowed down the same wire ends on both sides (ie upstairs and in the junction box)?

Hopefully, i should have my new Ecobee installed this weekend!
 
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Old 02-07-18, 07:43 PM
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I found 3 additional unused wires where you pointed out. That makes my life so much easier
Guess what, the wire that's going into the boiler from the junction box has one unused wire (green).
Great news.

I will also make sure the other end of the green wire is connected to "C" in the boiler
If you don't have C terminal in your boiler, R will be connected to one pole of the transformer and C will be just the other pole of the transformer. Follow the wire.

One question, as there are 3 unused wires of the same color, how can I ensure i have narrowed down the same wire ends on both sides (ie upstairs and in the junction box)?
If you have a multimeter, easiest way I can think of is following.

Pull all the wires on the thermostat side and make sure they are not shorted. Peel the insulation off unused wires.
Connect one of the unused wire to C.
With your multimeter set on AC voltage, measure voltage between R and one of the unused wires. When it reads 24V or something close to it, you found your C wire.
Mark it, and wire your thermostat.
 
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Old 02-07-18, 08:08 PM
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We need to see the boiler wiring. It may be an aquastat connection.

So you have five wires to the thermostat ?
If yes..... temporarily connect the two known wires together at both ends. You must keep these two together... tape would be ok. Connect any other wire to these two at one end. You can use alligator clips or anything to temporarily hold them together. Go to the other end and using your ohmmeter... check from the two wires to the spare wires. One will show continuity. Mark that spare wire as C at both end.

Keep your original two wires bundled together but not connected. Connect one of them to your C wire at one end. Go to the other end and using your ohmmeter.... check from C to either wire. When you find the correct wire...... mark it R at both ends. The remaining one from the original bundle will be marked W at both ends.
 
  #21  
Old 02-08-18, 11:05 AM
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Both great ideas!

I have attached a picture of transformer that i replaced in my boiler a couple of years ago. I will also post another picture tonight of the actual thing with all the existing wiring connected.
 
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Old 02-08-18, 04:11 PM
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With that controller, it will be as easy as just connecting to terminal C .

You will have 24V between R and C and these basically are each pole of the transformer.
G and Y terminal won't be used since G is fan and Y is air conditioner. You can utilize these for connecting a central A/C if you have it.
Often boiler and A/C are installed with separate thermostat, but it is not necessary to do so.
 
  #23  
Old 02-08-18, 07:50 PM
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Here is the actual picture. I am not sure why the W wire is not connected and Y and G are connected when i don't even have fan and air conditioner. My system is simple natural gas water boiler. The red and white wires from the thermostat (as marked in the pic) are connected to R and G. It looks like instead of W, G is being used. Are they both replaceable?

Anyways, I have marked where the unused green wire is. Is it as simple as connecting this green wire to where I marked C?
 
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Old 02-08-18, 08:01 PM
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That is called a fan center. It has a transformer and relay in it. The relay is for blower control and that is activated by powering the G (fan) terminal to the R (24v) terminal. Perfectly acceptable. Instead of powering a blower.... it's powering the burner.

Yes.... just connect the green wire to C.
 
  #25  
Old 02-09-18, 08:47 AM
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But at the thermostat, the G wire is not connected at all. The W wire is connected instead. Shouldn't both places be consistent? I have attached the thermostat picture below.

Just to be sure, at the thermostat I will connect the third wire (which is unused right now) to C (the same wire (green) that is connected to C in the boiler). Sorry for all these questions, just want to be extra careful when it comes to working with electricity.
 
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Old 02-09-18, 03:49 PM
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But at the thermostat, the G wire is not connected at all. The W wire is connected instead. Shouldn't both places be consistent? I have attached the thermostat picture below.
Looks like it was wired that way to utilize the relay.
Looking at he installation instruction, built in relay is wired between G and C. This means relay activates when G is energized (connected to R).
Other terminals W and Y are just being used as connection points.
The controller is meant for fan controlling. But, works perfectly fine for burner as well.

It would be nice to have a matching label, but doesn't have to if it is wired correctly.
Usually G (fan) terminal is also energized when AC or Heat is on. So, guess it will work even if it was wired to G terminal of the thermostat. Down side will be it will also be on when the AC is on or if that fan switch is turned on.

If really want to, you can move relay wire from G to W (requires soldering), probably there is no point in doing so.
 
  #27  
Old 02-09-18, 04:48 PM
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Currently at your stat..... your wiring is on R and W. That is 100% correct. On a call for heat the W terminal energizes the wire that goes to the G terminal on the fan center.

When you install your new stat...... you will be using R, W and C.
Pretty basic and straightforward.

There is no A/C involved so let's keep that out of the discussion.
This is a basic boiler only setup. No forced air involved.
 
  #28  
Old 02-11-18, 12:19 PM
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Installed and love it! Wanted to thank everyone and the DIY forum for the help on this.

Couple of observations:

1. I had to switch W and R. W in old thermostat had to be connected to R in the new and vice versa (i tried the old labeling first but thermostat wouldn't even turn on). i believe since i have the basic boiler set up, R and W were interchangeable in the old thermostat and it never caused any problems. I have attached a pic of new wiring (sorry the uploader keeps rotating the pic).

2. On the boiler side, I ended up soldering the C wire onto the C transformer port. I don't have much experience with crimping and did not have a crimper but if anyone see an issue with soldering, pls let me know.

The system has been operating flawlessly so far. Very happy with the new Ecobee.
 
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  #29  
Old 02-11-18, 12:53 PM
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1. I had to switch W and R. W in old thermostat had to be connected to R in the new and vice versa (i tried the old labeling first but thermostat wouldn't even turn on). i believe since i have the basic boiler set up, R and W were interchangeable in the old thermostat and it never caused any problems. I have attached a pic of new wiring (sorry the uploader keeps rotating the pic).
Your old thermostat was using battery to switch a relay which just closes connection between W and R. That is why it worked even if it was wired wrong.
It wouldn't have worked if you also had an A/C or a blower fan because the relay will be closing between Y and R or G and R.
The installer probably couldn't tell which is which since color has faded on the insulation.

2. On the boiler side, I ended up soldering the C wire onto the C transformer port. I don't have much experience with crimping and did not have a crimper but if anyone see an issue with soldering, pls let me know.
Soldering will work, but will make repair harder later. Also solder sometimes crack with a solid wire soldered to a terminal.
Soldering a stranded wire and connecting thermostat wire with a wire nut would work too.
That is what you would have had to do even with a terminal because thermostat wire doesn't make a strong connection with that crimp terminal.
 
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