# New thermostat with no 'C' wire



## homeownrnewb (Aug 30, 2012)

I bought a new Filtrete Thermostat today and ran into an issue before installation. It has Wifi and I understand that it requires a common wire to power it. My current thermostat is using only W, Y, G, and R. 

Is this something that I could add my self? I'm pretty comfortable snaking wires through walls and I've already found the point where everything is connected at my furnace. How hard would it be to run a new wire up to it or run a new 5-conductor line?


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## hvactech126 (Nov 11, 2010)

If you have an extra wire in the cable then it should be easy. If you need to run new cable then we have no idea how hard it will be to run new without being there. http://www.venstar.com/Thermostats/ACC0410/


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## gregzoll (Dec 25, 2006)

Really depends on if they stapled the original wire to the studs, when they pulled it up the wall. Only way to know, is to mark the wires, tie something to the end, or attach the new wiring, and start pulling back to the furnace.

Should take you no more than 5 maybe 10 minutes tops to do the pulling, a total 20 minutes to finish the job.


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## brunetmj (Oct 16, 2012)

I am having the same problem. I am under the impression that professionals do not know a lot about this because wireless thermostats are a fairly new thing. I know my local furnace guy could offer only very vauge advice.
I would suggest you search u tube for C wire issue (or similar).
The three choices you have are to jumper the green wire at the furnace control board, hook up a new single wire to the C connector on the furnace logic board or buy a 24 volt transformer as an add on. Utube offers more detailed instructions for these procedures. Obviously it requires you find the furnace logic board after turning of power.


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## beenthere (Oct 11, 2008)

brunetmj said:


> I am having the same problem. I am under the impression that professionals do not know a lot about this because wireless thermostats are a fairly new thing. I know my local furnace guy could offer only very vauge advice.
> I would suggest you search u tube for C wire issue (or similar).
> The three choices you have are to jumper the green wire at the furnace control board, hook up a new single wire to the C connector on the furnace logic board or buy a 24 volt transformer as an add on. Utube offers more detailed instructions for these procedures. Obviously it requires you find the furnace logic board after turning of power.


Adding on another transformer would still require him to run another thermostat wire.


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## brunetmj (Oct 16, 2012)

Notice how I referenced the u tube videos to get greater detail.
I did this to shorten my prescription . Obviously , for example , I left out that before jumping a green wire the logic board still needs to be checked to ascertain what color wire is being used on what terminal. The concept of an add on transformer involves the use of a new wire but does not involve a wire from the furnace. Remember the whole idea is to give the thermostat constant 24 volt power to run the wireless. It dosent care where that 24 volts come from. Again the videos give far greater details.


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## beenthere (Oct 11, 2008)

brunetmj said:


> Notice how I referenced the u tube videos to get greater detail.
> I did this to shorten my prescription . Obviously , for example , I left out that before jumping a green wire the logic board still needs to be checked to ascertain what color wire is being used on what terminal. The concept of an add on transformer involves the use of a new wire but does not involve a wire from the furnace. Remember the whole idea is to give the thermostat constant 24 volt power to run the wireless. It dosent care where that 24 volts come from. Again the videos give far greater details.



No, it doesn't care. It just won't run either the A/C or the heat.

last i knew it only has an RH and an RC, no R terminal, unless I'm thinking of the wrong thermostat.

24 volts must come from either the heating or cooling equipment. 

He's better off just running a new wire from the furnace, or using an add a wire.


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## brunetmj (Oct 16, 2012)

Your probably right about the best way. I just included the independent 24 volt source because one of the videos shows a detailed way of hooking that up. It would not be my choice either . Since he has an existing green wire at the thermostat the jumper method might work well for him, assumming the green is the fan power. There is a good video on that method. Since I only have a two wire ( red/white) system I personally will run new wires and just add the common wire at the logic board. Again I am not advocating any particular method because much depends on the posters comfort level and experience .


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## homeownrnewb (Aug 30, 2012)

I'm fine with running new wires as I do not want anything showing under or around the thermostat. Wire are my biggest annoyance and I'll go through anything to make sure they are hidden behind a wall. I believe the common wire is just to run the wireless so I don't think it matters where is comes from, but I think I'm better off buying some new wire and running it downstairs. 

My furnace is not too, too old . Should it have a 24v source that I can use? I'm heading downstairs now to investigate and take some pictures as well.


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## homeownrnewb (Aug 30, 2012)

Alright. So things look easier then I thought. 5 conductor wire is actually run from my furnace and joins what looks like telephone (4 conductor) wire via wire nuts. Then it is run up to the t-stat. I have a big whole directly below the t-stat from a speaker that was installed when the previous owners had a security system. The wire looks to to be free for the most part the only issue might be fishing through the fire block, but I figure that was already notched out for this wire to be run. 

The first picture is my furnace's logic board. The two conductor wire in the front is run to my AC Unit, the 5 conductor wire is obviously going to the t-stat (notice the blue wire just relaxing there), and the two conductor wire in the back is cut out side of the furnace. There is evidence of a whole house humidifier that was once installed (ties in to water pipe and a patch in the duct above the furnace) so it's possible that once went to switch on the humidifier when the furnace was running. Right? 

In the second picture you can see where the 5 conductor wire meets the telephone wire again with the blue wire just hanging out. I'm thinking I can just connect the blue wire to the 'C' terminal and run new wire from this junction point to my t-stat. Am I thinking correctly?


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## ben's plumbing (Oct 6, 2011)

but beenthere it came from the internet IT HAS TO BE RIGHT......RIGHT.....yea right......ben sr


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## beenthere (Oct 11, 2008)

mentalcase said:


> Alright. So things look easier then I thought. 5 conductor wire is actually run from my furnace and joins what looks like telephone (4 conductor) wire via wire nuts. Then it is run up to the t-stat. I have a big whole directly below the t-stat from a speaker that was installed when the previous owners had a security system. The wire looks to to be free for the most part the only issue might be fishing through the fire block, but I figure that was already notched out for this wire to be run.
> 
> The first picture is my furnace's logic board. The two conductor wire in the front is run to my AC Unit, the 5 conductor wire is obviously going to the t-stat (notice the blue wire just relaxing there), and the two conductor wire in the back is cut out side of the furnace. There is evidence of a whole house humidifier that was once installed (ties in to water pipe and a patch in the duct above the furnace) so it's possible that once went to switch on the humidifier when the furnace was running. Right?
> 
> In the second picture you can see where the 5 conductor wire meets the telephone wire again with the blue wire just hanging out. I'm thinking I can just connect the blue wire to the 'C' terminal and run new wire from this junction point to my t-stat. Am I thinking correctly?


The C on that terminal board is the common you need to connect to.


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## bubsmaster25 (Aug 31, 2012)

You don't have to use common on your t stat you can still use that stat and run it off batteries you only need a common wire going to your condenser unit outside from the furnace board


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## beenthere (Oct 11, 2008)

bubsmaster25 said:


> You don't have to use common on your t stat you can still use that stat and run it off batteries you only need a common wire going to your condenser unit outside from the furnace board


Not if he wants to use he WIFI ability.


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## gregzoll (Dec 25, 2006)

bubsmaster25 said:


> You don't have to use common on your t stat you can still use that stat and run it off batteries you only need a common wire going to your condenser unit outside from the furnace board


Problem is, with the WiFi module installed in the 3m Filtrete, it will run the batteries down shortly within 24 hours.


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## gregzoll (Dec 25, 2006)

mentalcase, watch out with the RTCOA radio that comes with the 3m Filtrete thermostat. I have a 3m-50, and within 3 months, the radio module died. I had nothing but problems with the RTCOA radio, so after it died, I got the Our Home Spaces radio and never have had any problems like I did with the RTCOA.


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## hvactech126 (Nov 11, 2010)

Did you look at the venstar link I had posted? http://www.venstar.com/Thermostats/ACC0410/


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## gregzoll (Dec 25, 2006)

Why go to all of that work & expense, when they already have the wire there. Even if they did not have the extra wire, it would be no problem for them to pull up a new wire set to allow them to attach to C.

hvactech126, you need to learn how to make work easier, not harder.


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## hvactech126 (Nov 11, 2010)

Greg, WHAT? There are 4 wires present and the module and Diode allow for use of the 4 wires to operate as 5.


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## gregzoll (Dec 25, 2006)

Again, why buy a extra item, that is not needed. Read post #10, the OP can do the fix without paying more for something that is not needed.


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## hvactech126 (Nov 11, 2010)

Greg, that is IF he decides to run new wire and if he CAN run wire. While it looks like the wire is not stapled he does not know for sure.


> The wire looks to to be free for the most part the only issue might be fishing through the fire block, but I figure that was already notched out for this wire to be run.


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## gregzoll (Dec 25, 2006)

Even if it is stapled, there should be no problem pulling the new wire, if you would read that they have a direct route from the thermostat to the basement.


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## hvactech126 (Nov 11, 2010)

calm down GREG.... Don't make mountains out of mole hills....


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## gregzoll (Dec 25, 2006)

hvactech126, who is getting upset. Looks like you are the only one getting all riled up, after someone pointed out that your effort is too much work & money.


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## sublime2 (Mar 21, 2012)

Deep breath guys.
Deeeeep breath.


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## rich0 (Oct 20, 2012)

*Jumpering Rh/Rc to save wire?*

I have a similar problem. I have a separate heater and A/C, and only five wires running up to the thermostat. One is Rh and one is Rc.

To free up a line for common, I'm considering jumpering Rh and Rc in the basement, and then running up a single R line. Then I'd wire a common to the A/C and send that up the now-free line.

I haven't opened up the controller on the furnace, but the A/C is basically a bunch of separate components attached by wire nuts. So, I should just be able to wire the common into the nut shared by everything on the transformer pole that isn't connected to Rc. As far as I can tell on the A/C neither pole on the 24V side is grounded.

As far as I can tell jumpering Rh and Rc in this way shouldn't create any safety issues - I imagine that more than a few homes are configured this way by accident (jumper left in place on thermostat for a 5-wire setup). If the common poles were grounded on both units there might be an issue if they weren't in phase from what I can find online.

Can anybody think of something that might go wrong with such a setup? If so then I'd probably just break down and run another cable.


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## beenthere (Oct 11, 2008)

Separate heater and A/C? 

Do you mean a standard split system. or you have a heating unit that doesn't even run its blower when your using the A/C.

Posting pics often helps.


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## rich0 (Oct 20, 2012)

beenthere said:


> Separate heater and A/C?
> 
> Do you mean a standard split system. or you have a heating unit that doesn't even run its blower when your using the A/C.
> 
> Posting pics often helps.


Heater is a summer/winter boiler, and the A/C is an A/C. They're in separate rooms actually. So, they're as separate as they get. The house is fairly old - the wiring for the heat is quite ancient, but I doubt there are any problems with it.

I've been doing more reading - it seems like jumpering two separate transformers is probably unwise. In theory it sounds like it should work as long as at least one of the 24V systems is not grounded. If both units are grounded on the common side and the reds are jumpered, then the transformers will be in parallel and likely short.

It also looks like if I don't jumper them then I can't use the common on the A/C either, since the thermostat expects it to be from the heat (though if they actually were connected by ground that wouldn't be a problem). My heater doesn't really have an accessible common (the transformer on that is on a circuit board and without taking the whole thing apart I doubt I could get to the other pole of the transformer and it has no common terminal).

I'm still interested in opinions, but the more I think about it the more likely it is that I'll probably just get a 24V AC adapter, run one more wire up to the thermostat, and jumper the other side of it to Rh.

A photo of the A/C would be amusing. The schematic on the panel is for a furnace, and inside it is basically just a bunch of anchored components attached by wires and nuts. That does make it easy to access the common on that, however.


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## beenthere (Oct 11, 2008)

Then can't do it the way you said. Each unit has its own transformer.

Just run a new set of wires from teh A/c and use its Common.


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## rich0 (Oct 20, 2012)

beenthere said:


> Then can't do it the way you said. Each unit has its own transformer.
> 
> Just run a new set of wires from teh A/c and use its Common.


That won't actually work. Most thermostats expect common to be paired with R - not Rc (though that doesn't matter if they're jumpered).

I was thinking about it and realized that no matter what I did I'd effectively have a transformer wired in parallel (a standalone one, or one from the heater/AC). There is a risk that a line is grounded in any of those cases.

I checked with a voltmeter and found 0V AC across the two R lines, which means there shouldn't be much current across them. I measured AC amps and got about 15uA, which is almost nothing. So, I went ahead and joined the lines freeing up a 5th wire for common, and ran that from the AC. Since the R's were now paired the common could come from either transformer.

All worked fine.

However, I do agree that this is something that carries a bit of risk with it, and anybody without a decent understanding of electricity should not attempt this, and anyone attempting it should check for voltage differences across any lines they intend to cross, and measure the leakage current. If somehow in the future both 24V circuits get grounded there is the potential to burn out a transformer, which is easily a $100 repair bill depending on the system and whether you call in help. 

Oh, and in the process I learned that my AC wiring wasn't that great to begin with - somebody crimped a spade connector of the wrong size and it was hanging by just a few strands of wire and came loose.


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