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Thermostat Common and Earth Ground

Posted by johnblank (My Page) on
Sat, Dec 22, 12 at 11:14

Common question these days. I've bought a new WiFi enabled thermostat which requires a C (Common) wire. My current connections only include R, G, Y, and W, no C. I saw a YouTube video from HoneyWell showing how to move the G (Fan) wire to C at the transformer, and use the G wire as C, however, they noted this only works for Gas furnaces because they automatically enable the fan. In addition it will disable the ability to manually turn the fan on. I do have a gas furnace for heating, but I have an electric A/C for cooling, so I don't know if this is even an option for me.

I looked at the possibility of adding another C wire and running it through the wall, but it's not a practical option. There is no attic space above and I'm in a condo on top floor so can't run it below either. To run a wire I would have to put holes in the drywall in various places. I would rather not.

Anyway, that's the background, here's the question, actually 2 questions. 1) The voltage from my R wire to the other wires (G, Y, and W) shows 27.5V, I thought it's supposed to be 24V, is this a problem? 2) There is a 3-way light switch right below the thermostat that has an Earth Ground wire in the box. I measured the voltage from the Thermostat R to the earth ground and it was 28V. Is there any reason, other than it's against code, that I can't replace the thermostat Common with the earth ground? I understand that earth ground is for safety and is designed to not have current running through it, however I really doubt the small amount of current the thermostat pulls will lower the safety of my home's wiring. I also understand that if the transformer is ever replaced this quick fix may not work anymore. Other than these things, are there any other considerations?


Follow-Up Postings:

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RE: Thermostat Common and Earth Ground

Did you check the wire bundle carefully? Sometimes there are extra wires which were not connected to the thermostat. You can use one of these spares.

You should be reading 24 volts AC. What is the house AC voltage? How accurate is your meter? It is on the high side, but it should be OK. See if the thermostat has a maximum voltage specification.

I don't think your idea of connecting to the earth ground in the 3-way light switch will work. The earth ground is connected to the 120V side of the transformer. It is isolated from the ground side of the transformer. The grounds may be at the same potential, but there is no electrical connection between the two. You would have to connect the two grounds together which I don't think is a good idea. Even if you did this, you may have problems with all the electrical noise on the ground side which could affect the WiFI system.

The G wire to C wire swap is a good solution. You give up control of the fan at the thermostat. You may have to jump the Y connection to the G connection on the furnace control board so that the fan will turn on with the AC. There is a Youtube video explaining this.

If you don't want to give up the fan control, then you could add a second 24volt transformer to power the thermostat. Below is the Youtube video explaining how you can do this.

Here is a link that might be useful: C Wire - How to power the thermostat via an additional transformer


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RE: Thermostat Common and Earth Ground

There are no extra wires, I can see where the bundle wrap is removed and the wires come out of the wrap. Plus I checked at the furnace and can see that no wire comes out of the C terminal on the furnace's board.

It is an older Fluke meter, a gray one, not yellow, Fluke 75 Series II Multimeter. I think it is accurate. The home voltage is reading 124VAC, and a AA battery is reading exactly 1.5VDC with the meter.

You really through me a curve ball when you said that connecting to earth ground wouldn't work because "The grounds may be at the same potential, but there is no electrical connection between the two." It violates my understanding of circuits, which is: if there is potential current will flow. So I got 2 AA batteries from my remote, both measuring 1.5VDC from their own positive and negative terminals. I measured the potential from the positive of one battery to the negative of the other battery, and it read 0 volts. So I even tried negative of one to the negative of the other, 0 volts. Positive of one to positive of the other, 0 volts. As I would have expected.

It looks like the ground of my furnace board transformer is connected to my home's ground.

I played it safe and swapped the C and G wires as you suggested. I checked and the fan kicks on for both AC and heat, but also like you said I can't control the fan manually. I rarely rarely every do this though. Thanks for your help!

See following link for another good answer to my question: http://www.doityourself.com/forum/thermostatic-controls/484695-thermostat-common-earth-ground.html#post2045774


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RE: Thermostat Common and Earth Ground

You have to have a closed circuit in order for current to flow. If you had connected the other two terminals of the batteries you would see the volt meter respond.

If you still want to control your fan you could install a switch near the furnace between the R and G terminals. I think you will be fine without it.


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RE: Thermostat Common and Earth Ground

You have to have a closed circuit in order for current to flow. If you had connected the other two terminals of the batteries you would see the volt meter respond.

If you still want to control your fan you could install a switch near the furnace between the R and G terminals. I think you will be fine without it.


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