The nest is sending a ghost signal which is causing the circulator to stay on. I attached to ground in order to wire a common, but the nest is telling me there is no power when I attach the common.

Any ideas on this?

 

Do you have a C terminal on the furnace control board? Ground is NOT always common. By circulator do you mean fan? We need more info on your setup. Have you tried a FULL reset of the nest after the wiring change?

 

That is correct. "Common" is NOT always Ground. If the manufacturer uses a transformer with an isolated secondary, the neutral side or "Common" will not be earth ground. If you do not have a Common terminal on your control board or relay box, you either need to call in an HVAC tech, or download the schematic of your controller, and find the point where the secondary neutral leaves the transformer pin. That path is "Common".

 

Let me explain how Nest uses the Red and White wire to draw power to charge its backup battery and to keep itself "alive".
As you know, if you touch the red and white wires together, you completed the circuit, the relay pulls in and the fan or circulator turns on. This is what a standard thermostat does by using a vial of mercury or some other switch. Nest completes that same circuit when it's on, but it tries to draw as little current as possible to prevent the relay from pulling in. When the Nest is connected to the circuit, current flows through the relay coil, but it's not enough to completely energize it. I imagine the Nest has ways of probling the attached relay to determine just how much current it can draw without activating the equipment. There are three current settings, 20ma, 40ma, and 100ma. A Nest connected with the common wire will draw 100ma. If you have it attached to just the red and white wires, you will see either 20ma when it's not calling for heat or cooling, or 40ma when the equipment is running. Occasionally Nest gets it wrong, and users report equipment that makes funny noises, will not turn off, or frequent cycling of fans, burner, or circulator pumps. It's because the relay circuit is being energized..... but just barely.

The problem is there are so many variables, that a lot of the time the two wire setup just doesn't work. Users are forced to pull new thermostat cable, search for a common point, hire a HVAC contractor and spend even more money, or throw their hands up and return the darn thing. If you look at other programmable thermostats, you will see that they all use utilize a "C" wire. Nest should examine their strategy in this area.
Now...if you can use the Common wire, Nest will have all the power it needs to run its business. If you look at the settings when using the Common wire you will see that Nest is pulling about 100 milliamps, and one tenth of an amp. Using this setup, Nest is completely satisfied and will not try to steal power from the Red and White wires. Again, I see a problem in this area of Nest marketing. Nest tries to keep it simple, and avoids any operational theory. "Hook up the wire and go", they basically say, but in a majority of cases it's not that simple.

When they alienate a potential customer, you can be sure that customer will tell all his or her friend of their bad experience with Nest. It's a great thermostat. I run two at my house. But it ain't that easy to install.