Issues adding C wire for Nest Learning Thermostat (3rd gen)

4 years ago

I bought a new 3rd gen Nest thermostat to replace an older battery powered Honeywell thermostat.

I have separate heating (Oil burner with hot water radiators) and cooling (central air) systems. My older thermostat had Rc, Y, G, Rh, and W wires connected.

When I set it up the same way on the Nest, all worked as expected, but the Nest wouldn't stay charged. A little research showed that I needed a C wire.

First, I made use of an unused wire wrapped out of the way coming from the heat cord as my C, and on the other end, I connected it to a bundle of common wires leading back to the C terminal on my 24V transformer.

Rh to C measured 24V as it should, but suddenly the Nest wouldn't recognize the Rh and Rc wires. When I removed the C, it would recognize them again. I did notice that I was getting about ~20V across Rc and C, rather than zero which seemed odd.

So, I scrapped the C wire from the heat side and I tried the method of using the fan wire as my C (and moving it to "B" on my A/C transformer, which seems to be what the outside unit returns its common to.

The system is now working as desired, but:

1. I'd prefer a solution that lets me run my fan independent of the A/C when desired, and

2. I'm still getting an error/warning that Rh is detected but not attached to equipment (Rh shows up as gray text on the wiring diagram while all others show up as black). In spite of this, Rh to W measures 24V as it should, Rh to C is zero (which I think is correct, since C is now coming from the A/C side) and the heating function all works as is should.

I'm really confused about what's going on. Any ideas?

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Comments (3)

mike_home
4 years ago
What is the voltage between Rh and Rc? It might be about 48 V. If it is then the two transformers are 180 degrees out of phase.
Thor Odhner
Original Author
4 years ago
Currently showing only ~20V between Rc and Hc, but showing ~50v between Rc and W.
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mike_home
4 years ago
The 20 V reading between Rc and Rh doesn't make sense unless the Nest thermostat is loading down the thermostat output. Try measuring the voltage between Rc and Rh with the wires disconnected from the Nest. The 50 V reading between Rc (which is connected to the AC transformer.) and the W which has a path to the boiler transformer supports my theory that the two transformers are power by two different legs of the 120 V supply. Rh and Rc are 180 degrees out of phase so instead of reading zero volts, you are reading the difference of 24 V - (-24 V) which adds up to about 50 V.

I am surprised the Nest is working properly and has survived this configuration with two transformers. Maybe the Nest engineers thought about this possibility and designed their circuit to withstand these voltages. It would be interesting to see the internal circuit. But unfortunately Nest does not publish that information.

You may be getting an error because the voltage between Rh and C is zero. The Nest may have a detection circuit that reports this as an error.

Nest has a power connector device which can be used when a C wire is not available. You should be able to install the power connector on the boiler side and eliminate the C wire. Then you can restore the wire back to as the fan control. I don't know if this will work but it is worth investigating. Here is the power connector installation video:

How to Install a Nest Power Connector for Your Nest Thermostat