Question about 3 wire connection configuration

[digdesdev]

Trying to help put a client installing a new Nest Learning Gen 3 thermostat to replace a  Radio Thermostat device that is no longer supported for remote control.

There are only three wires from the gas boiler control, connected as follows:

Boiler                                 Thermostat

R                Red                            Rh

W              White                        W1

C                Green                       C

The Nest keeps saying that there is no power on the C terminal.  However, there  is 24VAC between the C wire and the white wire.

Unless I'm missing something, this configuration should work to control the heating system (there is no air conditioner) just without independent fan control.  I.e., the fan is controlled only by the heating system, not the thermostat, and can't run independently.

If anyone can explain why it won't work, that would be appreciated.

[Jenelyn_O]

Hi digdesdev,

Thanks for reaching out for providing the information above. I appreciate you being a step ahead of us. If your Nest Thermostat isn’t detecting a wire, you should check that the wire end is straight, fully inserted into the wire connector, and isn't corroded. Google Nest thermostats detect wires by testing them in two ways: mechanically (is a wire inserted into the connector) and electrically (electrical current on the wire).

You can follow the guide below:

Strip and fix the wire

1. Important: Turn power to your system off at the system switch or fuse box. This helps protect your system and thermostat from possible damage.
2. Pull the thermostat display off the base.
3. Pull out one wire and inspect it.
   • The exposed part of the wire should be ⅓ to ½ inch long, clean and straight
   • Cut and re-strip any crooked or corroded wire ends
4. Re-insert the wire.
   Important: Make sure that the wire connector’s button stays down. If not, the wire isn’t fully inserted, and the thermostat won’t detect it.
5. Turn power to your system back on.

Let me know how it goes.

Best,

Jenelyn