Nest Learning Thermostat C wire not detected.

[Eric7789]

I have a 2nd generation Nest Learning Thermostat. It's been working fine for years. Last week I took it down to paint the wall, and when I put it back it said there was no power to the C wire. I checked the C wire with a volt meter, and it said 27v. After fighting with it for a week, I gave up and bought a Nest Power Connector. After hooking it up, it now says there IS power to the C wire, but NO power to the RH wire. I checked the connector to see if voltage is getting from one side of it to the other with the volt meter (it beeps if you touch both ends of the wire and current gets through). The red wire beeped, but the white did not. Is the base defective? Is the Power connector defective? Is the whole Nest system more time/money/frustration than it's worth? I'm starting to think the answer is "yes" to all three. Any ideas? 

[Patrick_Caezza]

@Eric7789 

Did you turn off the HVAC before you removed the thermostat?

If you didn't you might have blown the fuse on the control board in the furnace/air handler.

[Eric7789]

Yes, the power was off, and the fuse is working. It's also a boiler, not a furnace if that makes a difference. If I touch the red and white wires together, the boiler starts as expected, so everything seems to be working. However if I touch the red and white wires together after they go through the power connector, nothing happens. I feel like the C wire connector on the base might have failed, and the power connector itself is also defective.

[Patrick_Caezza]

@Eric7789 

Yes, a boiler does make a difference.

Boilers tend to be a pain to get working with any smart thermostat. They are looking for a switch closer (on/off) to turn them on while a furnace is looking for a 24vac signal.

\You said a C wire was installed. Is there a valve zone controller installed?

Do you happen to have a picture of the wiring before you removed it for painting?

Can you post a picture of the boiler controller and wiring?

[Eric7789]

I bought a 3rd gen model, and it's working just fine. I must have been right about the old unit being faulty. I bought it used and it's always been a bit temperamental. I appreciate your help nonetheless. 

[catheryn]

Hi @Eric7789, 

Thanks for letting us know. 

If you have more questions or concerns about any Nest product, feel free to post them. We will be more than happy to assist. 

Regards, 

Catheryn.