E79 with C wire and 28V between C and Rh

[justaguy]

I just bought a new house and moved my 3rd Gen Nest Learning Therm. I have a C wire, and when I test it when it comes with a multimeter, it reads 20V+. However, once I plug the C-wire into the backplate, it drops to <1V and I get an E79 error.

I've tried it with another backplate and haven't had any luck. I can't figure out what could cause this, but I saw another post with a similar issue: https://www.googlenestcommunity.com/t5/Nest-Thermostats/Nest-thermostat-e79-but-multimeter-showing-2...

The "widget" in the suggested response didn't lead anywhere, though.

[laurentbourg]

What happens if you simply don’t use the C wire, do you still get the e79 error?

[justaguy]

It’s working, but a week or two ago, the battery stopped charging and it had to disconnect from wifi. I ended up charging it with USB. It’s been fine since then. So, it may not be critical to have the C-wire, but it’s definitely preferred. 

[laurentbourg]

I agree, it is preferred, there are cases I know of where battery will drain faster than stealing current from R can provide, which consequently drains the battery - such as a temporary network outage which forces the thermostat to frequently attempt reconnections or other use case that is battery hungry. 

[Patrick_Caezza]

 Can you post a picture of the original thermostat's wiring before you swapped it out?

The C wire is just the common or return wire for the 24Vac that is on the R wire. When you measure the voltage across the R and C wire you are actually reading the voltage on the R wire.

[SimpleUser125]

Hi,

Facing a similar issue. When the furnace is turned off, the c wire shows power, but not the rh wire. However, when the furnace switch is turned on, it is the reverse (c showing no power, but rh is working). Would the nest charge through rh if I unplug the c wire to bypass the e79 error? Thanks in advance!