My recently-installed (about a week) Nest is throwing an E102 error (over-current on the W1 wire), and not calling for heat. It is correctly-wired with four signals, Y1, W1, G, and Rh.

I contacted Nest support that went through diagnostic procedures apparently intended to exclude the Nest itself as a causative agent. Their final response was "contact a HVAC guy, there's something wrong with the furnace itself."

My "HVAC guy" is a fine fellow, but he appears to have gone out of business, retired, whatever. So before seeking out a new HVAC repair person, I am turning to the boundless resources of SigForum.

My plan was to pull the Nest and return to the ritetemp 6022 that did a serviceable job for ten years without incident, but I'd like to get the Nest working the way it should (except see below).

Other misc. details that may have relevance: Rh is supplying 27.6VAC consistently, measured from Rh across W1, Y1, and G. Swapping W1 and Y1 changes the error thrown by the Nest to E103 (over-current on Y1), so it appears to actually be happening. The furnace's controller board is not flashing an error, nor are any control board fuses blowing. Furnace is a Carrier natural gas, forced air, unit.

For no other reason than it was easy to do, I connected the C terminal (blue wire coming off the furnace control board). This seems to have made the Nest happy, for now at least, but I am not confident in long-term operation. It did initially go a full week+ without throwing an error, after all.

Any wisdom that could be imparted would be welcome. Thanks in advance.

Not an HVAC, but 27.6 is within the range for 24v nominal. All a thermostat is a switch that puts 24v on fan and heat/cool. If calling for heat, you should get the same voltage on W as on R.

If you have the wiring to connect C, I would do that and you won't need batteries for your t-stat.

Worse case scenario, if you need heat wirenut RWG together for 20 minutes at a time, this is what the t-stat is actually doing. No temp control, but it will make heat.

Carrier's model number?

Have any add-on equipment, humidifier, electronic air filter?
What's the amp draw on W when calling for heat?

Some power stealing stats will act flakey, until a common wire is connected.
The Nest may require a neutral to function properly... I don't know and I'm done searching their buggy website for information.

By connecting the common wire the problems may be resolved, if not,
buy a Honeywell and your problems will disappear.

As a follow-up, adding the C signal to the Nest has suppressed the error message it was throwing for about 24 hours now.

I'm still uncertain as to why, but I do know a lot more about how a thermostat works than I did a few days ago.

Thanks everyone for your responses.

quote:

Originally posted by architect:
As a follow-up, adding the C signal to the Nest has suppressed the error message it was throwing for about 24 hours now.

I'm still uncertain as to why,

The C wire is a direct link to the common side of the transformer (24-). So the thermostat has a direct power source (R 24+ and C 24-).

Without C connected, the stat is getting a common from running through components in the furnace which are connected to the common, but adding resistance.

Honeywell WiFi stats require the C connection for operation.