Compatibility with 4th gen Nest thermostat

[dsingaraju]

My current Honeywell thermostat has two wires - R and W/Aux.  I also see a note for Max load per output: 1A  / 24 V AC.   I had questions about the right code for the W/Aux, and if I have a millivolt system.

- Are the wires compatible with the 4th gen Nest thermostat. My understanding is that I should read those wires as R (power for heating) and W (for the heating systems)

- Does the 24V imply that this isn't a millivolt furnace. My furnace is a Bryant plus 80t furnace.

I can't find an option to attach images 😕 When I try to drag/drop images, I get an error that I don't have permission to upload images. I verified that I'm signed in.

[dsingaraju]

The option to upload images finally showed up. Yay!

[Patrick_Caezza · Accepted solution]

@dsingaraju 

Your system is compatible with the 4th Gen Learning Thermostat and you are correct on the wire placement.

When you have a two-wire heat-only system, I always recommend adding a C wire by using a spare wire or installing the Google Nest Power Connector.

[dsingaraju]

Thanks for the response @Patrick_Caezza . What would the C wire add? Would there be any risk if I installed it without a C-wire or the Google nest power connector?

[Patrick_Caezza]

@dsingaraju 

The C wire is used to provide constant power to the thermostat which keeps its internal battery fully charged. Without a C wire, the thermostat uses a method called "power stealing" to keep the internal battery charged.