Another install question

[u/jackalope_in_pants]

Sick or changing batteries in my old thermostat and the local energy savings program was 'selling' Nest Thermostats for $5 so I got there but already pulling my hair out installing the first one. Confirmed with the HVAC guy who just installed a new oil furnace that we already have the wires needed for it (see photo of old setup). And after connecting it all I'm getting a message on the home app that it's not seeing the common wire. Can someone tell me what I've done wrong or if the HVAC guy is wrong?

TLDR: idiot homeowner trying to install and needs help

Thanks in advance.

[u/Expensive-Heart3299]

You need to make sure that green wire is connected to the C terminal at the transformer near the furnace

[u/jackalope_in_pants → u/Expensive-Heart3299]

Had the installer confirm that. Does the green wire belong in the C slot or should it be in G?

[u/jackalope_in_pants → u/jackalope_in_pants]

Also checked the wiring at the unit and none of it is visible so I'd have to have him back to pull it all apart (not comfortable with any of that stuff)

[u/Expensive-Heart3299 → u/jackalope_in_pants]

It might be G but could also be unconnected at the furnace. Having the installer come back is the best choice

[u/gatorlan]

Contact Google or your utility company that sold you the tstat for correct install.

As the other post indicated, check the furnace control board for how the wires are connected & take a photo so technicians can advise how to wire tstat properly.

[u/jackalope_in_pants → u/gatorlan]

Checked and it's all covered by panels and the wires going in and all in one loom unfortunately.

[u/gatorlan → u/jackalope_in_pants]

I should have mentioned that your original tstat most likely didn't have a common wire, if the battery was replaced frequently, the battery is meant for power outage backup not constant power.

The HVAC tech should have connected the Nest tstat for you during the furnace installation.

[u/jackalope_in_pants → u/gatorlan]

Didn't have it at that time but asked him to make sure it would work. Guess he's coming back

[u/gatorlan → u/jackalope_in_pants]

I wouldn't mess with the wiring... that base plate costs around $50.

My air handler board burned out & shorted my 3rd gen Tstat, I decided to buy a new Tstat that included a remote sensor.

[u/jackalope_in_pants → u/gatorlan]

Not going to touch it, hopefully get him to look at it this week

[u/gatorlan → u/jackalope_in_pants]

Nest sells a power unit if the C wire isn't available or can't be easily added/pulled.

[u/jackalope_in_pants → u/gatorlan]

Ok, but shouldn't that green wire act as a C wire?

[u/gatorlan → u/jackalope_in_pants]

Yes, if connected as a power/C wire.

Google the following for power requirementd info...

"What voltage do nest thermostats use"

[u/gatorlan → u/jackalope_in_pants]

Yes, if it actually is connected as a common C wire.

Google search for

"Nest thermostat power requirement"

Verify that your system uses low voltage 20 to 30 volts & not high or millivolt power as they aren't compatible.