Is C wire connector kit necessary here?

[twmissmt]

@CoolingWizard:  Attn: coolingwizard.  I disconnected my old thermostat and while there was a blue wire and a yellow wire present, they were NOT connected. The only connected wires were Red, White and Green. My Nest Therm says to use a C wire connector kit but isn't that blue wire a C wire?  I do not have a heat pump or AC, just a furnace.  I don't want to utilize the connector kit if not necessary.  10/30/23

[CoolingWizard]

@twmissmt , Let us go to the basics. The color of the wire insulation does not matter.  What is important is what the conductor wire is connected to in the thermostat it must be connect to the same in the HVAC equipment.  For convenience, thermostat cables are 18-gauge copper solid conductors with colored insulation that are Red, Yellow, White, Green, Orange, Blue, Black, Gray, Pink, and Brown, for convenience, the red wire is typically placed on R, the Yellow Wire is placed on Y, and the Green wire on G, and often Blue wire is on C. Some older thermostats for heat pumps had B and O along with the C terminal. These days Orange is often used for the heat pump reversing valve.  

Now then twmissmt, in your particular case, you have unused conductor wires.  So you do not need to use the Nest Power Connector. To help you best, I need to know more about your HVAC system. Do you have:

a) Packaged System; everything in a single unit outside. Sometimes on the roof, sometimes on the ground.
b) A split system, a compressor unit outside, and a blower unit indoors.

For Heating

 H1) Heat Pump, H2) Gas Furnace, H3) Oil Burning Boiler

AC Cooling Wizard

[twmissmt]

I have a package system I suppose.  Everything is in my crawl space.  And by everything I mean my furnace.  I do not have a heat pump, air conditioner or any other accessories, just a gas-fed, forced air furnace.

[CoolingWizard]

@twmissmt , attach the blue conductor to the C terminal in the Nest Backplate. As long as you feel comfortable, go to your attic, and then to the furnace. There you will see the thermostat cable entering the side of the furnace cabinet.  Look around and you should see an electrical disconnect.  Turn off or disconnect the power to the furnace.  Open the cabinet up be removing the front cover.  You should see the thermostat colored conductors.  Look for a control board.  There should be a C terminal on the control board.  Unwrap the blue  conductor, and attach to the C terminal screw.   Put the door coves back on. Turn the power back on.  

If your furnace does not have a control board. Take a picture of the wires attached to Red and White thermostat conductors.  I need to see the transformer and the wire coming out of the same side of the transformer as the Red wire is attached to. 

The AC Cooling Wizard

[twmissmt]

I pre-emptively connected the the blue wire to the Nest and the furnace fired up as did all the set-up items on the Nest.  I'm assuming this is an indication that that blue wire was already properly connected at the furnace?