Try to replace an old thermostat with a Nest thermostat

[jnthn123]

Hi. I am trying to replace an old Carrier thermostat, which only has 2 wires (V+ and Vg). I've called HVAC companies around and been getting various responses from "I don't think thermostats work with only 2 wires", to a $400 quote, to a "big job $1500" quote. Is it really a big expensive job to convert the 2 wire signals to 5 wire signals? I live in a small single family house with furnace in the attic and AC unit on the ground outside. If running wires are needed, the distance isn't that long. Just want to be sure what I need instead of trusting random companies that are trying to sell me their service.

[CoolingWizard]

@jnthn123 ,

When the thermostat uses only two wires labeled V+ or D+ for example, they are proprietary digital communications. Look at your thermostat cable, are there 5, 7, 8, or 10 conductor wires in the cable? The HVAC equipment that use digital communications also have conventional HVAC controls.  If you feel comfortable, I can assist you in converting to the conventional control by rewiring the indoor and outdoor unit.  

AC Cooling Wizard 

[jnthn123]

Thanks for the info. I am comfortable doing wiring myself (plus my budget can't swallow those quotes). I need to find some time to scope the inside of the wall (through the small hole) or trace the cable in the attic. I don't know how many conductor wires it has at the moment. Only 2 wires come out. I don't see the end of the other bundled wires. Is the other end connected to the control circuit board of the furnace and AC unit? Maybe a second option for me would be looking at that other end.

[CoolingWizard]

@jnthn123 ,
A typical thermostat cable has copper conductors with colored insulation contained inside an outer insulator. Yes indeed the other end will be at the indoor air handler.  You will see the cable enter the side of the air handler usually.  Inside the air handler you see a circuit control board.  Looking close you should see screw terminals with labels R, W, Y, Y2, C, O, G etc.  

AC Cooling Wizard