Do you know if the wires go all the way back to the furnace doubled up like that?
You will need to go look at what is connected to the furnace and post a pic.
Also you are going to need a C common wire connected to the common of the 24 v transformer at the furnace so if they ran double wires all the way back to the furnace you could use one of the wires as C since you do not need 2 reds and 2 whites only one wire per terminal on the nest.
So post some pics of furnace where the thermostat wires are connected and we can try and help you.
Hi Houptee - I posted more pictures. I think I have to ohm out each wire one by one in order to determine which ones go to R/G/W terminals. What puzzles me is why they ran two cable (two sets of wires) then when it goes to the furnace terminals they pull off one set (actually they connected the wires to a cable stub with the correct colors. Well, if you have any advice pls give it - thank you
This is a heating only system with no air conditioning correct?
The blue wire nuts is where the cable goes to the thermostat. How many different color wires are in the cable going up to the thermostat?
Is there a blue wire not connected? If yes then connect it with a wire nut to the blue going to the furnace and then you can later connect it to the screw in the furnace marked C common. Don't do that yet wait until you see if the blue goes to the old thermostat.
If you have it both ends you can proceed to install the nest.
Turn off the power switch to the furnace before connecting any wires because if they touch it can blow the fuse on the furnace board.
Hi Houptee - as you can see from my first photo at the thermostat location, I have 2 R, 2 G and 2 W wires connected to the R, G and W terminals. At this point, I have to assume that the the two cables leaving the thermostat go in the same direction down to the furnace. This is the 3rd furnace we've had in 34 years. However, from my posted pictures, it appears to be same two cables connected at the thermostat arriving out of the wall at the furnace. It is at this point that furnace installers made a jumper to go into the furnace and used twist downs to make the connection from the two old cables down to one jumper. The single cable jumper then goes into the furnace and the R, G and W wires are correctly connected.
My question at this point is do I connect the Nest (at the thermostat end) using the 2 R, 2 G and 2 W as is currently the case ? Will the Nest accept two wire ends per screw-down ?
If the answer is no, then somehow I will need to isolate the one cable of the two that is eliminated (not connected) via the twist-on transition to the jumper at the furnace. Any advice on how to do that will be welcome.
Please let me know and thank you so much for your advise.