We received a second generation Nest thermostat for Christmas and after excitedly installing it, having it set up an automatic schedule we ran into an issue where sometimes the heat would not turn on. Some days we would come home from work or wake up in the morning to an unpleasantly cold house that was only 55 degrees Fahrenheit (12 Celsius) and the thermostat display would indicate the power was low. After searching online to see if anyone else had a similar problem we determined the likely cause was the Nest thermostat not being able to draw enough power to maintain all of its functions, disconnecting from the network and therefore not starting up the furnace as programmed into the schedule. Our old battery powered thermostat did not have a “C” wire so when we hooked up the Nest thermostat it worked some of the time it just wasn’t getting enough power to stay on. Time to get to work installing a new thermostat wire!
We opted to install new 18/5 thermostat wire instead of just running a new “C” wire since the existing wires were beginning to show their age. It was also an opportunity to future proof the wire for when we eventually get an air conditioner. Our setup is very straight forward, yours may be different, and we are no HVAC experts so please take what you read with a grain of salt and contact a professional if you are unsure about anything. Installing a new thermostat wire is something you can do in a few hours with a few tools. Here are the steps we followed.
Installing the new thermostat wire
Remove the Nest thermostat and take a picture of the current setup in case you have to revert back to it. After you have documented the wires continue removing the base plate until you are only left with the thermostat wires protruding from the wall. If you are worried about the wires falling into the wall you can tape them off or attach them to a pencil so they don’t go down into the wall.
This next part is easier with two people so get someone to help you if you can. Drop your fish tape into the opening and have someone in the basement pull it through. Attach one end of the new thermostat wire to the end of the fish tape using electrical tape and pull it back through the opening.
At this point you will want to make sure that there is no power going to the furnace so we flipped off the furnace breaker. Take a picture of how the wires are currently connected to the furnace board. The next step is to disconnect the old thermostat wire from the furnace.
Run the new wire to the furnace and prepare the thermostat wire by cutting away some of the sheathing and exposing some of the copper wire with a multi purpose wiring tool. After consulting your furnace user manual, connect the thermostat wires to the furnace according to the manufacturers instructions. Back at the thermostat opening, prepare the thermostat cable to be connected to the Nest in the same way you did at the furnace by removing some of the cable sheathing and exposing about an inch of the copper.
Label the thermostat wires and connect them to the Nest according to the instructions provided. Flip the breaker on and the Nest should detect a wiring change. Follow the setup instructions on screen and test that everything is working well. Once you are satisfied that everything is working properly you can remove and discard the old wire.
Since installing a new thermostat wire the issue we were having with the Nest not getting enough power has been resolved and our house is nice and warm!
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