Wiring assistance

[Bimmerland]

I'm only using my furnace for heat. Currently, I have an old 2 wire thermostat. I'm converting to a Nest thermostat. I've already run a new cable from the furnace to the nest. Originally, I used W and R wires to control the furnace. If I add a C wire, will that be enough to power the nest. I will have W, R, and C wires.

[CoolingWizard · Accepted solution]

The only question that remains is to make sure your system uses 24V AC power for control. If the you add the C wire and it is connected to the common side of the 24VAC transformer, then all will be good. 

Ken, The AC Cooling Wizard

[Markjosephp]

Hello folks,

Thanks for helping, Ken.

@Bimmerland, I hope you've got the answer you're looking for. How's it going? Have you had a chance to connect the C wires to your Nest Thermostats? If you're still in need of assistance, feel free to let us know.

Regards,

Mark

[ATSpaz]

I have a similar situation; 2 wire (R,W) old tstat with 5-conductor (3 currently empty) attached. It’s a basement tstat used for heat only. 2 tstats upstairs are connected to furnace (heat) and 2 separate central air conditioner units.  I tried removing and replacing the R and W wires from the old basement tstat to a newly purchased nest tstat and then tried hooking up the spare blue wire to the c terminal at the tstat with the other end of the blue wire to either secondary (24 v) lead of the internal furnace transformer. Regardless of which transformer output lead I connect it to ( not sure which one is considered common) the nest tells me it doesn’t recognize a c wire hooked up and I can’t get any further.  The furnace has 3 zone valves and two circulator pumps, one pump hooded to the 3 house heating zones and one for the hot water external tank.  I just received a Nest power connector from Nest that they suggested I get to finish the installation.  I do not have typical nomenclature (color) control boards inside the oil furnace since there are pc boards installed for all the high efficiency options.  I don’t see how connecting the newly received Nest power connector is going to make a difference since I already have 5 wire conductor running to the tstat and have tried multiple configurations to the zone valve connectors and the internal transformer 24 volt power. Any suggestions for me? (ATSPAZ@ gmail.com) Thanks for your time.  

[Markjosephp]

Hello ATSpaz,

I'm sorry for the late reply, and thanks for letting me know about this. Could you please check the power readouts on your thermostat settings? Please go to "Technical Info" for the Nest Learning Thermostat or Nest Thermostat E and "Equipment" settings for the Nest Thermostat 2020 model. Also, could you share with us a photo of your system control board?

Best,

Mark

[Markjosephp]

Hello there,

I wanted to see if you still needed help. Please let us know if you have any other questions or concerns.

Regards,

Mark

[EmersonB]

Hi everyone,

Just one quick final check in here since activity has slowed down. We'll be locking the thread in the next 24 hours, but if you still need help, we would be happy to keep it open. If there's more we can do, just let us know.

I appreciate the help, CoolingWizard and Mark.

Best,
Emerson