Nest thermostat install C wire question

[RyanY]

 

I am trying to install a Nest thermostat to control my home’s heat pump, replacing a 20+ year old programmable thermostat. The installation guide stops when told that there is no common or “C” wire currently connected, and my research so far hasn’t found a definitive answer. There is an unused blue wire in the thermostat harness, and the controller on the heat pump has an empty space for a C wire - if the Neat needs a C wire would I be able to just connect the blue wire to the C terminal on the heat pump controller and have it work properly? Pic of existing thermostat wiring is attached. 

[CoolingWizard]

First of all what type of system do you have? is it a packaged unit and all located outside or a split system with part outside and part indoors?

Your thermostat is indeed old, lol.  Here what your wiring currently is:

R, Rh, Rc is the Red Wire, this is Power IN

O, B, O/B is the White wire, this controls the Heat Pump reversing valve 

Y, Y1 is the Yellow Wire, this wire controls the Compressor Contactor 

G, is Green wire, this controls the indoor Blower Fan motor 

the blue jumper between Y and W will be discarded. 
You say there is a blue wire in the cable that is not being used, I do not see it in the picture. Now, if you do indeed have a spare wire, you can connect to the the C terminal in the Heat Pump control board.  Now then, if you have a split system, that thermostat cable goes from the thermostat to the indoor air handler. The connection to C terminal will be inside the indoor unit.  If it is a packaged unit, the connection will be outside.

Ken, The AC Cooling Wizard

[RyanY]

Thanks Ken.

The unit is a package roof top unit.

The blue wire is tucked up behind the thermostat and is not connected on either end. If connecting it on both ends is all the thermostat needs (in addition to connecting the other wires obviously) then I should be good. 

[CoolingWizard]

RyanY,

if you feel comfortable with attaching the blue wire to the C terminal on the Roof Top Unit (RTU), then by all means, that is all you need to do to get the common working. 

Ken, The AC Cooling Wizard

[Jhonleanmel]

Hey folks, 

We appreciate your help here, CoolingWizard.
RyanY, I wanted to follow up and see if you are still in need of any help. Please let me know if you are still having any concerns or questions from here, as I would be happy to take a closer look and assist you further.

Thanks,
Mel

[Markjosephp]

Hello RyanY,

I'm checking back in — how's it going? Have you had a chance installing your Nest Thermostat? Please let us know if you need more help so we can assist you right away.

Regards,

Mark

[Markjosephp]

Hi RyanY,

I'm checking in to make sure that you've seen our response. Please let us know if you have any additional questions or concerns before we lock this thread in 24 hours.

Thanks,

Mark