we are planning to install a Nest learning thermostat in our house. On first installation attempt the device notified us that there was no power from the
Rc wires which from my understanding is because the thermostat cannot "power steal" any current into itself without the common wire.
The setup is wired in the following way (please excuse my bad diagram drawings):
The air handler is always turned off in the fall/winter season, so my question is:
Can I simply run another wire from the C terminal of the air handler directly to the thermostat or do I have to run a wire from the common of the boiler? I'm not entirely sure whether it matters if the handler is on/off since I believe the common is just completing the circuit and the 24VAC is sent to the Rh/Rc. The documentation for the boiler states that the common/hot are used for a low water cut off device, so I'm afraid to use it as a common to the thermostat without knowing for sure.
The air handler is a York F2RP030H06B and the manual can be found here. The PCB for the air handler has the following terminals: R, Y, G, W2, W1, O, and C. Of which only R, G, and C are connected. R and G go directly to thermostat. The C wire connects to a wire labeled "Outdoor temp sensor wire" in the Navien. (See diagram)
The heating system is a Navien NCB-240 which was installed within the past two years. The wiring diagram is located on page 69 of the manual. The labeling of the Navien is a bit different than what I have seen from other's installations in the past, so I'm just a bit confused.
Any input would be greatly appreciated.