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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 Rh or 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):

Diagram

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.

  • What do you mean by the air handler is "turned off" during heating season? Are there spare wires in the thermostat cabling? – ThreePhaseEel Nov 3 '19 at 2:57
  • @ThreePhaseEel There's a physical switch that disconnects power to the air handler. That's turned off when cooling is not required (winter/late fall). Yep, there are a few spare wires in the thermostat wiring. – kpjVideo Nov 3 '19 at 2:58
  • I take it the boiler feeds some other sort of fan-coil/radiator/baseboard type thing, or radiant heat for that matter? – ThreePhaseEel Nov 3 '19 at 3:06
  • @ThreePhaseEel Yep, You are exactly correct. Radiators. – kpjVideo Nov 3 '19 at 3:06
  • Hello, and welcome to Home Improvement. Great question with a great diagram! And, you should probably take our tour so you'll know how best to participate here. – Daniel Griscom Nov 3 '19 at 11:59
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From your picture it looks like the C from the air handler is simply connected to the boiler C. So you could pickup a C for the thermostat at either the air handler or the boiler.

The main problem you are going to have is related to turning off the electrical supply to the air handler. You will lose the 24VAC power feed to the thermostat if you do that. So instead you want to leave the air handler powered but program the new thermostat so that it does not turn on the demand connection to the air handler when you do not require its usage. This should result in very little standby electrical power consumption but allow your new thermostat to work.

The "demand connection" I refer to is the thermostat output that is turned on to request heat, cool or in some instances fan. You may need to check if the air handler has the fan running when you think the thermostat has both the heat and cool modes shut off.

Another thing you should check is if the thermostat will run with just one of the Rh and Rc wires connected. Check to see with the air handler power shut off if the Rx wire from the boiler still has 24VAC.

I see that you have the G wire connected to the air handler. As I recall the G wire is for running the fan. You need to see if the air handler stops blowing out air if you disconnect the G wire when the AC power is turned on to the handler. It could very well be that the thermostat activates the G wire in heat mode despite your boiler heat source not using an air circulation fan. A truly smart thermostat may have a programming option to not drive the fan in heat mode to support boiler applications.

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  • Thanks for the advise. As expected, 24VAC is only present when power is supplied to the air handler. The issue for us is that the air handler seems to push air out of the vents just by being turned on, even when the thermostat has the cooling and heating modes set to off. (It is quite audible) Perhaps it doesn't support the "demand connection"? Even though the Rh and Rc wires are separately supplied, does that mean the 2 commons aren't actually connected? – kpjVideo Nov 3 '19 at 18:25
  • @kpjVideo - The "demand connection" I refer to is the thermostat output that is turned on to request heat, cool or in some instances fan. You may need to check if the air handler has the fan running when you think the thermostat has both the heat and cool modes shut off. – Michael Karas Nov 3 '19 at 19:35
  • @kpjVideo - Another thing you should check is if the thermostat will run with just one of the Rh and Rc wires connected. With the air handler power shut off does the Rx wire from the boiler still have 24VAC? – Michael Karas Nov 3 '19 at 19:37
  • @kpjVideo - As I recall the G wire is for running the fan. You need to see if the air handler stops blowing out air if you disconnect the G wire when the AC power is turned on to the handler. It could very well be that the thermostat activates the G wire in heat mode despite your boiler heat source not using an air circulation fan. A truly smart thermostat may have a programming option to not drive the fan in heat mode to support boiler applications. – Michael Karas Nov 3 '19 at 19:44

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