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Nest Learning Thermostat - separate Heat & A/C wiring

SteveNYC
Community Member

I have 2 separate systems - one for heat (hydronic boiler - Rh and W1), and a central A/C system (Y1, G, Rc) that has been working with my Nest thermostats for a few years.  Suddenly, after the heating season (I think since April 2024) - the thermostat is unable to control my heat. 

 

On the thermostat display ,under equipment, the Equipment diagram shows the Rh is connected, but grayed out.  I also received a N72 error, and heat does not start the boiler up if selected.

What is curious - is that his has worked for years !! -- I have 2 Nest thermostats wired in this way, and both are now not seeing full power from the boiler.  I am connected to a Taco SR504-4 controller - nothing has changed.  "C" wire is coming from the A/C in this case.

I even installed a new Nest Learning thermostat, and have the same exact behavior.  I even left the out of the box firmware on the new unit, and it still did not work.  I am guessing some logic to detect the Rh changed in the back-end / cloud?

I have read posts on the forum with the same issue I am experiencing, but have not found a solution so far.

Thank you in advance.

5 REPLIES 5

chaconmorales
Community Specialist
Community Specialist

Hi @SteveNYC,

 

Thanks for reaching out to the Community. I am sorry that you are having issues with your Nest Thermostat, I’d be happy to help you. Let’s check it out.

The N72 error on your Nest thermostat indicates an issue with the Rh wire, which is responsible for supplying power to your heating system. Since your heat doesn't work when you select it, this error code is likely relevant. Here's how to troubleshoot the N72 error and get your heat working again:

Troubleshooting Steps:

  1. Check Wiring:
    • Turn off the power to your HVAC system at the breaker box for safety.
    • If comfortable: Double-check the Rh wire connection at both the thermostat base and the furnace control board. Ensure it's securely fastened in the correct terminal. (Note: If you're not comfortable working with electrical wiring, consult a qualified electrician).
  2. Nest Thermostat Settings:
    • Verify your Nest thermostat is set to "Heat" mode and the target temperature is higher than the current room temperature.
    • Ensure there are no scheduling conflicts preventing the heat from turning on.
  3. External Power Supply:
    • Some Nest thermostats require an external power supply (usually connected with the Rc wire). Make sure the connection for the Rc wire is secure and that the external power supply is plugged in and functioning properly.

If the above steps don't resolve the issue you can try Troubleshoot Nest thermostat help codes.

Let me know how it goes. 

 

Best regards,

Mario

Hi @SteveNYC,

 

Thanks for following up and thank you for following up all these important steps while making your best effort. I apologize for the inconvenience this has caused.

 

Since you've already tried more troubleshooting steps I suggest the following:

Let me know if you need more assistance.

 

Best regards,

Mario

SteveNYC
Community Member

Been down that procedure Mario already-   thanks.

#1, #2 - Been done, no dice.#3-There is no external power supply necessary, there the a "C" wire- this does not apply.

Remember - this had been working for 3+ years, I feel like something changed in software.  There are 2 of the same Nest thermostats in my house, and its the same behavior with same boiler.

Other thoughts?

chaconmorales
Community Specialist
Community Specialist

Hi @SteveNYC,

 

Thanks for following up, as far as we are concerned there has not been any software change that could affect your device. I apologize for the inconvenience this has caused.

 

Since you've already tried the basic troubleshooting steps I suggest the following:

  • Check the wires with a Voltmeter, this can help us to identify the voltage and the power received.
  • As the issue and behavior is the same for multiple thermostats, it is recommended to contact a HVAC technician to check the system/unit. For hiring a technician you can visit the next link: Nest Pro technicians.

Let me know how it goes. 

 

Best regards,

Mario

Thank you-

For the voltage check, what should it be between the Rh and W1 jumpers?  I will check that out.

No need for an HVAC tech, if i replace the thermostat, everything works fine.  And as indicated, it worked fine for years.  Its controlling the A/C compresser fine, A/C is running now as its very hot here in my area.

I think this is relating to the multitude of posts from other members with Nest picking up (enough) differential current/voltage on the Rh/W1 in my opinion.  I am using a Taco controller for the boiler, (which has not changed), but some of the articles seem to indicate that Nest confuses the "C" for the A/C and heat systems.

As a test, I have removed the A/C, and the boiler controls on Nest work , but I cannot have this configuration as Nest controls both heat and cool.

Thank you ahead of time for your continued support.