Nest 2020 thermostat and Navien combi-boiler constant call for heat issue

[Ckrankel92]

im having Nest 2020 thermostat and Navien combi-boiler constant call for heat issue. Was wondering if I put a 250 ohm pot on the W wire back to the navien unit with this resolve the issue of the nest thermostat constantly calling for heat.

[TandM]

Honestly i do not know. I will be following this topic.

Had seen another issue with a 2 wire hook up, (hot water heat) that the hub that the wires go into and screwed to the wall, it was shorting out the R and W, making the boiler turn on constantly (just a thought) 

(Have water heat and looking at upgrading to a system similar to yours)

Christmas Eve house full of people... Our Nest would not stop turning the boiler on! After trying to fix this issue, I just turned Nest off, and shut power off to the darn boiler. Outside temp about 46, inside 78 and climbing!!! We ending up all leaving and going to grandma's house just to have some darn relief...

After researching some more, I found the the history for the day was 0! the day before was just 1.5hrs of heat! so the App is not any real help. I have had no changes in wiring, boilers or all Thermostats for more than 5 years (2 are Nest 3rd gen) So something is wrong here, just have not pin pointed it yet

1st issue with Nest, but that was when it was new, turned out to be faulty from the start, which after many long phone calls and lots of email it was replaced.

I am wondering if the Temp sensor it self could be an issue? Just asking, since i inspected everything prior to firing up the boiler and that was almost 2 months ago!

Hope someone can help you on your issue though!

good luck

[Ckrankel92]

TanM.

I also have a two wire system at my boiler but pulled brand new 18/3 with no junctions from the thermostat to the Navien unit so I could add a C wire off of a 110 to 24 volt transformer to get the required power to the Nest thermostat  C lug. From what I read in another posts The Nest 2020 thermostat does not have enough resistance on its return power W terminal to interrupt operations of it calling for heat. That is why the heat is on constantly whether the thermostat is calling for it or not. Just didn't know if I put a pot with variable resistance in line on the W wire and tweaked it until the constant call for heat went away would be a viable solution.

[Markjosephp]

Hello everyone,

Thanks for reaching out here in the Google Nest Community.

I appreciate you sharing your experience with the Nest Thermostat, @TandM. 

@Ckrankel92, it sounds like you've put a lot of effort into upgrading your thermostat setup. Dealing with the Nest Thermostat's power requirements can be a bit tricky, but it's great that you've pulled new wiring and added a C wire from a transformer.

Regarding your question about adding a variable resistor to the W wire to adjust the resistance, it's not the most conventional solution, and there are potential risks. The W wire is responsible for signaling the heating system to turn on, and adjusting its resistance could have unintended consequences for the system's functionality. It might lead to issues with the thermostat accurately controlling the heating.

Consider some alternative solutions, such as the C-wire or the Nest Power Connector. Always be cautious when making modifications to your HVAC system, and if you're unsure, I'd suggest consulting with a professional HVAC technician. They can provide guidance based on your specific setup and ensure that any modifications are safe and effective.

Regards,
Mark

[Ckrankel92]

Mark, thanks for your helpful reply. I do have the nest power adapter and tried to run it off of my R and W wires coming from my Navien unit to the C lug on the nest base with the same result of constant call for heat. Wiring of the power adapter seems pretty straightforward unless I'm missing something in the wiring configuration of the power adapter. So any advice on how to wire the power adapter to make this work would be much appreciated like I said I've already got a 110 to 24 volt transformer and have had the nest thermostat up and running but constant call for heat won't stop. I have not tried to run the power adapter with the 24 volt Transformer don't really know how that would be wired up.

[Markjosephp]

Hey there,

If you're experiencing a constant call for heat even with the Nest power adapter, it might be worth double-checking the wiring configuration to ensure everything is set up correctly.

When using the Nest Power Connector, make sure the R and W wires from your Navien unit are connected to the corresponding terminals on the Nest Thermostat base. The C wire from the Navien should go to the C lug on the Nest base.

Considering you already have a 110 to 24-volt transformer, you can use it in conjunction with the Nest Power Connector. Connect the 110-volt side of the transformer to a power source, and connect the 24-volt side to the appropriate terminals on the Nest Power Connector. If the issue persists, it might be helpful to contact a local HVAC technician for the installation.

Regards,
Mark

[Ckrankel92]

Mark, The Nest power adapter is used to borrow power from the R wire to power the C terminal. The combination of the Nest thermostats low resistance at the W terminal and the high sensitivity of my Navien units W input is the real problem. The only way I see to fix this problem is to increase the resistance back to my boiler W input but not I'm willing to risk my HVAC equipment just to have a Nest thermostat so I have already put my old thermostats back in place. Thank you for the the help though.

[Ckrankel92]

 

[Markjosephp]

Hi Ckrankel92,

I'm sorry for the late reply, and thanks for sharing your experience with the Nest Power Connector and the challenges you encountered with your Navien units. It's unfortunate to hear that the combination of the Nest Thermostat's low resistance at the W terminal and the high sensitivity of your Navien unit's W input posed a real problem. It's always essential to prioritize the safety and functionality of your HVAC equipment.

If you have any further questions or if there's anything else I can assist you with, please feel free to reach out. Your safety and comfort are our top priorities, and we appreciate your understanding.

Warm regards,
Mark