Issue wiring new Navien combo boiler to existing Nest running A/C

[BeachBummer]

Just upgraded our home to a Navien combo boiler that provides heat to radiators and supplies our hot water. We tried wiring in the two-wire (red & white) to the Nest which currently controls our A/C. The moment we connect to the base it begins calling for heat. If we put on the faceplate, it shows no errors. My electrician is perplexed. We did call the support line, but we were getting nowhere with the tech who wouldn't escalate the call. I see some posts that the thermostats won't shut the boiler down, but no resolution. My electrician's answer is to junk the Nest and replace them with Honeywell units. I've been using Nests quite happily for years and not willing to change. Suggestions?

[Patrick_Caezza]

Are you in USA/Canada or UK/Europe? Continue reading if you are from US/Canada, and I can't help you if you are in the UK/Europe.

Only the Nest Learning Thermostat is compatible with your system as you have a dual transformer system, and only the learning thermostat can handle separate Rc and Rh wires. Also, your new combo boiler might just be looking for a switch closure which would require a relay to be installed.

[BeachBummer]

Yes Patrick, I'm in the USA using a Gen 1 and Gen 2 (2 zones) learning thermostat. The moment we connect the leads from the Navien to Nest it turns on and doesn't turn off. My electrician (wired the new Navien combo boiler) is totally perplexed. 

[EdwardT]

Hi folks,

@Patrick_Caezza, thanks for the help!

@BeachBummer, thanks for reaching out and I'm sorry to hear that you're having an issue with your Nest Thermostat. We'd like to know more about this. Please fill out this form so we could investigate further.

Thanks,

Edward

[Patrick_Caezza]

One big problem with many boilers is that they are looking for a switch closure, and the only way to get it to work is by adding a relay with an external 24 VAC transformer.

Something like this.

 

 

[BeachBummer]

Patrick, thanks for the suggestion and the wiring diagram. I fail to see why an external relay might be necessary when a traditional analog thermostat will do the job. And where would that relay go? I'm running a 2-zone system with air. Do multiple relays need to be placed inside the boiler combo? Somehow, I think the introduction of a third wire from the Navien to the Nest(s) may be the answer.

[Princesss]

Hi BeachBummer,

I've seen that you have been assisted via email. I'll consider this post as complete and will lock the thread in 24 hours.

Best,

Princess