Solved: Nest Thermostat and Navien Combi-boilers

[Wayupnorthguy]

(UPDATED)

Taking the advise from this thread I tested two additional configurations for a 2-wire thermostat with a Navien NCB-H combi-boiler.

#1 Using a 5W 220ohm resister wired between the W1 and C wires at the thermostat.  This configuration did allow the thermostat to charge and did stop the "call for heat" with just the Nest docking ring attached.
Solution confirmed.

#2 Using the Google Nest Power Connector (which I suspect is nothing more than a 220 ohm resistor in a fancy package.)  This solution also works.

Why Nest Tech Support offered none of these solutions to my problem is still a question.

I will say that there isn't much room to put the 5W resistors in the NCB-H due to the location of the connectors but this is the least expensive way to solve the problem.

The short answer you don't want to hear is "The Nest thermostat will not work with a Navien combi-boiler like the NCB-H series without some pretty significant hoop jumping (and extra equipment).

The problem lies in the fact that the built-in controller int he NCB series is simply too sensitive on the thermostat input side of the controller.  This means that the ~55k ohms of resistance in the Nest mounting ring (the small device that you attach the wires to first) presents enough of a load and current draw to the boiler as to keep it in constant call-for-heat mode.  

So what's the solution you ask?

#1 Don't bother calling or chatting with Nest.  They did nothing but fail to listen, fail to understand, fail to offer any solution or detail information about their products and, best of all, used terms like "let me make this simple for you" in their inept responses.

#2 You are going to need several parts if you are dead set on making the Nest thermostat work for you.

If you have at least 3 wires leading to your thermostats you will need one less piece of hardware. (eliminate the Nest Power Connector listed below)

If you only have 2 then you will need the following:

• A 24V AC transformer or power source (40VA or so if you are going to run more than one thermostat)  DO NOT USE the 24V that is available from your Navien furnace.
• A Nest Power Connector
• A RIBU1C Pilot relay
• Access to 120V AC near your furnace to power the 24V transformer

This solution will allow you to power your Nest thermostat and trigger your Navien furnace through the isolated relay in the RIBU1C.

The following is a drawing detailing how to connect all the components.  Each thermostat will need an RIBU1C and a Nest Power Connector.  The 24V transformer can power (at least) 2 devices.

Moderator edit: Subject updated for clarity and findability.

[CoolingWizard · Accepted solution]

@Wayupnorthguy ,

What you are describing is a power sharing problem. This power sharing problem requires the use of a 5-Watt, 220ohm resister in the NB-H. This resister will prevent the system from staying on when the Nest is charging. 

AC Cooling Wizard

[RenzJ]

Hi Wayupnorthguy,

I appreciate your effort, and thanks for sharing your resolution in the Community — It's a big help!

Cheers,

Renz

[CoolingWizard · Accepted solution]

@Wayupnorthguy ,

What you are describing is a power sharing problem. This power sharing problem requires the use of a 5-Watt, 220ohm resister in the NB-H. This resister will prevent the system from staying on when the Nest is charging. 

AC Cooling Wizard

[Akrugolets]

Hi. Thanks for the info. 

My Navien boiler has R W C connection. There is no G connection. I have the resister connection in parallel between  W & C but not sure how to connect the rest.

[ms82xp]

I only used R and W on mine. With the resistor between W and C on the Navien board. 

[Akrugolets]

Didn't work for me. Nest won't configure complaining that it needs C for power.  I'm thinking to connect C from Navien to C on Nest but not sure if that could damage Nest. Any thoughts? Appreciate. 

[Wayupnorthguy]

That, may indeed work for 3 wire installations.  Are you sure that works in a 2-wire solution (no - C-wire)?
How much do you know about the controller on the NCB-H boilers?  How do you explain the boiler turning on with only the Nest mounting ring installed and Rh-W attached?

[CoolingWizard · Accepted solution]

Yes indeed it does. Since your furnace is so sensitive to the 200mA  charging current, you need to have the resistor to pull the current to the ground and stop the transient back flow.

AC Cooling Wizard

[Wayupnorthguy]

I will try this once I get a 5w resistor.  If this works I would recommend that nest update their knowledge base.  Neither tech support from Navien or Google offered this as a solution.  On some threads...the "pros" were offering up only the relay solution for 2-wire.

[Wayupnorthguy]

So Google Nest also makes a power connector that looks like it may be nothing more than a resistor in a fancy package.  I tried to get information on that device to see if it would solve my issue but the support specialist couldn't answer any technical questions.  I ordered one just for testing and to try and help the community answer questions.  It will be here in a couple of days.

[Wayupnorthguy]

Just an update.  I installed (one at a time) the Nest Power Connector and the 5w,220ohm power resistor.  Both solutions work with a two wire installation.  The advantage of the Nest power connector is the ease of installation.  The advantage of using a resistor is one of $$$.  You could literally fix 50+ units using a resistor for the cost of one power connector.

[pauloutlaw]

Dealing with same issue now. I have a Google Nest Power Connector I will be attempting to install. Can you confirm that the Power Connector connects to the C, R, and W ports on the Navien, and then only the R and the W connect to the Nest Thermostat (only two wires to thermostat)? 

[Wayupnorthguy]

Yes,  That is correct.  

[ms82xp]

Can you post a pic of how you added the resistor?

[CoolingWizard]

@ms82xp , the location depends on which wire the nest is using for power common. The W or the Y. And it must be a 5 Watt resister rated for 220 Ohms.  

The AC Cooling Wizard

[ms82xp]

Am I adding it in the Navien? Wayupnorthguy wrote he added it on the Nest…

[Wayupnorthguy]

Yes.  At the thermostat connection.  On mine it's bottom right hand side of the unit internal..

[CoolingWizard]

You should not add it  into the Nest Backplate as the backplate will be damaged you attempt insert two condoctors.  You need to get the 5Watt 220Ohm resistor and place in the indoor air handler control board. 

The AC Cooling Wizard