Nest Thermostat With Navien NCB-240E

[theMezz]

I have a Navien NCB-240E connected via two wires to a traditional thermostat and all is well.

I wanted to replace that setup with a Nest.

I can run a new wire from the NCB-240E to the Nest.

I'm unsure of what type of wire to use and where the connections should go.

I can follow wiring diagrams - no problem.

Does anyone happen to have that configuration some place or maybe can scribble it down and attach ?

Thanks,
joe@theMezz.com

 

[Jadnashua]

You'd need to find the 'C' or common terminal on the unit. The gauge of the wire would only need to be something like 18-20g. Smaller might even work, depending on how long the run would need to be. To make things neater, you might want to run as much as a 5-wire cable. Then, should you want to add air conditioning, or manually control a fan on the unit, you'd have the wires, but to get power to the Nest thermostat, all you really need is just one more wire.

Functionally, the thermostat is a switch, so when it calls for heat, it connects the two existing wires together. You need the common wire to provide a return path for power.

Check where the thermostat wires are connected to the unit and see if there's a terminal labeled 'C' or common. If you can post a link to the installation manual, or a picture of the wiring diagram (often on the inside cover of the unit), that might lead to the right solution.

 

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[Leonard Bayard]

I have a Navien NHB-150 and have two Nests hooked up to it. I just have the red and white wires going into the nests and haven't had any issues, didn't need the C wire.

 

[Jadnashua]

While some electronic thermostats will work with no common wire, not every one will under all circumstances with all equipment. It depends on how often and how long the system calls for heating (or cooling in the summer). What happens then, is that when power is running through the wire to turn the system on, it siphons off some to recharge the battery in the thermostat. Some do that by shutting the power off momentarily and run it through the battery charging circuit, shutting off power to the device. That will work IF the thing is controlling a relay, as it takes a moment for the relay coil's magnetic field to collapse before the relay deenergizes. But, if the device is using a solid state control board, it can literally cause things to chatter at a high rate, and quickly cause it to fail. I've had that happen. By far, the safest way is to provide the thing a common so it has all of the power it needs. Otherwise, if, say you go on vacation and shut things down, the thermostat never calls for heat, and thus, no current is flowing through the thing, so the battery will discharge. You need current running through the thermostat to be able to draw off some to recharge the batteries. That only happens when it is energizing a relay (solid state or coil-based) at the HVAC equipment, whether that's the fan, the heating, or cooling. Otherwise, there's no path for current, and you cannot recharge the battery.

 

[PeterDux]

I don't think electricians or contractors exactly know with what hardware Nests will or will not be powered without a power wire. When I installed Nests in my house last year I had an old (24 years) 2 wire system for most of the thermostats, and had two thermostats that also ran a separate A/C system. None of them had power wires. Since the power wires would be difficult to run to some of the thermostats we elected to try them without. No problems. I replaced my boiler last Summer (oil hot water) and the electrician warned me that the thermostats may not work. They work fine. One zone in my system has NEVER called for heat since the new system was installed, yet the thermostat stills shows temp and by all appearances is adequately charged.

Of course running a power wire will eliminate all doubt, but all I'm saying is that it may not be necessary.

 

[FishinDVM]

I have a Navien NHB-150 and have two Nests hooked up to it. I just have the red and white wires going into the nests and haven't had any issues, didn't need the C wire.

Can you post a picture of your setup? My navien is not giving enough power to the nest. Nest says 5v. I am curious to see the wiring inside the Navien please??? Thank you, Chris

 

[Jadnashua]

One wire coming from the appliance is 24vac. The other goes through the control board of the appliance to the return or common to complete the path for power. So, with many electronic thermostats that don't use batteries, need that return path to have a complete circuit for their power.

There are ways to siphon off power as it goes through the electronic thermostat, but power does NOT flow unless it is calling for heat, fan, or cooling.

How long the internal battery will hold depends on a bunch of things: how often it calls for heat/cool/fan, how good the WiFi signal is, and how often you happen to walk by and cause the backlighting to turn on in the case of the Nest. The internal timing circuits don't take a lot of power, but you do want the battery to stay charged. That may not happen on all systems so the common is needed to have a complete circuit. The thermostat is similar to a light switch...you are not switching both the hot and return...you are only switching the hot so it can do what it needs to do elsewhere...IOW, you need a complete circuit to charge the battery.

If the appliance you're trying to control has a digital control board and is not directly controlling an analog relay, it may work fine. If it's something like a triac or some other electronic switching device rather than an analog relay, it probably won't.

There are ways to use an isolation relay and a separate 24vac transformer to keep the thermostat's battery charged, but often, it's easier to run a new cable or just common wire.