Nest Thermostat With Gas Fireplace (or Other Millivolt System)

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About: I would rather learn how to do it myself rather than pay someone else to do it for me.

This short tutorial will help you use your Nest thermostat with your gas fireplace or other millivolt type system. I found some other tutorials online but none really seemed to cover everything you'll need so here you go!

Step 1: Purchase a 24v AC Adapter

Your gas fireplace or other millivolt system does not operate using 24v AC like most HVAC systems. You have to supply the 24v using an external transformer. I got mine at Lowes for $7.99 (item number 168261).

Step 2: Purchase a Fan Relay

You will need a fan relay. I got mine at Grainger for $9.05 (item number 1N184). The Nest is going to switch the 24v source and this relay is going to switch the millivolt system in your gas fireplace.

Step 3: Install the Nest and Connect the Wiring

First, disconnect the thermostat wires from the gas fireplace -- you will use your existing thermostat wiring -- and it's always a good idea to disconnect things when working with electricity. Install the AC adapter -- I placed mine behind the wall in the laundry room (very convenient there was an outlet right there for me) and ran the wiring up the wall and through the same hole as the thermostat wire.

Connect one lead from the AC adapter to the RH terminal on the Nest backplate (you will probably have to tin the end of the wire so the Nest can sense there is a wire connected), splice the other lead from the AC adapter to the existing RED thermostat wire and finally the WHITE thermostat wire to the W1 terminal on the Nest backplate.

Step 4: Connect the Relay

Install the relay in or near the gas fireplace. You may need to have some female spade terminals handy for this step if you don't already have them. Put those on all the wires -- two wires from the Nest and the two wires in the fireplace -- first if needed.

Connect your existing thermostat wires from the Nest to terminals 1 and 3 on the relay. Connect the thermostat leads from the fireplace to terminals 2 and 4 on the relay.

Step 5: Plug in the AC Adapter and Connect the Nest

Plug in the AC adapter. Connect the Nest thermostat to the backplate.

If all is well, you should now have a fully functional Nest thermostat to operate your gas fireplace. It took a while to fully charge the battery in the Nest so that I could configure the WiFi network settings and access the thermostat from the app.

2 People Made This Project!

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43 Discussions

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TomF151

Question 20 days ago

msenn77
I am going to attempt this on my Mendota DXV-45 gas fireplace.....I know this seems like a simple question but does my simple schematic look right......The reason i ask is your Nest picture does not either reflect your schematic or i am interpreting it wrong.

Any comments would be appreciated.....Thanks in advance...tfalse99@gmail.com

Nest Fireplace.jpg
6 more answers
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msenn77TomF151

Reply 19 days ago

The first image in Step 4 of my instructable shows the wiring diagram. It matches yours.

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TomF151msenn77

Reply 16 days ago

just hook this up and I'm getting a E72 error on the nest... Any thoughts?can this be overcome by a jumper?

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TomF151msenn77

Reply 15 days ago

Thanks...i was able to get this working using the Rc connectionj instead of the Rh..Do you see any issues with this?...Works great now...

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msenn77TomF151

Reply 15 days ago

Based on the error code you gave me I believe you're running a 3rd Gen Nest and this tutorial was written based on the 2nd Gen. I can only assume there are differences in setup/installation options and function of the thermostat. I will say that if it's working that it's probably fine as the Nest requires the 24v supply to power on, charge and operate correctly so it must be getting that and the only function the Nest serves beyond looking good is to turn your appliance on and off. So, if both of those are occurring and your house hasn't burned down it's most likely just fine. :)

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TomF151msenn77

Reply 15 days ago

yes thanks for your comments it's a third-generation.... One interesting thing is.... I went to Lowes to buy some cabling to dress my final solution up a best... And when I checked my phone/nest application... My fireplace showed E72 offline... As you can imagine I was probably the saddest person in Lowes.... Long story short I ran a jumper from RC-RH and as of now it's back working......

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Boink99

24 days ago

What a life saver you are! I struggled with my Nest for three days trying to get it to control my gas fired wall gravity heater. After I realized my heater was a “millivolt” system (never heard of “millivolt” before), I bought the relay you recommended (I already had the 24 volt transformer). I am not a professional HIVAC, but I can follow instructions. I followed your instructions exactly. Now my Nest and heater are working together like a charm! Thank you again!

3 replies
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TomF151Boink99

Reply 20 days ago

Glad you had success..Can you take a look at my simple schematic then see if that's basically how you wired it

Nest Fireplace.jpg
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Boink99TomF151

Reply 19 days ago

No, that is not how I connected the wires. The primary difference is that you took one
of the leads from the 24 volt transformer directly to the relay. I did not. Here is what I did.





At the NEST:





  • Connect
    one lead from the AC adapter to the RH terminal on the Nest backplate.





  • Splice
    the other lead from the AC adapter together with the existing RED thermostat
    wire coming out of the wall at the Nest.





  • Connect
    the WHITE thermostat wire coming out of the wall at the Nest to the W1
    terminal on the Nest backplate.








You should now have only a RED wire and a WHITE wire coming
out of the wall near the relay. These
two wires are the wires you connect to the relay.









At the relay:





  • Connect
    the RED and WHITE wires to terminals 1 and 3.





  • Connect a
    new set of RED and WHITE wires from terminals 2 and 4 to your fireplace.



Good luck!
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msenn77Boink99

Reply 24 days ago

You're welcome. I'm not an electrical engineer nor an HVAC tech either. I just like figuring things out and sharing when I do. There was no complete instruction for doing this prior to mine. I'm glad it's helped a few people out there who wanted to use the Nest with their fireplace.

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Instructor57

4 weeks ago

This works great with my gas-fired woodstove, so thanks. One thing: I can't figure out how to use the nest feature that will start the fire going early ("Early On") to meet the schedule. I don't have a fan on this unit, so the Nest isn't confusing my system with an air system (which does supposedly enable the Early On feature). My system is also not a radiant system, so the early on type feature that works for that isn't enabled, either. Got any suggestions?

3 replies
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msenn77Instructor57

Reply 24 days ago

"Early On" is a feature of the Nest that turns on the heating appliance ahead of your scheduled time so that the desired room temperature is achieved AT your scheduled time.

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Instructor57msenn77

Reply 24 days ago

Exactly, and my point is that my system, built according to these instructions, won't heat up early. It does calculate how long it will take to heat up the room, but won't use that information to actually turn the heat on early to allow it to come to temperature by the scheduled time. "Early On" isn't engaging. My question is How can I get it to function?

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msenn77Instructor57

Reply 24 days ago

Ah, I definitely misunderstood. I apologize. I don't know what much about my Nest honestly. It works so well that I never have to think about it. I just read some of the following article -- https://nest.com/support/article/What-is-Early-On -- which is more than I ever knew about the Early On feature. I haven't had to troubleshoot mine I set it up and it has just been working.

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AlecY3

Question 8 weeks ago

Thanks for the instructions! I have 6 wires going to my thermostat. Do you know if I could use the yellow, blue, green or brown wires to connect the AC Adapter to RH?
Also, is there a way to wire a blower to the Nest as the fan?

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msenn77AlecY3

Reply 7 weeks ago

This tutorial covers a very specific installation scenario to make the Nest thermostat work with a "standard" gas forced air fireplace a fan with a millivolt control system and an existing two-wire setup. Yes, there are probably dozens of exceptions and variations that could work or be made to work. As I am not familiar with your fireplace or wiring specifically I would need more information to help you out. Do you know the make/model of your fireplace? Does it already have a blower/fan or are you looking to add one? Are all of the six wires you mentioned used for something already or are they just hanging in the wall and available?

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AlecY3msenn77

Reply 6 weeks ago

The fireplace is a Kozy Heat #56701 TRF.
Don't have a blower, but looking to get one.
All six wires are available hanging in the wall.

Thanks!!

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afalzon

7 weeks ago

Thanks for the instructable!
I am in Australia, and the nest thermostats are not officially available here. I managed to get one on eBay but then discovered even though my thermostat looked like it was compatible, I had millivolt system.
After finding this instructable I had a better look at my heater to find these was already a 24vac transformer in it, so all I had to get was a relay. I managed to get one from RS components for about $7.
And now my nest is working!