Introduction: Nest Thermostat With Gas Fireplace (or Other Millivolt System)

Picture of Nest Thermostat With Gas Fireplace (or Other Millivolt System)

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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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.


AdamS477 made it! (author)2018-01-03

This was perfect. I used this to setup my Nest-E Thermostat with a Napoleon Knightsbridge. It works flawlessly.

AdamS477 (author)AdamS4772018-01-09


My Nest Thermostat voltage is currently at 3.88 Volts, 6 days after installation. (That's great -
according to a Nest Support Rep - as the max voltage is 3.9.)

my voltage did drop to 3.73 on two occasions in the past week, causing
the Thermostat to go into low power mode and disable Wifi. (I was
concerned both the thermostat and furnace would shut down, so I pulled the thermostat off the faceplate and
plugged it into a USB port to charge for about 30min to get the voltage
back up.)

These voltage drops occurred during extreme weather of -10F, when the furnace was running nearly 24 hours in the day.

When does the Nest Thermostat get to charge? Is it 24/7, or when the relay is/ isn't activated?

A related note, the wall the Thermostat was installed on is slightly cooler than the rest of the building, so if I had the Thermostat set to 50 while I was away for an extended time, the Thermostat was actually on a 40 degree wall. I think being on that cold wall may also have caused the Voltage drop. I will likely move it.

AdamS477 (author)AdamS4772018-01-09

I reviewed the 'Power' details in the 'Technical Info' section with Nest further, and confirmed the battery is actively used while the furnace is on. (You can see this by turning the furnace on and off manually via the Nest.) Hence, if the furnace runs 24hr/day, it is not charging as it does when the furnace is off, and could be an issue in the long-term for the battery.

msenn77 (author)AdamS4772018-01-09

Interesting. I tend to be a little cynical at times -- ok, a lot. I read this as the Nest is designed then, primarily, for brand new homes with R-12000 insulation ratings and high efficiency furnaces that will only run 10 minutes a week. :/ All joking aside, I didn't not see any issues with charging or voltage drops and WiFi disconnects in my last home which was built in the 50s nor in my current home built in the 70s -- neither of which have great windows nor insulation and the fireplace does run quite a bit. A couple fo weeks ago when it was -15 outside, the fireplace was on between 13 and 16 hours a day and I didn't notice any drops. Mine is mounted on an interior wall that stays as warm as the ambient temperature. You may consider moving the location of your thermostat to see if that helps.

msenn77 (author)AdamS4772018-01-09

I'm not sure on the voltage drops. I did notice that it seemed like it took a very long time for the Nest to charge fully using this setup but my Nest had been in storage for a while prior to doing this so I wasn't sure the cause. I just read this article which may have something to do with it:

AdamS477 (author)2018-01-03

For the Nest-E Thermostat, does R = Rh?

Also, would there be a method to relay a fan controller to a 120v fan?

Thank you for taking the time to do this! I have been searching all over the web for a holistic guide, as well as talking to Nest and my Fireplace manufacturer to no avail. Bits and pieces were everywhere, not one guide. I'll be sharing this.

msenn77 (author)AdamS4772018-01-03

There are two notes in the Nest E documentation: 1) Note: If you have an R, Rc or Rh wire, you can put it into the R connector on the Nest Thermostat; and 2) IMPORTANT: If you have Rh and Rc wires, you have a dual transformer system. The Nest Thermostat E is not compatible with dual transformer systems, but you may be able to use the 3rd generation Nest Learning Thermostat. If you are running a gas fireplace -- and it sounds like you are -- I'm guessing you do not have dual transformers (i.e.: your gas fireplace is only user for heat). Regarding your second question about the 120v fan control, I will also guess that yes this setup would work because the relay featured here will switch up to 240v. Since you haven't provided a wiring diagram for your setup, I am only guessing that you'd have to use two relays wired in parallel from the Nest to the 1 & 3 terminals to switch the millivolt system that switches the gas valve with one and your 120v fan using the second.

AdamS477 made it! (author)msenn772018-01-09

The Nest-E detected the single wire in R as Power, not heat.
The W1 wire was detected as Heat. (See attached image.)

Is there an issue with that?

yes, my fireplace is only used for heat, and just has an on/off state.
The intensity of the heater is controlled with a manual knob on the gas
unit. I assume that means I do not have dual transformer.

msenn77 (author)AdamS4772018-01-09

This setup is only switching the 24VAC from the transformer so I really don't think it matters what the Nest thinks it's switching or which wire is on which terminal on the back plate. If it's functioning, I believe you're good to go!

AdamS477 (author)AdamS4772018-01-03

I confirmed R (on the Nest-E) = Rh (on the Nest).

AdamP217 (author)2017-12-28

Thank you!

msenn77 (author)AdamP2172017-12-29

You're welcome!

DIY Hacks and How Tos (author)2017-12-16

Great tutorial. I need to get a Nest at my house. I still have the original mercury thermostat.

I have a version 1 one I could sell you.

About This Instructable



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