Nest Thermostat With Gas Fireplace (or Other Millivolt System)





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

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.

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18 Discussions

​Thanks to your post, I have successfully installed a Nest 2nd Generation Thermostat that controls a Heat & Glo Supreme I-30 natural gas fireplace insert. I do have a couple tips that will clarify and help finish the install. When wiring the nest backplate, I put the all black wire from the 24AC V adapter into the Rh terminal, and spliced the black/white dashed wire from adapter with the red thermostat wire (of course the white thermostat wire still goes into the W1 terminal). I was having low battery issues on my Nest, so I ran a jumper wire from Rh to C, and another jumper wire from Rc to the red thermostat wire/black-white wire splice. This quick fix powers the Nest 24/7 and eliminates any chance of the Nest disconnecting from Wi-Fi due to a dying battery. In the setup under Equipment, I chose Gas for fuel, and Radiator for heat type. This is an important tip as I first chose Forced-Air, and I couldn't get the nest to turn on the fireplace. I tried Radiator, and the fireplace fired right up. In my setup, I had access to the back of the Nest as I placed it on a wall of a laundry closet. I didn't want to mess with the wiring under the fireplace, so I put the relay in the closet which was about 15' away. I have the Nest app running on my iPhone, and the whole setup is sweet. Thanks again for the original post/idea, and i hope my tips will help anyone else who attempts this hack.

3 replies

I'm having low battery issues. Can you clarify your set up? How did you implement jumpers? Two wires in the back plate receptacles? Your black and white wire runs to relay post 1?

​Sorry I don't have a picture, but it looks just like the image in step 3 plus the 2 jumper wires as described in my writeup. For jumpers, I just cut a small single wire from extra thermocouple cable w/ 2 wires, stripped a 1/4" from both ends, and simply insert into each terminal from Rh to C and another wire from RC to red thermostat wire/black-white wire splice. For the latter, I wrapped em all around each other and taped em up. Hppe this is clear enough. After 5 weeks, my Nest has never dropped Wi-fi connection.
​Sorry I don't have a picture, but it looks just like the image in step 3 plus the 2 jumper wires as described in my writeup. For jumpers, I just cut a small single wire from extra thermocouple cable w/ 2 wires, stripped a 1/4" from both ends, and simply insert into each terminal from Rh to C and another wire from RC to red thermostat wire/black-white wire splice. For the latter, I wrapped em all around each other and taped em up. Hppe this is clear enough. After 5 weeks, my Nest has never dropped Wi-fi connection.

Thanks for the response. My battery died again yesterday so I'll give your layout a whirl. I diagrammed it. Yeah, I did that.


I have a gas fireplace that uses an RF ultrasonic remote to control the fireplace... fireplace model 738BVN by Miles Industries.
Is it possible to eliminate the ultrasonic receiver and directly wire the Nest (or equivalent ie. Ecobee) thermostat to such a fireplace?

1 more answer

I should add that the receiver is a Mertik Maxitrol G30-ZRRTT (battery pack and receiver) which has 4 wires connected to the valve on the fireplace.
I’m aware that I’ll have to run low voltage wires from the smart thermostat to the fireplace.
Would this setup allow for variable flame control or is this just a binary on/off setup?

Hello, I have two 2005 Quadra Fire Garnet gas stoves, and I was hoping to connect a Nest Thermostat E to each of them. I just came across your tutorial, and it's very helpful, but I do have a few questions since every setup is slightly different.

The Nest Thermostat E has a different back plate with different connections then the standard Nest. The two Garnet's are operated by remotes, with the receiving terminal on the stove being powered by 4x AA batteries. The connections on the unit show a "TH" , "TP", and a second set of "TH" and "TP".

Another box (last picture) contains a single AA battery with 3x connections, a "HV" off the top, and a "Wa" off the top left, and a "Wb" off the bottom left. I believe this is what powers the ignitor.

Could you help guide me through this installation?

1 more answer

I'm sorry for the delay. I have had a couple of busy weeks at work and took a brief hiatus from the internet. I just did a quick Google search for your gas fireplace make/model and found this information in the user manual: "A thermostat may be installed to regulate the Garnet. It is important
to use a thermostat designed for millivolt operation. Do not connect
the heater to a thermostat serving any other appliance. Bedroom
installation in Canada requires this heater to be connected to a
Connect the thermostat wires to the outside valve terminals labeled
"TH" and "TPTH". Turn the manual switch on the control panel to
"OFF"." I have to assume because the user manual does not explicitly state otherwise that you will not be able to use the remote control setup and the Nest simultaneously. Another user here has built/installed his Nest E thermostat using these instructions with success. You will use the W1 and R terminals on the Nest E backplate.

Hi, an HVAC person is telling me I don't need a relay for my Pellet stove, as the Nest just switches it on, and the millivolt thermostat is essentially doing the same thing, any thoughts? I would just need the 24V power supply.

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First, the fine print. If neither you nor your HVAC technician are familiar with some very basic principles of electricity and electrical devices such as the relay, I would strongly suggest against performing this installation or any modifications to your heating and cooling system. Having said that, a relay is simply an electrically operated switch. The device uses one circuit (typically low voltage or amperage) to switch another circuit (typically higher voltage or amperage). Think of it almost exactly like the wall switches in your house where you would represent the low voltage circuit that flips the switch (relay) that turns on the 120V circuit (lights, fans, etc.). The switch is necessary because, while it would probably work at least temporarily, you would not want to have the wires hanging out of the wall and complete the circuit yourself by grabbing both wires and hoping for the best. The lights would probably come on (at least temporarily) but some serious damage would be done to both you and most likely the devices connected to the circuit though luckily the breaker would probably blow before you were dead or the house burned down. In the instance of this Instructable, the two circuits we are talking about are the 24VAC circuit that you need to power/charge the Nest and the millivolt or 120V circuit on which your pellet stove operates is the second circuit. In a typical furnace and central air setup, the furnace provides the 24VAC required by the Nest so no relay is necessary. The furnace would be connected directly to the Nest which steals power from that internal 24VAC source when no heat is called for and uses the circuit to turn on the furnace when it is. Most pellet stoves and gas fireplaces DO NOT have an internal transformer to provide this 24VAC. In the case of your Quadrafire pellet stove and based on the wiring diagram on their website it looks as if your stove operates entirely on 120VAC. You should be able to verify this if you have a multimeter handy. Set the control knob to AC (probably says 200 VAC or V~ or a number close to 200) and touch the leads to the thermostat connection terminals on the back of your stove. If it reads 24V or something very close you should not need the relay. But, I am guessing that it should read either a very low number (millivolt) or around 100-120V. If so, the relay will be necessary if you want to use your Nest thermostat. You can see below that several people have used this tutorial to successfully install and use a Nest thermostat with their gas and pellet stoves and you should have no issues doing so yourself.

Thanks for the quick response. It’s a Quadra Fire Santa Fe Pellet Stove.

I thought the relay was more about protecting the stove not the thermostat. I guess if we don’t know whether the nest isolates the power and switching circuits, it’s hard to know whether not having a relay would be safe right?

I'm not sure I can fully answer the question(s) with the limited details you've provided. First, the Nest requires a 24VAC source to power and charge itself. While the Nest may, indeed, be capable of switching the millivolt supply and/or the 120V source (depending on your pellet stove internals), neither one will power and charge the Nest. I would be a bit hesitant to connect 120V to the Nest because I would be a sad panda if I fried my $250 thermostat. Having said this, I am happy to help so please provide me with your pellet stove make and model number and I'll see what I can dig up for you.

With my fireplace, the fan turns on automatically once fireplace heats up. Will I still need to add the relayin order to hook up the Nest? Thanks in advance

1 more answer

Yes, most likely, as long as your fireplace operates on a millivolt system. The Nest will not operate using only the millivolt source from your fireplace. You need to provide a separate 24v source to power the Nest thermostat. Even though I am using what is called a "fan relay" in the HVAC world as part of the installation, it's not actually used to switch the fan. As noted in Step 2, the Nest is going to switch the 24v source to activate the relay and the relay will switch the fireplace's millivolt system. Honestly, if you are not familiar nor comfortable with some of these electrical and electronic component fundamentals, I would highly recommend that you do not attempt this installation yourself and seek help from a qualified, licensed and insured HVAC technician or electrician.

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

1 reply