Nest (Gen 2) Thermostat With Gas Fireplace (or Other Millivolt System)

Intro: Nest (Gen 2) Thermostat With Gas Fireplace (or Other Millivolt System)

This short tutorial will help you use your Gen 2 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 activate the control side of this relay which in turn will switch on/off 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.

Comments

[filmil_]

Wait, why is the relay switch even necessary? It seems to be possible to separate the millivolt circuit from the power to the Nest device by connecting the AC adaptor to C and Rc, while connecting the millivolt circuit wires to W1 and Rh. This way you don't seem to need the relay. What am I missing?

[JeredF]

The fan relay is necessary because the millivolt valve controller is incredibly sensitive to leakage current, and the thermostat uses a MOSFET (a power transistor, or a sort of solid-state relay) which has a tiny bit of leakage current even when in the "off" state.

With my gas fireplace, this exhibits as the fireplace never turning all the way off when the thermostat is done. This likely also reduces the lifespan of the valve.

[msenn77]

It's been a while since I set up this project here but if I recall I tried it and it sent 24v to the fireplace. You're welcome to give it a go yourself to see if it works with your appliance. Please let me know how what you find out!

[filmil_]

I think your setup sends 24V to the fireplace because the 24V AC adapter is connected in series to the relay control circuit. This is what your wiring diagram shows, and this is why the relay control circuit actuates the relay.

But if you don't connect 24V AC to the control circuit, then (at least ostensibly) it won't be able to do that. I'll see if I can figure this out safely without trying it on the appliance itself, since my appliance is a propane stove, and I don't want to be sending 24V anywhere near propane gas. (In that respect your relay-actuated design makes a lot of sense!)

[sfjava]

FWIW, you could avoid "sending 24V anywhere near propane gas" by locating the relay near the thermostat (and 24 power-source) -- and running only the (millivolt) long switch-wires from the appliance over to the relay (switch) terminals. That way, the only current/voltage running near the heater will be the (very low) one that is generated by the thermopile in the heater-unit already, as intended.

I think the author suggested mounting the relay inside the heater-unit simply so that the relay could be "hidden" somewhere out of the way; and since that's also perhaps closer to a place where one could also plug-in the 24V adapter/transformer.

Hope that helps! Best, --dk

[filmil_]

Sadly in my case it's a propane stove (space heater), not a central heater, so there is no "inside" for the relay to be mounted in.

[adambomb2000]

Msenn77, Thank you so much for posting this. I followed the instructions, and it is working like a charm - Nest Learning Thermostat + Jotul millivolt gas fireplace.

Very clear instructions and a lot of helpful comments from folks like 'GG of the North' to help with some of the hookup details.

Much appreciated!

[benetoh]

Hello and thanks for posting this. I set up the wiring exactly as described and I am getting an equipment error n261. It is telling me I need a stable power source. It does work for about a month at a time and the Google Home App occasionally sends me a notification about the error. Then after about a month it loses its network connection as the battery is low. I do have a different Nest version than is pictured here. I included an image of my backplate.

[msenn77]

The N261 error indicates (basically) that you aren't getting power. What power adapter are you using?

[benetoh]

Thanks forbthe reply. I am using this one:

https://www.amazon.com/Transformer-Thermostats-Compatible-Honeywell-Thermostat/dp/B07MT5VWQN?pd_rd_w=vsGOh&content-id=amzn1.sym.3cb9b85f-43ef-4d4b-94d0-0a8efe2194e9&pf_rd_p=3cb9b85f-43ef-4d4b-94d0-0a8efe2194e9&pf_rd_r=YDYA60BRRN3VBQS5FHVD&pd_rd_wg=mRnOB&pd_rd_r=394a383f-f23b-403f-96b5-c59efd4308c7&pd_rd_i=B07MT5VWQN&psc=1&ref_=pd_bap_m_grid_rp_0_1_i

[msenn77]

Could be the output amperage. The one you've got is 300mA and the one recommended here is 750mA. Or, it is possible you need to use the C connection on the back of your thermostat but for this you'd need a third wire for the W (heat) connection as well which would require modifying the wiring diagram I've posted above.

** Fine print: I don't have a version of your thermostat to test here so I'm throwing out ideas. Continue at your own peril.

[benetoh]

Understood on difference in amperage. I was considering swapping the power supply out as my next troubleshooting step. The thermostat power test does consistently show 30-32 volts but I do not remember the amps or if it even displays that. I will check it out next time I am there.

I have read other articles suggesting connecting one of the power supply wires to the C. What would the other changes to wiring be for this?

[gantho]

I attempted this project in a room with a newly installed fireplace as well as room that never had a thermostat installed before. Here is a diagram of what I did in order to get this to work.

[Wsom]

Thanks for such a great clear diagram and that's what I'll be using for my install (Emerson Sensi Smart-55 thermostat). I've only got 3 wires to my millivolt fireplace system and the thermostat requires the C wire in order to keep the wifi connect (just the Rh and W1 work just fine without the wifi control if there is no C wire). You posted this a year ago and I just wanted to ask if it's still working fine and what was your reasoning for adding the C wire if all you needed was the Rh and W1 to get the nest working. .

[TSchu]

I'm looking at using this method to power either the same thermostat as you (Emerson Sensi Smart 55) or the LUX Geo-WH, both of which can also be battery powered. I've read the wifi rips through batteries ridiculously fast so I'm interested in using the adapter to power the Thermostat. With that said, are you using batteries as a back up in case of losing power? If so, does the thermostat not use the batteries up until power is lost? Also, if ALL power is lost will this still act as a mechanical thermostat and turn the fireplace on at a certain temp? This is all important to me because this is for a vacation home in the woods and the propane fireplace is my primary heat source during the cold winter months. Thanks for any help.

[sfjava]

I've only just now come across this post :) but I think I can help answer your question, TSchu -- given what I think you're asking. :))

If you just need to be able to simply "override" the thermostat that you're wiring-in here -- such that if you lose AC power you can still turn on your heater -- then the easiest thing would be to simply wire an "override switch" across the original red/white wires that go to the heater; i.e. across the output of the relay that makes that connection per this project.

Or, if you want a "no-power thermostat control" (instead of just an override "heat on" switch), add a "non-powered" (i.e. millivolt compatible) thermostat right next the "fancy" one -- which can also close the same circuit (but, say, for a given temp). These thermostats usually have an on/off switch as well (or heat/cool/off) -- so that can be set to "off" when you want only the fancy one to control the heater (i.e. most of the time, when you have power).

Hope that helps!

[msenn77]

My gas fireplace is a millivolt system which means that it does not provide the typical 24 VAC needed to power the Nest thermostat like say most forced air gas furnaces would. I had to add the additional AC power adapter to provide this voltage which was the reason for adding the relay as well. I needed to isolate the 24 VAC powering the Nest from the millivolt circuitry in the fireplace. And yes this system is still working today with the same setup and components mentioned in the tutorial.

[terminationshok]

This was the missing part for me. I understand how to use the C wire now. Thanks!

[amwild66]

Thanks for a great tutorial. I was able to wire my natural gas fireplace to my nest in my sunroom. Life is good!

[msenn77]

You're very welcome! Thanks for posting a picture - it's nice to see the results of everyone's labor!