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.

3 People Made This Project!

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56 Discussions

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Krzy56

13 hours ago

Recently I purchased two Nests of 3th generation and while connecting it and setting up in my primary home in Mississauga, Ontario, Canada, installing it in my recreation house at the Georgian Bay (some 120km of north), was a bit of challenge. To heat up my cottage I have there an old style natural gas burning stove with logs. Our fully winterized cottage serves during skiing season, but going there was always asociated with way too long time to warm it up. So Nest appeared a perfect solution. But the stove does not use any external power to turn on the burner. But Nest needs some power to operate. That was easy. 24 Volt AC adapter (in fact, transformer of few wats, i.e. 15Watts) was waiting few years in my garage. Stripped wires and connected them to the proper terminals of the Nest's base. One wire to Rc and other to C. Since this is AC (Alternative Current) it doesn't matter which wire where.

Bigger problem was with only two wires coming from my stove. Voltage measured accross them was 0,570V DC (or 570 milivolts). Little exploration of internet has shown that problem can be solved with needs to use additional relay. I was wondering. Really? Lets take a little look into the Nest.
I was studying manual of Nest to find solution. One of the advise was that Nest isn't suitable for milivolts operating devices.
I did installation of the Nest simulating real conditions. Nest has started as expected. Now I could simulate installation of Nest working with different devices, which allowed me to check results of turning on and off in variety of configuration.
Important conclusion comes after. Nest does not have any relay inside which would be turning on/off gas in the stove, as the old thermostat did.

If this is not relay that means it must be a transistor. Hence, you don't need ANY additional relay. Again - ANY. That 570 of milivolt DC comes from thermocouple inside the stove. That is further used to turn on gas inside of stove, but first must be turned on by our thermostat. Our Nest.

Only what you need is to place proper wire to proper terminal. In my case I used W1 and Rh. How to check which wire goes where? Since we are dealing with milivolts of DC (Direct Current) and transistor inside of Nest as the electronic switch, POLARITY is playing here the fundamental role. So, if at random placed wires is not resulted with turning on your stove, simply flip them and you shall have positive result.

One I must say. THIS IS SOLUTION FOR MILLIVOLTS OPERATING DEVICES ONLY. And this is absolutely safe for both, your Nest and stove. And is absolutely compatible, but 24Volt AC transformer/adapter must be the source of power for Nest only. Make sure during installation do not connect AC wires with any of those milivolts carrying wires.

I am writing my post in the cottage which was waiting warm as never before the Nest era. (dated Jan. 19. 2019) Cheers!

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goo2

Question 10 days ago

I set this up but recently had a power outage and the nest thermostat stopped working showing the wiring diagram and a link to tech info. Should this setup still work off the nest battery?

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msenn77goo2

Reply 10 days ago

When AC power is lost the Nest will show an error code. That is normal and would happen whether the device is powered by your HVAC system or using this power adapter "hack". This "hack" is basically invisible to the Nest and the battery that powers the Nest for a brief period of time during an outage is no different than the battery in any other device you have in that it has a limited amount of storage capacity and a limited life span. I would check the appropriate error code here: https://nest.com/support/article/What-do-Nest-s-error-codes-mean. Unfortunately, you probably won't get any tech support from Nest if needed because i'm almost 100% sure they won't officially support this "hack".

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walterw66

Question 4 weeks ago on Step 2

I have a thermostat hooked up to my gas fireplace
next to the thermostat is an on/off type light switch that controls the power to the fan separately
If I turn the fan switch on , the fan will not start working until the fireplace has heated for a few minutes. It will turn off if the fireplace cools off.

considering that the fan is thermally controlled and that there is a separate power toggle for the fan; how do you think i should connect power to the Nest E third generation?

thanks

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msenn77walterw66

Reply 4 weeks ago

considering only the information that you've provided first i would probably just recommend that you remove the light switch that controls the fan. i cannot conceive of a situation where you would want to use that as i imagine that it could seriously damage the fireplace if the unit was run without the fan due to overheating and ultimately could result in fire and injury or loss of life. secondly, i don't think this setup would make any difference in how the Nest (or standard thermostat) would be connect or function with your fireplace as the switch is not interrupting anything that controls the flow of gas, ignition, etc.

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redheelerdog

7 weeks ago on Step 2

Anyone know if there is a solid state replacement option for the White Rodgers 90-370 184-916 relay? Where I have mine installed the relay switch is rather loud? Thanks

Fan Relay White-Rodgers rbm type 184.JPG
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redheelerdogmsenn77

Reply 7 weeks ago

So the SSR numbered inputs and outputs are the same as the White Rodgers?
For the SSR connect 2 and 4 to my gas stove, and 3 to W1, and 1 to B&W AC adapter ? (like diagram attached) Thanks for your help!

Nest Diagram.jpg
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redheelerdogmsenn77

Reply 7 weeks ago

Sorry to be a PIA, does that relay need 80V minimum to switch? Will the 24V PS be enough for it to work. Thanks again.

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msenn77redheelerdog

Reply 7 weeks ago

oh wow. it appears i am batting zero -- i have been struggling with the stomach flu the last couple of days. yes, it probably won't switch at 24v so you'd need to find a relay that works at the 24vac supplied by the adapter that powers the Nest. in hindsight, this may have been another reason why i went with the electromechanical relay.

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redheelerdog

8 weeks ago on Step 5

Hey msenn77, thanks a ton for your post and the Nest Tstat info. I have a millivolt Quadrafire Hudson Bay gas log stove. I applied your instructions to a Honeywell Lyric T5 wifi thermostat. Everything works great.

I also used NathanL118's power jumper info, and the diagram by JaredT37.

Excellent info, thanks a ton!

-redheelerdog in Montana

1 reply
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TomF151

Question 2 months 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
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msenn77TomF151

Reply 2 months ago

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

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TomF151msenn77

Reply 2 months 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 2 months 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 2 months 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 2 months 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......