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!
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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.