If you, like me, heat your home exclusively with a gas fireplace or a free-standing gas stove, and you would like to control your system using a Nest thermostat, then this is the Instructable for you!
The only reason that the nest thermostat wont work on these types of systems is that they do not run enough voltage though their thermostat wires to power the Nest. So the trick is to power it another way.
First off, if you have an air conditioning system, then the Nest should be able to draw the power it wants from that and you should be good to go. If you do not, then you must trick the Nest into thinking that you do.
Just connect the thermostat wires as shown above and connect a 24 AC transformer (see next step) to provide the power. The positive lead from the transformer should go to the Rc terminal and the negative should go to the C terminal. In this way, you can trick the Nest into powering itself off of a transformer without putting any unwanted voltage over your thermostat wires.
These types of transformers are commonly available as sprinkler system components at your local hardware store.
I have been using this setup to reliably control my gas stove for over a year now with no issues, but of course, neither I nor Instructables can warranty your Nest or your stove, so hack at your own risk.
Step 1: 24 V AC Transformer
These are commonly available at your hardware store, usually in the sprinkler systems section and will provide the right voltage for your Nest.
I hid mine in the closet behind the wall where the thermostat lives and ran the wires through the wall for a clean setup.