electricalthermostat
I have a fireplace wired up to a switch (simple off/on), and I want to throw a Nest thermostat on there.
It has a black and white wire coming out of the wall. I think the white goes to the W1 and the black goes to the RH?
I may have to run a common wire as well I read.
But is there any risk replacing the switch? Do I have my hook up correct?
You're going to have to take a look inside the furnace, where you should find the control board where all these wires terminate. Each terminal on the control board should be labeled, so just make a list of letters and colors. Then go back to the thermostat, and use the list to connect the wires to the proper terminals.
If you post the make and model (model number would also help) of the furnace, or a quality picture of the furnace wiring diagram (should be printed on the inside of the access panel), somebody may be able to offer a better answer.
See below, if you jumper Yellow to White the heat strips will run in both cool and heat as Yellow is energized in both modes, what makes it a cooler or heater is the O being energized in cool mode Rheems use the B terminals to be energized in heat.
Best Answer
You need both a transformer and a relay to do this
The Nest cannot switch a millivolt system on its own, even when supplied with 24VAC, as it has solid-state output switches instead of mechanical relays. Hence, you'll need both a 24VAC transformer and a 24VAC fan relay in order to control your fireplace from your Nest. The transformer secondary goes to Rh and C, while the relay coil is connected to W and C, and the relay contacts connect to the existing switch wires.