Wiring 2 wire milliamp system and 2020 Nest

[Spar]

This is about controlling a gas fireplace with a millivolt gas valve and automatic fan. The pilot is always on. So all I need is on/off in response to set point.

I'm looking to replace an existing 2-wire thermostat (Acumen model K) with a new Nest. I have a 24 VAC transformer to provide charging power. From the geniuses on YouTube I've absorbed that I should attach the the transformer wires  to C and R on the mounting plate. Since we're dealing with AC, polarity is not an issue. 

Some sources indicate the thermostat (millivolt) wire should go to W and R, that is, two wires in R.

Questions 

1. Any missing or incorrect assumptions?

2. Do I need a relay to isolate the Nest?

Thanks!

[johnCNA · Accepted solution]

The C terminal is normally wired as a common in systems.  If it were DC voltage, it would be "ground".  Or if you're versed in A/C wiring, think of R as Hot, and C as Neutral.

Thermostats are a simple switch which connects R to W when heat is called for.  W feeds 24vac to the high side of a furnace board relay, the other side of the relay is connected to C.  You would need two terminals on the gas valve; W and C.  I have no idea of what current level is being used though, nor how much current the gas valve would require.  Personally, I'd use a low-current 24v relay to switch the gas valve, just to be sure I didn't overload the Nest.  But yeah, it could work.

[johnCNA · Accepted solution]

The C terminal is normally wired as a common in systems.  If it were DC voltage, it would be "ground".  Or if you're versed in A/C wiring, think of R as Hot, and C as Neutral.

Thermostats are a simple switch which connects R to W when heat is called for.  W feeds 24vac to the high side of a furnace board relay, the other side of the relay is connected to C.  You would need two terminals on the gas valve; W and C.  I have no idea of what current level is being used though, nor how much current the gas valve would require.  Personally, I'd use a low-current 24v relay to switch the gas valve, just to be sure I didn't overload the Nest.  But yeah, it could work.

[MelbaDT]

Hey folks,

Thanks for answering to this and for your suggestions, johnCNA. 

@Spar, I just wanted to drop by and ensure everything is covered here. Feel free to reply back if you have more questions. 

Best, 

Melba

[Patrick_Caezza · Accepted solution]

You have to install a relay to control your millivolt system.  A circuit like this will work.

[Spar]

Thanks John and Patrick. Installed the relay today and lo and behold it works. Feeling accomplished.😉

[Ryan_G]

Hey folks,

Thanks for visiting the Google Nest Community. 

Since this thread hasn't had activity in a while, we're going to close it to keep content fresh.

If you have additional questions, feel free to submit another post and provide as many details as possible so that others can lend a hand. 

Hope this helps!

Kind regards,

Ryan