Nest learning thermostat

[pcgneim]

I have a 1978 Honeywell mercury type thermostat with only two wires connected to it.  one red, one grey.  will this work with any of the Nest thermostats being offered today?

[kingtubo]

You can put a multimeter on the wires and confirm if they are around 24v.  Those are the really skinny little wires.  If you have 110v electric wires like the ones in the walls to your outlets, then it won't work.

[GBD · Accepted solution]

@pcgneim in addition to what @kingtubo said, the Nest Thermostat (2020) model is not compatible with systems that don't have a "C" wire. To use a nest thermostat with your 2 wire system, it will have to be the Nest Learning Thermostat or Nest Thermostat E. These thermostats are designed to work without a "C" wire but there will be some systems where the thermostat can't blead off enough power to keep itself charged.

Another thing to keep in mind, is that the old 2 wire mercury thermostats don't care about wire placement so either wire could be the power wire. To operate, the Nest thermostat will require the power wire to be connected to "RH" and the other wire connected to "W or W1".