I am battling a Nest and losing at every turn. I have a 2 zone system and was able to install a Nest on the 1st floor by repurposing a wire as a COMMON wire. Unfortunately, the 2nd floor only has 3 wires (POWER - RC/RH; Y - HEAT; W = A/C) and it is on the other side of the house relative to the zone controller in the basement. Instead of running a 24V COMMON / C wire from the basement to the master bedroom on the 2nd floor, can I use an "add-a-wire" product such as Venstar (https://venstar.com/thermostats/accessories/add-a-wire/). I searched a bit and reading it only works with 4 wire applications. I have an outlet right below it so there must be something I can do without running lines through the wall from the basement.
Don't even bother with that goofy multiplexer. Just get a small 24VAC transformer - 10 VA would suffice (common ones are 40 VA, that's fine too).
On the secondary side, connect one of the transformer secondary wires to R. You do this on the Nest's wall plate, you don't even need to install the Nest proper for any of this.
Now, check the phasing of the transformer. Measure AC voltage between the remaining transformer wire and the R terminal (the place you just connected the first wire). Note that voltage, it's gonna be around 24V, because it's coming right off the new transformer. We just need it for a reference: it should be the highest voltage we see.
Now, measure between that remaining wire and all the other terminals on the Nest one at a time. All these should be equal or lower. If any or all of these voltages are higher (you might even see 48V), then you picked the wrong wire in the first step. Switch wires and start over.
Now that phase is correct, connect the remaining wire to C.
Don't connect any other wires from this transformer to anything else. Don't connect C to anything but this transformer.
If someone rewires the furnace, changes the furnace or replaces or rewires the furnace's transformer, I would redo this phasing step in 2-3. The idea is to have these transformers work alongside each other, not "stack" to 48V, which asks a lot of the insulation on the Nest and technically violates the low-voltage wiring rules. (I'm sure it's built for that, but why tempt fate).