I am looking at the installation instructions and they all show that the power connector goes back to the furnace/AC control panelIf this is installed just to make sure the thermostat gets enough power to recharge and properly connect to wifi why do I need to go back to furnace for this???

For power only I could use power outlet near the thermostat. In my case I have the furnace/ac in the basement and the thermostat on the main floor. So there is no C wire running between the two floors. From the picture below (my case) I am seeing the adapter interjected between the furnace/AC and the thermostat BUT I do not see what that is adding for the thermostat since the same RWY wires will go back to the thermostat. enter image description here
Are they saying that there is not enough amperage on the RY wires to charge/power up the Nest Thermostat and this is what the power adapter does ?
More about my particular wiring here
Why can't my Trane/Honeywell thermostat be replaced with a Google Nest non learning thermostat?

1 Answer 1


The power connector changed the protocol between the furnace and the thermostat.

The regular protocol is simply shorting R with either W or Y for heating/cooling. This traditionally closes a circuit through a relay that then activates the heating and cooling. However a relay designed to be pulled in for an extended period of time only lets through a few milliamps and requires a certain voltage to be across its own coil. This means that the thermostat only has (delivered voltage - minimal voltage)*(current through coil) power to leach power from in ideal circumstances. More advanced furnaces might be using electronics to sense the short between R and the heating/cooling wire, which means even less current needs to flow and even less power can leached. There is also almost no way for a thermostat to differentiate between these furnaces

The power connector instead uses the thermostat-side R and W/Y wires as a dual power/communication line and uses either powerline ethernet or wifi (or something else entirely) to communicate whether the power connector should short its furnace-side R and W/Y.

The thermostat R will likely be connected straight through so that shorting that with another furnace-side signal wire (for example the W in a heating/cooling furnace) will work properly.

  • Your explaination is pretty much what I assumed it is happening but you seem to be wrong in regards to the W line. As you see in the picture it is still connected straight to the furnace. Do you have any references for this?
    – MiniMe
    Commented Nov 27, 2023 at 11:32
  • 1
    I referred to the non-R wire as W/Y because when it us used for heating only that's the wire being connected. Commented Nov 27, 2023 at 12:31

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