Question regarding using an external 24V power supply to provide power to a smart/WiFi thermostat. I’m not clear on why both Honeywell and Emerson/Sensi docs state that the use of an external power supply only works “with heat only systems.”

I live in a condo with what I believe is referred to as an “air handler” unit in a closet. The unit is a cooling only system. 1 coil. A compressor, fan, and control board. The building supplies water flow through the unit as well as a condensate drain. FYI the heating for the condo is supplied by the building via discrete baseboard.

Currently my basic AC unit thermostat uses 3 wires: Red, Green, and Yellow. The thermostat is set to “Cooling Only” in the advanced setup menu. Red wire is connected to R. Rc is not used and the terminals are jumped. Green is fan and connected to the Green terminal. Yellow is cooling and connected to the Y terminal. The system has been running in this configuration for years with no issues. BTW - the AC unit's internal control board does not have a "C" terminal.

Now I want to add a smart thermostat that requires a C wire. I understand the concept. What I don’t understand is why the use of an external 24V PS wouldn’t work here.

For example - if I disengage the R terminal(s) jumper - I will now have an open Rc terminal and a single C terminal on the proposed WiFi thermostat. Remember the system will never call for heat. So - If I leave the thermostat config. wiring for cooling as described and then connect one lead from an external PS to the open thermostat Rc terminal and connect the second PS lead to the thermostat C terminal … why would this have any adverse effect on the AC functionality?

Basically what I feel like I’m doing is simply adding a second/discrete constant 24V power source to the thermostat that has nothing to do with the AC configuration. In essence I'd be nulling the necessity for using batteries to power the thermostat and instead will supply constant power via the external PS. They say this would only work with heat only systems? I don’t understand why.

Maybe my AC only unit is a rarity hence the lack of documented support for this wiring configuration? Not sure.

Anyway - I’m really curious to know if I’m off track with my hypothesis.


  • Can you provide links to the manuals that say you can use an independent power supply but only in heat-only mode? You can't connect the A/C's red wire to Rh. That won't work. You can connect it to Rc, and remove the jumper, and TRY to connect your power to Rh and C. If the manual actually says that won't work, it probably won't but who knows. You could instead get a C wire adapter. Those are cheap and easy. That your board doesn't have a C terminal isn't a problem .... you'll have to cut a wire somewhere or find it somewhere, but it definitely has a C.
    – jay613
    Commented May 11 at 23:34
  • Thanks. I'm aware of the C- wire adapter solution. I'm just not grasping why any combination of using R and Rc in my example above wouldn't work w/ external power supply if the thermostat is set to work with a cool only system. To be specific - I would remove jumper, connect red wire to Rc and use Rh + C for the external power adapter. Have a look here right under the "Compatibility" heading: sensi.copeland.com/en-us/support/… ... as far as Honeywell - I spoke directly to a tech via their telco support line.
    – Paul
    Commented May 12 at 0:05
  • Tx for the link. Apparently at least some thermostats take their internal power across Rc-C. The prevalent situation lacking a C wire is an old old furnace with a two wire thermostat. So makes sense to design for that scenario by using the unused Rc for power. You can't do that. When the thermostat calls for cooling it will connect Rc to Y. Not Rh.
    – jay613
    Commented May 12 at 0:35
  • I see. Since I have an available unused wire at thermostat through the wall and leading into AC unit's control board area - I see a step down transformer and relay. I'm guessing one side of transformer is 120 line voltage coming in and on opposite side is 24V coming out. In theory can I tap into 24V common output from transformer - splice in my unused wire ... and use other end located at thermostat as a compatible C? In essence I wouldn't need to use discussed C-wire adapter kit. BTW I wouldn't attempt to do this w/out supervision by a tech. I just want to make sure I understand the concepts.
    – Paul
    Commented May 12 at 0:57
  • Can you post photos of the wiring at your air-handler please, including any wiring diagram that may be present? Commented May 12 at 6:08

1 Answer 1


Question 1: Why can't you use an external 24V PSU to power the thermostat in your cooling-only application? Because, extrapolating a little from the thermostat's documentation, it derives its internal power from the Rc-C terminals. In a heat-only application you can use Rh-W for control and Rc-C for power. But in a cool-only application, the Rc-Y terminals are used for control, and there is nowhere else you can apply external power. You cannot use Rh to control cooling, that just isn't how the thermostat works.

Question 2: What should you do? Based on the photos and comments you added to the question, you have spare wires to the thermostat. You can easily connect a C wire, and don't need a new PSU. It would be better to use the blue wire if you can rescue it, because that's a bit of a standard, but otherwise use the white one. In the A/C unit, connect it here with the blue wire on the 24V side of the transformer. Either end of the blue wire or cut it and splice it all together. Be careful most of the wiring and unprotected terminals inside this unit are high voltage, apparently 208/240V. This is a very dangerous environment. Test the voltages, make sure you know I'm right (don't just believe me), turn off the breaker before touching anything and test again after. If you don't know how to do these tests, hire an electrician to do this.

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  • Great. Thanks so much for this. I appreciate everyone who helped me understand what I have going on and what I need to do.
    – Paul
    Commented May 13 at 14:45

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