I'm upgrading to a smart thermostat, which requires a C wire. My current thermostat cable has 4 wires, no C wire. The controller in the furnace does have a C connection. The system has both heat and AC.

So, I could replace the cable with a 5-wire cable, and all would be good. It runs through the crawl space, so it's accessible, but I'd rather not have to spend all that time on my knees and belly. I'll do it if there's no good alternative.

The thermostat is a Honeywell T9, and it comes with a C-wire adapter for this kind of situation. 5 wires go to the furnace controller, and 4 go to the thermostat. There's an installation manual that explains in great detail how to install the adapter.

The superficial explanation I've seen about what this adapter does is that it combines the G and Y wires, and calls the combination K. So, if I understand that correctly, there's a single wire, the K wire, that controls the AC and the fan. I can see how the adapter could have some internal logic to recognize that both K and R are active, and turn on the heat and the fan and not the AC.

We run the fan full time; the house needs that circulation. Does that K wire mean that I lose manual fan control? The new thermostat also can run the fan at random times; would I lose the ability to do that?

1 Answer 1


The "K" wire is a Honeywell-specific one that is used only with the Honeywell C-wire adapter and compatible Honeywell thermostats. The T9 is compatible. Everything will work the way it should. You can keep your fan running all the time, or use the T9's nice feature to run the fan randomly, or, since it's a smart thermostat, you can set up rules in your smart home hub to run the fan only when Jupiter and Mars are both visible, or when there is a big red bus parked in your driveway, or whatever you want.

The Honeywell adapter is "one more thing to go wrong" .. and note, it's a little harder to diagnose problems. There is signalling on the K wire so you can't just use a voltmeter to look for 0 or 24 V and for continuity. There is more going on and I don't know if it's even documented.

If I could run a new cable without breaking walls that is what I'd do.

Note you have some possibly better options:

  • There are C-wire adapters that include gadgets at BOTH ends of the wire (one you install at the furnace, like yours, and another little widget you install at the thermostat location) but they are not Honeywell-specific, they will work with any thermostat.
  • You don't have to crawl around so much. For the price of the Honeywell C-wire adapter you can buy a nice little wire fishing kit and run a new cable. Or you could use some wire hangers and broom sticks, and use the money to buy yourself beer for after the install.
  • Okay, so the thermostat "knows" about the K wire; it's not just combining two things, but there's communication on the K wire between the thermostat and the adapter? Commented Nov 22, 2021 at 20:42
  • 1
    Yes. Well. It is just combining the two things, by using the "K" wire to signal both of them to the adapter. It is "multiplexing". One wire, two signals.
    – jay613
    Commented Nov 22, 2021 at 20:52
  • Thanks for the information. I decided to skip the beer and go with the adapter. It's working just fine. And, for those who are considering an upgrade like this, the instructions that come with the adapter are excellent. Commented Nov 24, 2021 at 14:52

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