How is an air handler's fan controlled if its control board doesn't have a G terminal?

Home Improvement Asked on January 5, 2022


I have a 2-stage cooling heat pump unit from Bryant, installed ~2006 (central Florida):

  • Condenser model 698BNX048-B
  • Handler model FE4ANB006

Shortly after moving in ~5 years ago, our evap coil was freezing over when cooling and the OEM Bryant thermostat was determined to be the issue. The home warranty company covered the cost of getting a new thermostat put in. The original Bryant thermostat (model SYSTXBBUID01-A) is a communicating unit. This thermostat was replaced by a basic Honeywell unit, which as I’ve recently come to learn, doesn’t allow for any of the advanced capabilities of the system (e.g., 2-stage cooling).

The thermostat wiring was changed from the original 4-wire communicating-style setup to a "standard wiring" scheme (R/C/Y/W/etc.) Ideally I could reinstall an original Bryant thermostat, restoring all its features, but those run ~$800 new (or ~$400 used on eBay); I don’t see myself spending that on a 15 year old unit. Instead, I’m looking into other (cheaper) options, hoping I can at least restore 2-stage cooling at the compressor, which is currently running 2nd stage around the clock.

"standard wiring" schematics I’ve seen online align with how our system is wired (again, no longer communicating style). We have 8 wires from both the thermostat and condenser:

  • Brown (unused everywhere)
  • Green (connected to G terminal at thermostat, but unused everywhere else)
  • Red (connected to R terminals at thermostat, handler and condenser)
  • Blue (connected to C terminals at thermostat, handler and condenser)
  • Orange (connected to O terminals at thermostat, handler and condenser)
  • White (connected to W or W1 terminals at thermostat, handler and condenser)
  • Yellow (connected to Y or Y1 terminals at thermostat, handler and condenser)
  • Black (connected to Y2 terminal at thermostat, but unused everywhere else)

Air handler board CEBD430510

CEBD430510 board

  • Note: no G terminal (what I’m trying to understand)
  • Note: HUM terminal is unused

Air handler board wiring diagram

CEBD430510 wiring diagram

Condenser board CEBD430439

CEBD430439 board

  • Note: A, B, C, D terminals are unused

As mentioned, 2-stage cooling is not currently supported with our basic Honeywell thermostat. Our condenser board does not have the Black wire connected to a terminal; instead the Y2 terminal has a jumper wire from Y1 to Y2. Here is our condenser board’s current wiring:

CEBD430439 2-stage jumper wire

  • Note: Black wire is not connected to condenser terminal, but has continuity from thermostat’s Y2 terminal

Restating my goal of restoring 2-stage cooling at the compressor, I thought this would be as simple as:

  1. Purchase and install a new thermostat with 2-stage cooling capability
  2. Remove the brown jumper wire between Y1 and Y2 at the condenser
  3. Connect the floating Black wire to the Y2 terminal
  4. Save money & extend compressor life


For the setup above, how is the blower fan in the air handler being controlled? Everything I’ve read talks about air handler boards having a G terminal to control the fan, but my air handler doesn’t have a G terminal and the green wires from the thermostat and condenser are unused.

My current assumptions are that:

  • Since the fan runs in both cooling and heat modes, the handler must know to start the fan when it has a signal from either Y or W terminals. If that’s the case, how would the fan know to turn on in a 2-stage cooling scenario where Black wire is energized?
  • Any time the Black wire is energized, the Yellow wire must also be energized (controlled via thermostat) — this would allow the fan to come on. Energizing the Black wire has no impact on the air handler, only the condenser/compressor via the Y2 terminal.

I know what they say about assumptions, so I’m looking for some more informed feedback. Thanks in advance!

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