I have a Rheem RHPL-HM2421JC air handler, a Rheem 1.5 ton heat pump condensing unit, and a comfort control thermostat.

The heat and fan worked well over winter. I can also turn the fan mode to "on" and the fan will run. However, if the system is in cool mode and the fan is set to 'auto', nothing happens. I swapped out the circuit board, but the same problem persist. All the wires in the thermostat seem connected, so i am at a lost for the cause. Any suggestions?

  • Has it ever worked? Do you have a multimeter? Are you comfortable opening, and poking around in HVAC equipment?
    – Tester101
    Commented May 22, 2016 at 19:13
  • Yes, worked great last season, and yes i have multimeters and love poking around
    – Britt
    Commented May 22, 2016 at 19:14
  • 1
    Does the outdoor unit come on?
    – Tester101
    Commented May 22, 2016 at 19:29
  • I'll have to check this tomorrow. I don't this so in cooling mode, but since the heat works, I expect that it does in heating mode... but maybe it is the aux heating coils in the air handler. Will check this tomorrow. Thanks
    – Britt
    Commented May 23, 2016 at 2:22

1 Answer 1


Usually A/C, heat, and fan run the blower at different speeds. It's possible that the speed the A/C uses is dead.

Any easy test, is to swap the motor speed wires connected to HEAT and COOL on the board. Then see if the blower starts with the A/C. If it does, that means the speed used by A/C is dead. Depending on the system, you might have to replace the blower, or simply run it at a different speed for A/C.

WARNING: Don't run the A/C with the wrong blower speed for long, as it could damage the system.

If it's an ECM motor, there may not be a way to select/change the speed manually. Or the speed selection might be adjustable using dip switches, or jumpers.

  • "swap the motor speed wires connected to HEAT and COOL on the board"-- Do you mean on the circuit board in the air handler?
    – Britt
    Commented May 23, 2016 at 2:20
  • @Britt yes, but this only works if it's not an ECM motor.
    – Tester101
    Commented May 23, 2016 at 2:30

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