Rheem Humidifier Control Output Failure

The thermostat is not controlling a whole-house humidifier connected to it; humidity does not increase when expected.

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Safety Warning

This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.

If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.

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Possible Causes

Incorrect wiring to HUM terminals, humidifier powered from the wrong source, failed humidifier relay in the thermostat or indoor board, humidifier configuration disabled in settings.

How to Fix / Troubleshooting

Safety first: Turn off power to the indoor unit before working on control wiring.

Steps:

  • Verify configuration: In the thermostat installer menu, ensure the humidifier is enabled and set to the correct type (powered vs. bypass).
  • Check HUM wiring: Confirm the humidifier is wired to the correct HUM or accessory terminals on the indoor board or thermostat as per Rheem's diagram.
  • Test output: With a call for humidity, measure for 24VAC (or switched line voltage, depending on the design) at the humidifier control terminals.
  • Bypass test: Temporarily jumper the humidifier control terminals (per manufacturer instructions) to see if the humidifier runs. If it does, the control output is the issue.
  • If no output from thermostat/board: The humidifier relay or driver may be failed and require board or thermostat replacement.
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Repair Difficulty

Hard 4/5

Required Part

Humidifier Control Relay
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