E15
American Standard
Humidifier Control Circuit Fault
Thermostat displays E15 and fails to control whole-home humidifier
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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
Open or shorted humidifier control wiring, Incorrect HUM/H terminals used, Failed internal humidifier relay, Incompatible humidifier type selected
How to Fix / Troubleshooting
Safety: Turn off power to the furnace/air handler at the breaker before working on HUM wiring.
- Verify humidifier type: In installer settings, confirm that the correct humidifier type (powered, bypass, steam) is selected according to the installed American Standard or compatible humidifier.
- Check wiring: At the furnace control board and thermostat, verify that the humidifier is wired to the correct HUM or accessory terminals as per American Standard diagrams. Tighten any loose connections.
- Inspect for shorts: Look for humidifier wires touching metal or each other. Repair damaged insulation.
- Bypass test (for pros): A technician can temporarily jumper the humidifier terminals to verify the humidifier itself operates, isolating the thermostat as the fault.
Resolution: If wiring and configuration are correct and E15 persists, the thermostat’s humidifier relay may be defective.
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Repair Difficulty
Hard
4/5
Required Part
American Standard thermostat humidifier relay/output
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