Aprilaire humidifier continues running even when humidity setpoint is reached

Safety first: Turn off furnace power before working on control wiring.

• Check actual humidity: Use a separate hygrometer to verify room humidity. If it is significantly higher than the setpoint, the control is not regulating correctly.
• Inspect wiring: Ensure the humidistat/control wires are not shorted together. Remove the wires from the humidistat and see if the humidifier stops; if it continues, the short is elsewhere.
• Test humidistat: For mechanical models, use a multimeter to check continuity across the control terminals while adjusting the dial. It should open when set below room humidity.
• Replace control: If the humidistat remains closed or a digital control does not respond to setpoint changes, replace the Aprilaire humidistat/digital control.
• Verify outdoor sensor: For automatic controls, ensure the outdoor sensor is reading correctly; a failed sensor can cause over-humidification.