Rust stains or water marks inside the furnace cabinet or on the burner area.

Safety: Turn off power and water supply to any connected humidifier before inspection.

Steps:

• Inspect humidifier: If a bypass or powered humidifier is mounted on the furnace, check for leaks at the water line, drain, and distribution tray.
• Check flue connections: Look for water trails from the vent pipe into the furnace. Ensure vent joints are sealed and properly sloped back to the furnace (for condensing models) or away (for non-condensing, per manual).
• Clear condensate drain: Flush the condensate trap and hoses as described in the condensate leak section.

When to call a technician: Persistent internal moisture can damage the heat exchanger and controls. A professional should evaluate venting and humidifier installation.