An auxiliary drain pan for installation vertically below a humidifier. The pan has a base and a peripheral wall around the base and a single opening formed in the base adapted to form an anti-backflow connection with a conduit from the humidifier for directing liquid from the humidifier through the opening and a drainage connector, the anti-backflow connection preventing a reverse flow of liquid from the opening into the humidifier. The pan has a structure for supporting the base and having connecting features for connection with a vertical surface in an installed position, with the base secured in a substantially horizontal position in the installed position.