Here's a drain for a condensing furnace in an attic. It is tied into a sewage vent from a bathroom below (against code.) There is no emergency pan below it, so the water dripping down on the ceiling below soaked the wallboard and the cabinet above the sink fell off. There is a rubber tube lying on top of the plastic white semicircle, so the water pours over the edge to the floor.

The white flecks below are water dripping on the wood flooring above the ceiling.