Aprilaire 4231 Replacement Humidifier Yellow Orifice
These are the tiny nylon components which, accurately meter the flow of water into your humidifier.They allow a specific amount of water to go into the humidifier, some of which is evaporated and some which performs the important flushing action. This keeps the Water Panel clean and reduces maintenance to a minimal amount. For use with models 400, 500,  550, 550A, 558, 560, 560A, 568 and 600.