The Aprilaire 4021 red orifice regulates the correct amount of water onto the water panel within the humidifier. The orifice is installed on the inlet side of the feed tube, which is attached to the outbound side of the solenoid valve. Over the course of time, chemicals within the water, as well as the flow of water will enlarge the opening, causing excess water to pass through the orifice. This leads to wasted water going down the drain, or potentially causing water to overflow and leak into your furnace. For optimal performance, the orifice should be replaced annually.