Illinois State Plumbing Practice Exam

Drainage Fixture Units measure the expected wastewater load a fixture places on the drainage system, guiding how large the piping must be. For a water closet, the code table used in this context assigns 4 DFU, reflecting the relatively high discharge involved with a toilet flush, especially for flushometer-valve style toilets common in commercial settings. This higher value ensures the waste line is sized to handle peak flow without siphon or backing up. While some tables may list smaller DFU values for other toilet types (like gravity-feed models), the standard for this material is 4 DFU for a water closet. When sizing a project, you would add 4 DFU for each water closet and combine with DFUs from other fixtures to determine total demand on a branch or stack, then apply any applicable demand factors.