Unlike a toilet, urinals use very little water per flush (sometimes as little as 0.125 gallons). This low-flow design is water-efficient but creates a specific vulnerability: . Solid waste and sediment sit in the trap longer, allowing build-up to harden.
Professional plumbers use inhibited hydrochloric or sulfamic acid designed for urinals. The water is evacuated, the acid is poured in and left for 8 hours, and then flushed. This dissolves uric scale completely. This is dangerous for DIY because the fumes are caustic and it can destroy chrome finishes. blocked urinal