Low water pressure in shower only — San Francisco, California

When water pressure drops in only one shower while the rest of the house has normal pressure, the problem is almost always isolated to that fixture rather than the home's main supply. The most common culprit is a clogged showerhead — mineral deposits from hard water build up in the small spray holes and on the internal flow restrictor. Unscrew the showerhead and soak it overnight in white vinegar, then scrub the holes with an old toothbrush. If pressure improves but is still weak, the flow restrictor inside (a small plastic disc) may need cleaning or replacing. The next suspect is the shower valve cartridge — these contain small ports that clog with sediment, especially after city water main repairs that stir up debris. Cartridge replacement is a 30-minute job for a plumber and runs $150–$350 including parts. Single-handle thermostatic valves have additional check valves and filters that may need cleaning. If multiple fixtures lose pressure simultaneously, the issue is the main pressure regulator and you need a plumber to test and replace it.