Why Is My Water Pressure So Low? (LA Home Diagnosis)

Step 1: Test the Pressure Actually Coming Into the Home

A $12 pressure gauge that threads onto any hose bib tells you what DWP is delivering. Normal residential pressure for LA is 55–75 PSI. If you're at 35 PSI with a gauge on the outdoor hose bib closest to the main, the problem is DWP-side or at the pressure regulator. If you're at 70 PSI at the hose bib but 25 PSI at a bathroom faucet, the problem is internal to the house.

Step 2: Check the Pressure Regulator (PRV)

California building code requires a pressure regulator (PRV) on any home where DWP supply exceeds 80 PSI — which is most of LA. The PRV is a brass valve usually located at the main shutoff at the front of the house. They fail in two predictable ways:

• Stuck closed / narrowed. The internal diaphragm fouls with mineral deposits (LA hard water strikes again) and drops supply pressure to 20–35 PSI. Whole-house pressure crashes.
• Stuck open. The PRV no longer regulates. Pressure creeps up to 100+ PSI. You'll see other symptoms: T&P valve weeping on the water heater, occasional hammering in pipes, shorter fixture life.

PRV replacement is a $250–$450 job — cheapest fix on this list by far. If your home is 15+ years old and still has the original PRV, replace it prophylactically.

Step 3: Consider Galvanized Pipe Narrowing

This is the most common cause of "pressure that's dropped over the years" in SFV homes. Galvanized steel supply lines narrow from the inside as rust and scale accumulate. A 3/4" pipe after 60 years often has 3/8" of actual open passage. Flow at fixtures is dramatically reduced even though the supply-side pressure is fine.

Signs it's galvanized narrowing:

• Home built 1920–1965.
• Pressure drops when multiple fixtures run (the narrow pipe can't supply flow fast enough).
• Rust-colored water on first flow.
• The gauge at the hose bib reads 65 PSI but the shower dribbles.

The fix: whole-home repipe. Patching individual narrowed sections doesn't help — the system is uniformly narrowed. This is when pressure complaints become repipe quotes.

Step 4: Check the Main Shutoff and Meter Valve

This one sounds silly but happens: the main shutoff at the house isn't fully open. Usually after a repair or a gardener accident. Opens with a quarter-turn. Sometimes the DWP meter valve is also partially closed — which DWP will open for free if you call them.

Step 5: Look for a Slab Leak or Hidden Supply Leak

A supply leak under the slab or in a wall cavity bleeds pressure continuously. Symptoms: pressure normal when nothing is running but drops rapidly as soon as any fixture is on; water bill trending up; sound of running water when everything is off. We'll diagnose slab and hidden leaks before any demolition starts.

Rare But Real: Fixture-Level Issues

Sometimes the pressure complaint is fixture-specific:

• Clogged aerator or shower head. Mineral buildup on the screen. Soak in vinegar or replace. $3 fix.
• Failed angle stop. The shutoff valve under a sink has corroded internally and is partially restricted. Replace ($85–$150).
• Failed cartridge. A modern single-handle faucet that's 15+ years old may have a scaled cartridge. Replacement restores flow.
• Clogged fill valve on a toilet. The toilet takes forever to fill — tells you the angle stop or the fill valve itself is restricted.

If only one fixture is weak, start here. If every fixture is weak, go back to PRV / galvanized / main shutoff / slab leak.

Realistic Diagnostic Flow

1. Buy a $12 pressure gauge. Test the outdoor hose bib closest to the main.
2. If below 45 PSI: suspect PRV or main shutoff. Call us or replace the PRV.
3. If 55–75 PSI at the hose bib but weak inside: look at galvanized status. Home is pre-1965? Probable galvanized narrowing. Quote a repipe.
4. If 55–75 PSI at the hose bib, home is post-1970, one fixture is weak: fixture-level issue. Check aerator, cartridge, angle stop.
5. If pressure drops only when fixtures run, water bill rising: call for leak detection.