If every faucet, shower, and toilet fill in your house has gone weak at the same time, the problem is upstream of your fixtures. Something between the city main and your plumbing is restricting flow. The three most common culprits are a failing pressure reducing valve (PRV), a partially closed main shutoff valve, and buildup inside aging galvanized pipes.
Check the Easy Stuff First
Before calling anyone, walk to your main shutoff valve. It’s usually near the front of the house, in a utility closet, or at the water meter box at the curb. If it’s a gate valve (round wheel handle), make sure it’s turned fully counterclockwise. If it’s a ball valve (lever handle), it should be parallel to the pipe. A valve that’s even a quarter turn closed can cut pressure noticeably across the whole house.
Also check whether your neighbors have pressure. If they do and you don’t, the problem is on your side of the meter. If they’re weak too, call your water utility first, because main-line issues are their responsibility to fix.
The PRV: Most Likely Culprit After 10–15 Years
A pressure reducing valve is a brass fitting, usually installed where the main line enters the house, that steps down incoming pressure (city mains can run well above 80 psi, sometimes over 100 psi) to a safe residential level, typically 50–70 psi. They wear out. The diaphragm inside hardens or gets coated with mineral scale, and flow drops.
Signs it’s the PRV:
- Pressure was fine for years and dropped gradually, or dropped suddenly after a period of high sediment or debris in the water
- A pressure gauge on a hose bib shows under 40 psi (you can buy a simple gauge at any hardware store for around $10–15; it threads onto an outdoor spigot)
- Hot and cold are both weak, not just one
PRVs are adjustable, but do not touch the setting without first measuring your actual pressure with a gauge. Increasing the PRV without knowing your starting point can push the house above 80 psi, which stresses pipes, water heaters, and appliances. Have a licensed plumber confirm the reading and handle the adjustment or replacement.
Galvanized Pipe: The Long-Term Buildup Problem
Galvanized steel pipe, common in homes built before the 1970s, corrodes from the inside out. The rust and mineral scale don’t just weaken the pipe, they narrow the interior diameter over decades until water can barely push through. You might have full-diameter pipe at the street and a fraction of that opening by the time water reaches your fixtures.
How to tell if this is likely your issue:
- The home is older, pre-1970 construction
- Water discoloration (brown or reddish tinge) when you first run a faucet
- Pressure has been declining slowly over years, not suddenly
- Hot water pressure is similar to cold (if hot were weaker, the water heater would be a more likely culprit)
There’s no fix for heavily corroded galvanized pipe short of replacement. Partial replacements on accessible sections can help, but if the main runs inside walls or under a slab are galvanized, a licensed plumber will need to assess the full scope. This is not a DIY job.
How a Plumber Diagnoses It
A plumber will typically start by measuring static pressure at a hose bib with a gauge, with all fixtures off. Then they’ll measure pressure again while a fixture is running. The difference tells them whether the issue is restriction (a blockage or PRV) or supply (main pressure low).
From there they’ll check:
- PRV age, condition, and adjustment range
- Main shutoff position and valve condition
- Visible pipe material and any sections they can inspect
- Water heater inlet if hot-only pressure is affected
If the pipes are galvanized and old, they may recommend a camera inspection or pressure test to understand how much restriction exists before recommending repiping.
What You Can Safely Do Yourself
- Check and fully open the main shutoff valve
- Test static pressure with a hose bib gauge (around $10–15 at a hardware store)
- Run a faucet and note whether hot and cold are equally weak or only one side
- Check if the water softener or whole-house filter has a bypass valve and try running with it bypassed temporarily (a clogged filter housing can kill whole-house pressure)
What to leave to a licensed plumber:
- Adjusting or replacing the PRV
- Any work on the main shutoff or meter side
- Pipe replacement, soldering, or anything behind a wall
- Diagnosing whether a pressure drop is structural or a supply issue
When to Call a Licensed Plumber
If checking the shutoff valve and running a hose bib pressure test doesn’t point you to an obvious open valve or clogged filter, the next step is a licensed plumber. PRV replacement and galvanized repiping both require a permit and licensed work in California.
Before hiring anyone, verify the contractor holds a C-36 Plumbing license at cslb.ca.gov. Low whole-house pressure almost always has a clear mechanical cause, and a licensed plumber can pin it down with actual pressure readings and a look at the pipe condition, which is more reliable than guessing at settings.