Low Water Pressure in Your House? Causes, Checks and Fixes

Low water pressure in a San Jose or South Bay house usually comes from one of five places: a clogged aerator or fixture, a valve that isn’t fully open, a failing pressure regulator, a hidden leak, or old galvanized pipe narrowed by corrosion. Whether it’s one faucet or every faucet tells you which.

This guide covers the safe checks you can do in a few minutes and when it’s time for a licensed plumber in San Jose or the South Bay.

What Does Low Water Pressure Mean?

Start by noticing the pattern. Where it happens, and whether it came on suddenly or slowly, narrows the cause before anyone opens a wall.

The causes in the table come from San Jose Water’s water pressure guide and general plumbing practice. Checked October 2026.

What Is Normal Water Pressure for a House?

There’s no single “right” number, but there are limits:

• Upper limit: 80 psi. The California Plumbing Code (2025 edition, section 608.2) requires a pressure regulator where static pressure in the supply piping exceeds 80 psi, and an expansion tank downstream of each regulator. San Jose Water notes that the rule applies to new buildings, and suggests an existing home above 80 psi talk to a licensed plumber about one.
• Lower limit: 15 psi while water flows. Section 608.1 of the same code calls for at least 15 psi of residual pressure at fixtures, after friction losses, or a tank and pump where the supply can’t deliver it.
• A comfortable target. San Jose Water says a regulator can bring indoor pressure down to about 50 to 60 psi.

Static pressure (nothing running) and flow are different things. A home can read a healthy 60 psi on a gauge and still have a weak shower if corroded pipe chokes the flow, which is why a plumber measures both.

What Safe Checks Can You Do Yourself?

These take a few minutes and need no tools beyond a pair of pliers and an inexpensive pressure gauge. None of them involves opening the water company’s meter box, which San Jose Water asks customers not to do.

1. Check One Fixture Against the Rest

Run cold, then hot, at a few faucets on different floors and at an outdoor hose bib. One weak fixture points to that fixture. Every fixture weak points to the main, the regulator, the pipes or the street.

2. Clean the Aerator or Showerhead

Unscrew the aerator at the tip of the faucet (wrap the pliers in a cloth) and rinse out grit and scale. Soak a showerhead in vinegar. Low-flow fixtures aren’t the problem on their own: EPA WaterSense certifies its labeled showerheads for spray force and for keeping a minimum flow across household pressures.

3. Make Sure the Valves Are Fully Open

After any plumbing work, check that the main shutoff at the house is fully open, and the small stop valves under the sink or toilet. A gate valve that has been turned many times can stick partly closed inside even with the handle open; if it won’t turn freely, leave it for a plumber rather than force it.

4. Measure the Static Pressure

San Jose Water recommends measuring with a pressure gauge screwed onto an outdoor hose bib while no water is running anywhere in the house. It also suggests installing regulators downstream of the outdoor hose bibs, so in some homes the hose bib reads street pressure rather than what reaches the fixtures inside. If the reading looks fine but the house is weak, a plumber can measure inside too.

• Above 80 psi: pressure is too high, not too low; see water hammer and noisy pipes for what high pressure does.
• Normal on the gauge, weak at the taps: the restriction is in the pipes, a valve or a fixture.
• Low on the gauge: the regulator, the service line or the street supply.

5. Do the Two-Hour Meter Test for Leaks

A leak between the meter and the fixtures steals pressure and shows up on your bill. EPA WaterSense suggests reading the meter before and after a two-hour period when no water is being used. If it changed at all, you probably have a leak. See how to detect water leaks for the full check.

6. Bypass a Water Softener or Whole-House Filter

San Jose Water lists a malfunctioning softener as a cause of sudden pressure loss and suggests bypassing it until it’s repaired. A clogged whole-house filter cartridge does the same.

What Not to Do

• Don’t adjust or open the regulator yourself if you’re not sure what you’re doing. A regulator set too high exposes every pipe and appliance to excess pressure, and San Jose Water recommends a licensed professional for regulator adjustments.
• Don’t shut water off at the meter. Use your house’s main shutoff valve; the meter belongs to the water company.
• Don’t add a booster pump without having the pressure and flow measured first. A pump on a leaking or corroded system makes the leak worse.

What Are the Most Common Causes of Low Water Pressure?

Clogged Aerators, Showerheads and Cartridges

The most common cause of a single weak fixture. Scale and grit from pipe work collect in the screen. Single-handle faucets also have a cartridge that can clog or wear.

A Partly Closed Valve

The main shutoff, a meter valve or a fixture stop that wasn’t fully reopened after a repair. Old gate valves can also fail inside, with the gate stuck partway down.

A Failing Pressure Regulator

Homes on high street pressure have a regulator (pressure-reducing valve), usually near where the water line enters the house. When one wears out it can drop pressure, let it creep too high, or swing between the two. San Jose Water lists regulator failure as a cause of both sudden and gradual pressure loss, and regulators are installed and maintained by the homeowner. A plumber confirms it with gauges on both sides and replaces it, along with the expansion tank the code calls for. See pressure regulator replacement; CPI also installs regulators with water line replacements.

A Hidden Leak

A leak in the service line, under the slab or in a wall diverts water before it reaches your fixtures. Other signs: a higher bill, a meter that moves with everything off, warm spots on the floor, damp drywall or the sound of running water. Leak detection finds it with acoustic, thermal and pressure testing, so only the leak is opened. See signs you need leak detection and slab leak signs.

Corroded Galvanized Pipe

Galvanized steel pipe rusts from the inside, and the rust narrows the opening until even good street pressure can’t push enough water through. It’s common in older South Bay homes and in old service lines; San Jose Water names a deteriorating galvanized service line as a cause of gradual decline. The lasting fix is new pipe: see how repiping improves water pressure and what repiping involves.

A Water Heater Problem (Hot Water Only)

Weak hot water with normal cold points to the water heater: a partly closed inlet valve, sediment, or corroded fittings. See signs your water heater needs repair.

Low Pressure From the Street

Pressure depends on where you are. San Jose Water explains that homes lower than the storage tank get more pressure, roughly ½ psi for every foot of drop, and that in hilly areas near a tank the only fix is a booster system. A main break or repair can also drop pressure for a whole street for a while.

Low Water Pressure in San Jose Homes

• Know your water company. San José has three water retailers: San Jose Water, San José Municipal Water and Great Oaks Water, each with its own service area. San Jose Water’s Customer Service is (408) 279-7900, 24 hours a day.
• Know where your side starts. San Jose Water is responsible for pressure at the meter and owns the meter and the line from the main. The line from the meter to the house, the regulator and everything inside are the owner’s to maintain.
• Older homes, older pipe. Many South Bay homes still have galvanized supply pipe or an original service line, the classic cause of pressure that fades over the years.
• Hills and flats. Foothill homes near a tank can have low pressure from elevation alone, while homes far below one may need a regulator to stay under 80 psi.
• Permits. A repipe or a new water service in San José needs a City plumbing permit and inspection, which we handle.

When Should You Call a Plumber for Low Water Pressure?

Call a licensed plumber when:

• the whole house is weak and the valves, softener and aerators check out
• the gauge reads low, or above 80 psi
• the meter test shows a leak, or you see damp spots, hear running water or the bill jumped
• the pressure is fine on the gauge but drops badly when two fixtures run
• your home has galvanized pipe and the pressure keeps getting worse
• you’ve had more than one leak in a year

A plumber measures static pressure and flow, checks the regulator from both sides, tests for leaks and inspects the pipe. CPI Service is licensed (CSLB License #1101278), offers same-day service and 24/7 emergency service, and backs its work with a 1-year labor warranty.

What Does It Cost to Fix Low Water Pressure in San Jose?

It depends entirely on the cause, so diagnosis comes first. Typical San Jose prices, as of October 2026:

Jobs are quoted as a fixed price before we start, typically based on $150 to $250 per hour plus parts and drive time, so you know the total up front. See the leak detection cost guide and the repiping cost guide for what moves those prices.

CPI Service fixes low water pressure throughout San Jose and the South Bay, from pressure regulators and leaks to whole-house repiping. Request a free estimate or call (408) 978-3093.

Sources

• San Jose Water: Water Pressure
• San Jose Water: Who’s Responsible for What
• City of San José: Water Customer Service (three water retailers)
• City of San José: Repipe and Water Service Projects
• California Plumbing Code 2025 (Title 24, Part 5), free online edition, sections 608.1 and 608.2
• California Building Standards Commission: current codes (2025 edition, effective January 1, 2026)
• EPA WaterSense: Fix a Leak Week
• EPA WaterSense: Showerheads

Checked October 2026.