Skip the scope-narrowing triage and you'll change the wrong part. I've watched homeowners replace a $300 PRV when the problem was a $4 aerator clogged with municipal flush debris. Five minutes of diagnosis saves three hours and three hundred dollars.
First question: is it one fixture or the whole house?
Answer four questions in order. Each one eliminates 75% of possible causes.
- One faucet only? → Aerator screen at THAT fixture, or supply-stop valve under THAT sink. Stop reading and go unscrew the aerator. 5 minutes.
- Only HOT water everywhere? → Water heater. Sediment in the tank or a broken dip tube. Cold side is fine, so the main, PRV, and municipal supply are all eliminated.
- Whole house, hot AND cold, all fixtures? → Main valve, PRV, municipal supply, or pipe corrosion. This is where the gauge comes out.
- Did it change recently — last 24-72 hours? → Call your water utility FIRST. POCO water-main work, a neighbor's excavation cutting a service line, or a partial frozen-pipe restriction is far more likely than a part suddenly failing.
The 7 causes ranked by field frequency
(1) Clogged aerators — 1 fixture only. The screen at the tip of the faucet catches every flake of pipe scale, sediment, and rubber from the supply line. Unscrew it counter-clockwise by hand (use a rag for grip), soak in white vinegar 30 min, rinse, reinstall. 5 minutes, $0.
(2) Failing pressure-reducing valve (PRV) — whole house drops. The PRV at your main service line is supposed to step municipal pressure (often 100+ psi) down to a safe 50-60 psi for your house. When the internal diaphragm fails, output pressure drops to 30 psi or less. Common brands: Watts N45B / LFN45B (~$80-$160, adjustable 25-75 psi, factory-set 50 psi), Zurn Wilkins NR3XL, Honeywell DS06. Lifespan: 10-15 years.
(3) Partially closed main valve. If a plumber, meter reader, or city worker touched your main shutoff or curb stop in the last few weeks and didn't open it fully, you'll get reduced flow that mimics a failing PRV. Free fix — open the valve fully (counter-clockwise, turn until it stops).
(4) Galvanized pipe corrosion — pre-1970s homes. Galvanized steel pipes corrode from the inside out. The zinc coating wears off, rust accumulates, and a 3/4" pipe's interior shrinks to 3/8" or less. Lifespan is 40-70 years — every galvanized house built before 1970 is now living on borrowed time. Tell: low pressure worst at the fixtures farthest from the main. Fix: whole-house repipe in PEX or copper ($4,000-$15,000).
(5) Sediment-clogged water heater — hot-only low pressure. Calcium and magnesium settle at the bottom of the tank. Sediment can clog the dip tube outlet, restrict flow, or break the dip tube itself (snapped dip tube also drops your hot water temperature because cold mixes at the top). Confirm by running cold-only at the same fixture — normal cold flow + weak hot flow = water heater. Flush the tank annually as preventive maintenance.
(6) Municipal supply issue. Call your water department's non-emergency line, then check their website's "service alerts" page. Main repairs, hydrant flushing, and pressure-zone re-routing all drop neighborhood pressure temporarily. If your neighbor also has low pressure, this is your cause.
(7) Hidden leak — slab or irrigation. Shut every fixture in the house. Check your water meter. If the leak indicator (the small triangle or star) is still moving, water is going somewhere. Confirm with a water bill spike vs last month. Slab leaks need a leak-detection pro; irrigation breaks are often visible as a soggy patch in the yard.
Buy the $10 gauge — do this before anything else
The hose-bib pressure gauge ($10-15 on Amazon — Rainbird, Camco, or generic). Thread it onto an outdoor spigot. Shut off everything in the house. Open the spigot fully. Wait 30 seconds for the needle to settle. Read:
- 40-60 psi: Normal. Look elsewhere.
- 60-80 psi: High but legal. Not your problem.
- Below 40 psi: Confirmed low. Move to PRV diagnosis.
- Above 80 psi: Code violation under IAPMO Uniform Plumbing Code §608.2 — you need a PRV (or your existing one is stuck open).
The water-meter leak check. Shut everything off. Walk to the meter. Watch the low-flow indicator (the triangle, star, or red dial) for 60 seconds. Any movement = active leak. Free, takes 2 minutes, rules out the most expensive cause on the list.
The PRV test — the #1 missed diagnosis
The PRV is a brass valve typically installed at the main water entry, right after the main shutoff. Bell-shaped brass body with an adjustment screw on top.
Test it in three steps:
- Gauge at the hose bib upstream of the PRV (if your outdoor spigot is fed from before the PRV — rare but check).
- Gauge at any hose bib downstream (most outdoor spigots, washing-machine hookups).
- Compare. If upstream reads 80+ psi and downstream reads under 40 psi, the PRV diaphragm is failing.
Adjustment: Some PRVs are adjustable in place. Turn the screw clockwise to raise pressure, counter-clockwise to lower. 1/4 turn at a time, re-test. If adjustment does nothing, replace.
Replacement cost: $80-$160 part (Watts LFN45B ~$130), $300-$500 plumber labor, or DIY if you can sweat copper / use SharkBite. Requires shutting the main and bleeding the system. Lead-free brass is now code-required.
When to call a pro
- Whole-house repipe for galvanized — permits, drywall, multi-day job
- Slab leak detection — acoustic listening equipment, $300-$500 just to locate
- PRV install from scratch if you've never sweat copper and your system uses threaded brass (SharkBite makes PRV swap DIY-friendly, but a first install with no existing PRV needs a plumber)
- Anything municipal-side of the meter — that's the utility's pipe, don't touch it
FAQ
Can low water pressure damage my appliances?
No. Low pressure under-performs; doesn't damage. High pressure (above 80 psi) cracks supply lines and fails dishwasher solenoids.
Why did my pressure drop overnight with no warning?
PRV diaphragm finally tore, municipal main work, or partial frozen pipe restricting flow.
Is 40 psi enough for a two-story house?
Barely. Each floor costs ~0.43 psi per foot. A second-floor shower from a 40 psi main is sitting at ~35 psi at the head — weak but functional. Target 55-60 psi at the gauge.
Pressure fine but flow weak — what's the difference?
Pressure (psi) is potential; flow (gpm) is delivery. Good pressure + bad flow = a restriction (corroded pipe, partially closed valve, sediment).
Will a booster pump fix low municipal pressure?
Last resort. Whole-house booster (Grundfos, Davey) runs $800-$2,500 installed. Only worth it if the utility's supply is genuinely under-spec and they refuse to fix it.
Field-pattern callout
"Replaced the PRV three years ago. Pressure great for a year, then slowly dropped to 35 psi over six months. New PRV in, back to 60. Apparently they just don't last like they used to."
Al's read: Residential PRVs from the last decade are not lasting the 15 years they used to. If you're in a hard-water area and your PRV is past year 8, treat it as a wear item, not a permanent fixture.
Related guides
- Water heater banging popping — sediment buildup that also causes hot-only low pressure
- Toilet keeps running — separate symptom, often confused
- Best tankless water heaters 2026 — gas-line sizing affects hot-water flow rate
Editorial standards: Cited authorities include IAPMO Uniform Plumbing Code §608.2 (excessive water pressure code), Watts N45B PRV technical documentation. Reviewed by Al, Building Doctor — IUOE Local 39 Stationary Engineer.