Water pressure that fades over years is usually pipe. Water pressure that drops suddenly, over a day or a week, is usually one specific failed part, and there are not many candidates. That is good news, because it means this is findable.
The first question, and it takes two minutes: is it the whole house, or one fixture?
Whole house or one tap
Run the cold tap in a bathroom sink. Then the kitchen. Then a tub or a shower. Then go outside and open a hose bib.
If one fixture is weak and everything else is normal, it is that fixture. Usually an aerator clogged with mineral grit, or a cartridge, or an angle stop that got bumped half closed. Unscrew the aerator, rinse the screen, put it back. That solves it more often than people expect.
If hot is weak everywhere but cold is fine, the problem is at or after the water heater. Sediment in the tank, a partly closed valve on the hot outlet, or a failing mixing valve.
If everything is weak everywhere, keep reading. That is a house-level problem.
Measure it before you guess
You cannot troubleshoot a feeling. A pressure gauge that screws onto a hose bib costs about fifteen dollars, and it turns this whole article into a number.
Thread it onto an outside spigot or the laundry tub, open the valve fully with no other water running in the house, and read it.
- Under 40 psi: genuinely low. Something is restricting or failing.
- 40 to 60 psi: normal, and if it feels weak the problem is flow rather than pressure (see below).
- 60 to 80 psi: fine, at the upper end.
- Over 80 psi: too high. Code caps residential systems at 80 for a reason, and above it you are steadily destroying fill valves, supply hoses, appliance inlets and the water heater.
Write the number down. It changes everything about what happens next.
The five usual causes of a sudden drop
A failed pressure reducing valve. Most Northern Virginia homes have a PRV where the water enters, because street pressure here often runs well above what a house should see. PRVs have a service life, typically ten to fifteen years, and when they fail they usually fail closed, which strangles the whole house at once. This is the single most common cause of an overnight drop, and the symptom that points at it is exactly what you are describing.
A valve that got moved. The main shutoff, or the meter valve, half closed after other work in the house. Free to check and embarrassing to skip. Follow the line from the meter and make sure every handle is fully open.
A clogged whole-house filter. If you have one and nobody has changed the cartridge in a year, that is your answer, and it is a fifteen minute fix.
A leak, usually underground. A break in the service line between the meter and the house bleeds off pressure before it ever reaches you. The tells: a soft or unusually green patch in the yard, the sound of running water with everything off, or a water bill that jumped for no reason. Check the meter with every fixture in the house closed. If the dial is still moving, water is going somewhere.
Galvanized pipe finally closing up. This one is not sudden, but people notice it suddenly. Older galvanized steel corrodes inward until the opening is the diameter of a pencil. If your house still has galvanized supply lines, the fix is repiping, not a valve.
Pressure versus flow, because they get confused
Pressure is force. Flow is volume. A gauge can read a perfectly healthy 60 psi while your shower still feels weak, because the pipe feeding it is too small, or clogged, or because three fixtures are running at once on a line that can only serve one well.
If your gauge says the pressure is fine and it still feels bad, the problem is flow, and the fix is about pipe sizing or restriction rather than pressure. Different job, different quote, and worth knowing before you buy the wrong one.
The high side matters more than people think
Half the houses we test have the opposite problem. Pressure over 80 psi does slow, expensive damage: it is why the toilet fill valve keeps failing, why the washing machine hose burst, why the water heater’s relief valve keeps weeping, and why the pipes bang when a faucet closes.
If your gauge reads high, a PRV is the fix, and it protects everything downstream of it. It is one of the few plumbing purchases that pays for itself in things that do not break.
What to check yourself, in order
- Is it one fixture or all of them?
- Clean the aerator if it is one fixture.
- Put a gauge on a hose bib and write down the number.
- Walk the main line from the meter and confirm every valve is fully open.
- Change the whole-house filter cartridge if you have one.
- Close everything and watch the water meter for movement.
If you get to the end of that list and still have a number under 40, you have a failed PRV or a leak, and both need somebody with the parts.
Getting it sorted
SwiftPro tests pressure, replaces PRVs, finds service line leaks and repipes across Northern Virginia. We put a gauge on it before we quote anything, because the number decides the job. Call (703) 997-9757 or book online.
