Short answer: If the pressure is low in one bathroom and normal elsewhere, the problem is between that bathroom and the rest of the house: a clogged aerator or shower head, a shutoff valve under the sink or at the toilet that is not fully open, a worn faucet cartridge, or (in older houses) a corroded galvanized branch line. The first three are ten-minute fixes. If the pressure is low everywhere, it is the pressure-reducing valve where the water enters the house, and that is a plumber.
What is actually going on
Water pressure is set once, at the main, and it is the same everywhere in the house up to the point where something restricts it. So "low in one place" is always a restriction on that branch, and the restriction is almost always at the end of the line where you can reach it.
- Aerator or shower head. Grit, pipe scale from a repair, and mineral flakes collect in the screen. Charleston and York County water is soft, so heavy limescale is not the usual cause here; sediment from the water heater or from a recent utility repair is.
- Shutoff valve not fully open. The little valve under the sink or behind the toilet got bumped, or was left half-closed after a repair. Old multi-turn valves also fail internally and restrict flow while looking open.
- Faucet cartridge or shower valve. A single-handle faucet or shower has a cartridge that wears and clogs. Low flow on both hot and cold at one fixture, with a good aerator, is usually this.
- Hot only, or cold only. Hot-only low pressure at one fixture points at sediment in the hot line or the water heater outlet; cold-only points at the cold branch or its valve.
- Galvanized branch line. In houses from before the 1970s (downtown, West Ashley, older Rock Hill), the steel branch to the far bathroom can be rusted nearly shut inside. Pressure is low at every fixture in that bathroom, and it has been getting worse for years.
How to check in five minutes
- Is it every fixture in that bathroom, or one? One fixture: aerator, cartridge or its valve. All of them: the branch or a valve feeding the bathroom.
- Unscrew the aerator (a pair of pliers with tape on the jaws) and run the tap without it. Full flow means the aerator was the problem; rinse it or replace it.
- Open the shutoff valves under the sink and at the toilet all the way. If a valve will not turn or drips when you touch it, it needs replacing.
- Hot vs cold. Run each alone. If one is fine, you have halved the search.
- Check the whole house. A $10 pressure gauge on the outside hose bib reads the house pressure: 50–70 psi is normal. Under 40 everywhere is the regulator, not the bathroom.
The honest DIY version
Aerator: unscrew, rinse, reinstall or replace ($5). Shower head: unscrew, soak in vinegar overnight or replace. Shutoff valve: open it; if it is the old multi-turn kind and it is stiff or leaks, replacing it with a quarter-turn valve is a beginner plumbing job with the water off. Cartridge: the manufacturer publishes the part number; it is a $20–60 part and a half hour with the water off, and it fixes most single-fixture pressure complaints in a house under twenty years old.
Do not do it yourself: the pressure-reducing valve, any galvanized pipe replacement, or anything behind a tiled wall. Those are a plumber, and the last one may be a bathroom project.
When it is serious
- Low pressure everywhere, or pressure that swings: the pressure-reducing valve (PRV) at the main. See below for why this fails early in Charleston.
- Pressure that drops when another fixture runs: undersized or corroded supply, or a PRV on its way out.
- Brown or rusty water with the low pressure in a pre-1970s house: galvanized pipe corroding from the inside. It is on its way to leaking, and the fix is replacing the branch (a repipe of that run).
- A wet spot anywhere along the route to that bathroom, or a water bill that jumped: a leak in the branch line is also a pressure drop.
What is left after the plumber leaves — and who does it
If the answer is a corroded branch line to that bathroom, the plumber replaces the pipe, and to reach it they open the wall or the floor. What is left is the wall: drywall, tile, trim and paint, and in a tiled shower it can be the whole wet wall. That is general-contractor work, and if the bathroom is thirty years old it is often the moment it becomes a bathroom remodel rather than a patch. We coordinate the plumber and the finish work as one job. For pressure problems generally, our plumbing division covers PRVs, repipes and the emergency version.
Charleston and York County notes
Charleston has a specific PRV problem. Charleston Water System has fitted a dual check valve on every residential service since 1985, which makes each house a closed system: water cannot push back toward the street. When the water heater heats, the water expands and has nowhere to go, so the pressure in the house spikes every heating cycle. That cycling wears out pressure-reducing valves and water heater relief valves early, and it is why a house here should have a thermal expansion tank. If your whole-house pressure is erratic, ask the plumber to check the expansion tank as well as the PRV. York County systems (Rock Hill and the utilities it supplies) do not all have the same check-valve setup; in Fort Mill and Tega Cay the common whole-house cause is simply a PRV at the end of its life on a house from the early 2000s.
And the hard-water myth: both metros are inside the USGS "soft" band (Charleston Water System publishes about 54–58 ppm; Rock Hill about 35). Limescale clogging your aerators is not the story here; sediment is.
Sources
- Charleston Water System — FAQs (dual check valve, closed systems): charlestonwater.com/171/FAQs
- USGS — hardness of water (the soft / hard scale): usgs.gov/special-topics/water-science-scho…
General guidance for homeowners in Charleston and York County, South Carolina, written from what we see on jobs. Conditions vary house to house; anything structural, gas or electrical is licensed work in South Carolina and should be looked at in person.
Frequently Asked Questions
Why is the water pressure low in just one sink?
A clogged aerator, a shutoff valve under the sink that is not fully open, or a worn faucet cartridge. Unscrew the aerator and run the tap: if the flow is fine without it, that was the problem.
Why is water pressure low in one bathroom but not the rest of the house?
Something restricts the branch feeding that bathroom: a partly closed valve, sediment, or in older houses a corroded galvanized pipe. If every fixture in that bathroom is low, look at the branch and its valves rather than the fixtures.
Is low water pressure everywhere the city or my house?
Put a gauge on an outside hose bib. Under about 40 psi with the city normal means your pressure-reducing valve has failed. In Charleston, thermal expansion in the closed system wears PRVs early; an expansion tank protects the replacement.
Does hard water cause low pressure in Charleston?
Rarely. Charleston and York County water is soft by USGS standards (roughly 54–58 ppm and 35 ppm). Low pressure at a fixture here is usually sediment, a valve or a cartridge, not limescale.
Who fixes low water pressure in one bathroom?
Aerators, valves and cartridges are DIY or a plumber. A corroded branch line is a plumber, and opening the wall to reach it makes the tile, drywall and paint afterward general-contractor work, which Southeastern Renovation does.