Why is the water flow low at just one faucet?

The aerator is the small fitting on the end of your faucet spout. It mixes air into the water and shapes a smooth, even stream while holding flow to a set rate. Inside it sits a fine mesh screen plus a stack of small washers and discs. That screen is the first place debris collects, which makes it the first place flow drops.

Three things plug an aerator. Scale is hardened mineral deposit, mostly calcium and magnesium, that crusts onto the metal. Sediment is loose grit, sand, or pipe flakes that the screen catches. Rust flakes off older galvanized pipe and lodges in the mesh. Any of the three can cut flow at a single tap while every other fixture stays strong.

Phoenix water makes this worse than most places. The City of Phoenix reports total hardness around 170 to 284 mg/L, which works out to roughly 10 to 17 grains per gallon. The USGS scale rates water as "very hard" above 180 mg/L, so much of the Phoenix supply lands at the top of "hard" and into "very hard." Those minerals drop out of the water and cake onto screens, valves, and fixtures over time. EPA lists hardness among its secondary standards, the aesthetic and nuisance limits that cover scale and staining rather than health risks. Hard water is the single biggest reason a Phoenix aerator clogs.

The buildup is gradual, so the flow loss creeps up on you. A screen that was clear last year slowly fills with a chalky white crust until one day the stream is a weak fan instead of a full flow. Because the aerator sits right at the spout tip, it catches debris that the rest of the plumbing never sees, which is why a single faucet can drop off while everything else holds steady. The screen is also the easiest part to reach and clean, so it is the right place to start every time.

Cleaning the aerator solves most single-faucet flow problems and takes about an hour, most of it soaking time. Here is the order of work.

1. 1Close the drain or lay a towel in the sink so small parts cannot fall in.
2. 2Unscrew the aerator from the spout tip. Turn it by hand counterclockwise first. If scale has locked it on, wrap the fitting in a cloth to protect the finish and use pliers gently.
3. 3Disassemble the parts. Push the insert out and lay the screen, washer, and discs in order so you can stack them back the same way.
4. 4Soak the parts in white vinegar for 30 to 60 minutes. The mild acid dissolves the mineral scale. Bad buildup may need the full hour.
5. 5Scrub the screen with an old toothbrush. Clear each hole in the screen with a pin or a stiff brush so water passes freely.
6. 6Rinse everything in clean water.
7. 7Reassemble in the same order and screw the aerator back on hand-tight. Turn the water on and check the stream.

If the metal is corroded, pitted, or cracked, replace the aerator instead of fighting it. They are inexpensive, sold at any hardware store, and come in standard sizes. Bring the old one to match the thread and size. There are two thread styles, male and female, so the matched part is the surest way to avoid a second trip.

In a hard-water home, plan to clean the aerator on a schedule rather than waiting for the flow to fall off. Many Phoenix homeowners pull and soak their kitchen and bathroom aerators every few months, which keeps the screen clear and the stream full. It is a five-minute habit once you have done it once, and it heads off the slow decline before it becomes noticeable.

If the flow is still weak after a clean aerator goes back on, move upstream. The next suspect is that fixture's supply stop valve, the small shutoff under the sink or behind the toilet. These valves can be partly closed, or their internal parts can corrode and restrict flow. Make sure the valve is opened all the way. If it feels gritty or will not open fully, it may need replacing.

Next, look at the supply line, the flexible hose running from the stop valve to the faucet. These hoses can kink behind the cabinet or collapse internally with age, pinching the flow. A kinked or swollen line is easy to spot and cheap to swap.

For a single-handle or cartridge faucet, the cartridge itself can clog with the same scale and sediment that fouls an aerator. The cartridge controls flow and temperature inside the faucet body. When it packs with mineral grit, flow drops even with a clean aerator and an open valve. Cartridges are model-specific, so match the brand and model before buying a replacement.

Work through these in order: aerator, then stop valve, then supply line, then cartridge. Most single-faucet flow problems stop at the aerator. The rest are quick checks that rule out the next likely cause.

One handy test separates an aerator clog from a problem deeper in the faucet. Unscrew the aerator entirely and turn the water on with the bare spout. If the flow surges back to full, the aerator was the choke point and a clean or new one fixes it. If the bare spout still runs weak, the restriction is upstream in the valve, line, or cartridge, and you can skip straight to those checks.

The number of affected fixtures tells you where the problem lives, and this is the most useful thing to know before you start. One weak fixture means a local issue. The trouble sits in that faucet's own parts: the aerator, the stop valve, the supply line, or the cartridge. Everything between the wall and that one spout is fair game, and nothing else in the house is involved.

Weak flow at every fixture means a pressure problem that affects the whole system. That points to something shared: a failing pressure regulator, a partly closed main shutoff, a clogged whole-house filter, or a supply-side issue. Cleaning one aerator will not touch a house-wide pressure drop. For that, see our page on low water pressure throughout the whole house, which walks through the system-wide causes.

So before you take anything apart, run a quick test. Turn on the hot and cold at the slow faucet, then test two or three other fixtures around the house. If the others are strong, you have a local clog and the steps above will fix it. If they are all weak, the problem is pressure, not a clogged screen.

One more distinction worth noting: low flow is not the same as a drip. If the faucet leaks or drips when shut off, that is a worn washer, O-ring, or cartridge seal, covered on our page about how to fix a dripping faucet. Low flow is too little water when the faucet is open; a drip is water escaping when it is closed.

Knowing the target rate helps you judge whether a faucet is truly weak or just meeting a modern limit. The EPA's WaterSense program sets bathroom faucet flow at a maximum of 1.5 gallons per minute at 60 psi, compared with the older standard of 2.2 gallons per minute. According to EPA WaterSense, the goal is to "reduce a sink's water flow by 30 percent or more from the standard flow of 2.2 gpm without sacrificing performance."

That matters for diagnosis. A WaterSense faucet is built to run at a lower rate by design, so it will feel calmer than an old high-flow tap. That is normal and not a clog. What you are watching for is a clear change: a faucet that used to run strong and now trickles. A sudden drop at one fixture is the scale, sediment, or rust signature, and it almost always traces back to the aerator first.

Across a home, small flow problems add up to real waste. EPA WaterSense reports that household leaks waste nearly 1 trillion gallons of water nationally each year, and that the average home loses more than 9,300 gallons annually to leaks and running fixtures. Keeping aerators clean and fixtures in good order is part of holding that number down, and in hard-water Phoenix it also keeps your faucets running the way they should.