Low Pressure at One Faucet Only: Start at the Spout
Whole-house pressure problems are a supply issue. One weak faucet is something between the wall and the spout, and most of the candidates take five minutes to check.
August 16, 2026 · 5 min read
If every tap in the house is weak, the problem is at the main, the pressure reducing valve or the service line. If one tap is weak and the others are fine, the problem is somewhere between the branch pipe and the spout of that one fixture, and the list of suspects is short. Here it is, in the order we would check.
1. The aerator
The screen at the tip of the spout. In our hard water it collects scale, and it catches any grit that comes through the line after work on the pipes. Unscrew it by hand or with a cloth and pliers, look at the screen, and run the tap without it. If the flow is strong with the aerator off, soak it in vinegar for an hour or replace it. This is the cause more often than everything else combined.
Shower heads are the same story. Unscrew the head, run the arm bare, and if the flow is strong, descale the head.
2. Hot only or cold only
Run the tap on full hot and then full cold. If one side is weak and the other is fine, the problem is on that side's supply. Weak hot with strong cold at one fixture is often a clogged hot supply line or a partially closed hot stop valve. Weak hot at every fixture is a water heater problem, usually sediment or a failing dip tube, and belongs in a different article.
3. The supply stop valves
The small valves under the sink or behind the toilet. They get bumped by cleaning supplies and half-closed by whoever last worked on the fixture. Turn each fully open. If a stop valve is old, it can also fail internally, with the stem turning but the valve not opening; a valve that feels loose and makes no difference in either direction is due for replacement.
4. The supply lines
Flexible braided lines can kink behind a cabinet, and older rubber lines can delaminate inside, with a flap of lining that acts as a check valve. With the stop closed, disconnect the line at the faucet, point it into a bucket, and open the stop briefly. Strong flow from the line means the fault is in the faucet. Weak flow means it is the line or upstream.
5. The cartridge or valve body
Single-handle faucets and shower valves have a cartridge inside that controls mix and flow. Scale and debris lodge in its passages. Some faucets have an inlet screen on the cartridge that clogs. Pulling the cartridge, flushing it and reinstalling often restores full flow; if the cartridge is worn, replacing it is a short job with the right part. Pressure-balancing shower valves have a small spool that seizes with scale and produces exactly this symptom.
6. The branch pipe itself
If the aerator is clean, both sides are weak, the stops are open, the supply lines flow well into a bucket and a new cartridge makes no difference, the restriction is in the pipe feeding the fixture. Causes we find: a solder blob or a crushed section at a fitting from the original construction, a PEX line kinked behind drywall, a section of copper narrowed by scale on a long hot run, or a galvanized nipple at the wall that has rusted almost shut. Locating that takes a pressure test at the fixture and sometimes a small access opening, which is where we come in.
Family-owned in Rockwall, licensed and insured. TX Master Plumber TXM11693. (469) 338-5533.
Frequently asked
Why did one faucet get weak right after plumbing work in the house?
Debris dislodged by the work travelled to the nearest screen. Clean the aerator and the cartridge inlet screen. It is normal and usually a one-time clean.
The kitchen sprayer is weak but the main spout is fine. What is that?
The diverter valve inside the faucet, or a clogged sprayer head. Both are common and both are repairable; on many faucets the diverter is a small replaceable part.
Can a water softener cause low flow at one fixture?
A softener affects the whole house, not one tap. If a softener was recently installed and one fixture is weak, resin beads from a damaged softener may have reached that aerator; check the screen for tiny spheres.
Is it worth replacing an old faucet rather than repairing it?
If the faucet is more than fifteen years old and parts are hard to find, often yes. A modern faucet with a new cartridge and supply lines resolves several possible causes at once.