The guest bathroom tap had been weak for a year, and everyone in the house had quietly decided that the plumbing was just like that. It was not. Every other tap in the house ran hard, and when the aerator finally came off, a felt disc of grit and scale came out with it. Four minutes of soaking and brushing put the flow back to normal, which is the whole lesson: weak flow at one sink is almost always a local problem, and the order you check things in decides whether you spend four minutes or four hundred dollars.
Work out whether it is one sink or the whole house
Before touching anything, do a thirty second comparison. Run the cold tap at the weak sink, then the cold tap in the kitchen, and then a second bathroom if you have one. If every outlet is weak, the problem is upstream of the sink and none of the steps below will help. If only this sink is weak, it is local, and you can work through the list in order, from the cheapest and most likely to the least. One more question narrows it further: compare hot and cold at the same tap. Weak on hot only points at the cartridge or the water heater side, weak on cold only points at the supply side, and weak on both points at the aerator or the inlet screens.
Start with the aerator
The aerator is the little screen assembly screwed onto the end of the spout, and it is the single most common cause of a weak bathroom tap. It does two jobs - it mixes air into the stream and it restricts flow to save water - which means grit, scale and pipe debris all collect inside it. Unscrew it by hand if you can, or use a rubber strap rather than pliers, because the chrome dents easily. Inside you will usually find a plastic insert and one or two screens; soak the whole thing in white vinegar for half an hour, scrub it with an old toothbrush, and rinse. If the screens are crusted solid or the plastic has crumbled, a replacement aerator costs a few dollars and screws straight on. If it will not budge at all, do not force it: tape the jaws of a wrench, or soak the spout end in a bag of vinegar overnight, which dissolves enough scale to break the grip.
Check the shutoff valve and the supply line
If the aerator is clean and the flow is still poor, look under the sink. Each tap has a small stop valve on the wall or the cabinet floor, and a valve that is only part way open behaves exactly like low water pressure. Open both fully, then check the braided supply line from the valve to the faucet for a kink, a sharp bend behind the cabinet, or a bulge - a bulging line should be replaced rather than straightened. While you are down there, run the tap and watch the connections. An old gate valve that has not been turned in twenty years often will not close again once you move it, so if the valve feels gritty, plan to replace it rather than exercise it repeatedly.
If the problem turns out to be the drain rather than the supply, that is a different list: a bathroom sink that drains slowly is about the waste side, and what the overflow hole does explains the other opening in the basin.
Look at the cartridge and the inlet screens
In a single handle faucet, the cartridge does the mixing, and in hard water areas the hot side scales up first, which is why a tap can be fine on cold and weak on hot. Before replacing anything, compare flow with the handle on full cold and full hot; if the difference is obvious, the cartridge or its inlet screens are the suspects. Many faucets also have small screen washers where the supply lines meet the valve body, and those clog faster than the aerator because nobody ever looks at them. Shut the stop valves, undo the supply nuts with a pan underneath, pull the screens out with tweezers, rinse and refit. If the cartridge itself is scaled, soaking it in vinegar sometimes revives it, but a replacement cartridge is the honest fix.
When it is the whole house
If several taps are weak, or the pressure drops whenever a shower or an appliance runs, the sink is innocent. The usual causes are a main valve left partly closed after some work, a pressure reducing valve that has failed, a well pump that is short cycling, or old galvanised pipe that has furred up from the inside. A cheap screw-on pressure gauge on an outdoor spigot or a laundry tap will tell you the static pressure in a minute; if it reads well below forty psi with nothing running, the problem is upstream and it is worth calling someone rather than replacing fixtures at random.
| What you see | Most likely cause | What to do |
|---|---|---|
| Weak on hot only | Cartridge scale or the hot supply | Clean or replace the cartridge; check the heater valve |
| Weak on cold only | Stop valve or supply line | Open the valve fully; look for a kinked braided line |
| Weak on both, one tap | Aerator or inlet screens | Remove the aerator, soak it in vinegar, rinse the screens |
| Weak at every tap since some work | A valve left part open | Check the main valve and any new stop valves |
| Fine, then fades when something else runs | Shared demand or a failing reducing valve | Test at another outlet; have the valve checked |
| Spitting and uneven flow | Air in the line after a shutoff | Run every tap for a minute |
The bottom line
Work from the cheapest cause outward: compare taps to confirm it is local, clean the aerator, open the stop valves fully, check the supply lines, then look at the cartridge and its screens. Only when several outlets are weak at once does the answer live in the house supply rather than in the faucet. And if the cause turns out to be a clogged aerator, keep the faucet you have: the cheapest fix is usually the right one.
Common questions
Why is my bathroom sink water pressure suddenly low?
A sudden loss at one tap is usually the aerator or a stop valve that is only part open, particularly after any work on the plumbing. Clean the aerator first, then check the valve under the sink.
Can a clogged aerator cause low water pressure?
Yes. The aerator is a screen and a flow restrictor in one, so grit and scale collect behind it and throttle the stream. Soaking it in white vinegar and brushing it out restores most taps in a few minutes.
Why is only the hot water weak in my bathroom sink?
That points at the cartridge or the hot supply rather than the shared parts: scale builds faster on the hot side, and the stop valve or the heater outlet can be restricted. Compare full hot with full cold at the same tap to confirm.
How do I fix low water pressure in the whole house?
Check the main valve and any pressure reducing valve first, and confirm with a screw-on gauge - below about 40 psi with nothing running is a supply problem, not a fixture problem. At that point it is worth calling a plumber.
Related reading
- Waterfall bathroom faucet guide: is the matte black look right for you?
- Bathroom refresh on a budget: what is worth changing
- Why does my bathroom sink drain slowly? (And what actually fixes it)
If the faucet is the part you are replacing anyway, the one we make is our matte black waterfall bathroom faucet - and if the cause was a clogged aerator, keep the faucet and clean it instead.