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Poor water pressure is a nuisance in the shower and a warning sign elsewhere. The cause depends on where it happens: one fixture, the hot water only, or the whole house. Work through it in that order.
Only one fixture
A clogged aerator or shower head. The screen at the tip of a faucet, and the holes in a shower head, collect mineral scale and grit. Unscrew the aerator by hand, or with pliers wrapped in tape, and soak it in white vinegar for an hour. Do the same with a shower head, or seal it in a bag of vinegar overnight if it will not come off. If it is beyond saving, a new shower head costs little.
A partly closed shut-off valve. Check the valves under the sink or behind the toilet. They can be nudged closed by things stored in the cabinet.
A blocked cartridge. In a single-handle faucet, grit lodges in the cartridge. Removing it and rinsing it, as in our guide to dripping faucets, usually restores the flow.
A kinked supply line. Flexible lines under a sink can be pinched by whatever is stored beside them.
Only the hot water
If cold is strong and hot is weak everywhere, the problem is at the water heater.
- A partly closed valve on the cold inlet to the heater. Open it fully.
- Sediment. A tank heater full of sediment restricts the flow. Flushing it helps.
- A clogged inlet or outlet. On older heaters, corrosion narrows the fittings at the top of the tank. A plumber can replace them, or it may be time for a new heater.
- Galvanized pipe. In houses with old galvanized steel pipes, the hot pipes corrode fastest from the inside. Weak hot water throughout is a common sign that repiping is getting close.
The whole house, all the time
Start at the main shut-off. A gate valve that was closed and only partly reopened, perhaps after some other repair, is a common cause. Open it fully, then back a quarter turn.
Measure the pressure. Screw a pressure gauge onto an outdoor faucet. Normal is 40 to 60 psi.
- Below 40 psi with town water: the pressure-reducing valve, a bell-shaped brass fitting near the main shut-off, may have failed or be set low. Some can be adjusted with a screw on top, but if it is old, replace it. The town's own pressure in your street can also be low, and your water supplier can tell you what it should be.
- Below 40 psi with a well: the pressure switch on the pressure tank may be set low, the tank may have lost its air charge, or the pump may be wearing out. Well work is a job for a well contractor or a plumber who does wells.
Old galvanized pipes. If the house was built before about 1960 and still has its original steel pipes, they are probably narrowed by decades of rust. Pressure is fine at the main and poor everywhere else, and it gets worse each year. The cure is repiping in copper or PEX, which is a serious job, but it transforms the house.
A leak. A big drop in pressure that appeared suddenly can mean a leak in the supply line between the street and the house. Look for a wet patch in the lawn, and check the water meter with everything turned off: if it still moves, water is going somewhere.
Pressure that comes and goes
- Fine until someone else runs water: the supply pipe is too small, or the house needs a larger main line. Common in houses that have been extended.
- Weak at certain times of day: the town supply, especially in summer when sprinklers run.
- Weak for a few seconds then fine: air in the pipes, or a failing pressure-reducing valve.
When to call a plumber
- The pressure at the outdoor faucet is under 40 psi or over 80 psi
- Weak hot water everywhere, with an old heater
- Galvanized pipes
- A suspected leak in the main supply line
- Any well pump or pressure tank problem
Find a licensed plumber near your town.