- One fixture: usually the fixture. Whole house: supply side.
- Check neighbors to rule out a City issue.
- Whole-house drop plus a moving meter points to a service line leak or break.
Low pressure is one of the most common plumbing complaints and one of the most misdiagnosed. The fix for a clogged aerator costs nothing; the fix for a broken water main costs a great deal. Here's how to tell them apart, starting with the cheapest explanations.
Step 1: One fixture or the whole house?
Run several taps, inside and out, including a hose bib near the street. If only one is weak, look at that fixture: a clogged aerator or showerhead, a partly closed stop valve under the sink, or a failing cartridge. If everything is weak, the problem is on the supply side and the rest of this guide applies.
Step 2: Is it just you?
Ask a neighbor or two. If the whole street has low pressure or no water, it's likely a City main issue, planned work or a break in the public system, and the City handles it. If it's only your property, keep going.
Step 3: Check your own valves
- Building shut-off: after any plumbing work, it's easy for this valve to be left partly closed.
- Customer valve in the meter box: on the house side of the meter; make sure it's fully open (don't touch the City's valve on the street side).
- Pressure regulator: if your home has a pressure-reducing valve near the main entry, a failing regulator can cause low (or high) pressure.
Step 4: Look for a leak on the supply line
A leak between the meter and the house diverts water before it reaches your fixtures. Small leaks barely affect pressure; a significant leak or a full break can make a big difference. Run the meter test: if the meter keeps moving with the house and irrigation shut off, there's a leak on the service line. Wet ground between the meter and the house is further confirmation.
Step 5: Consider the pipe itself
If there's no leak and pressure has faded gradually over months or years, an old galvanized service line may be the cause. Rust builds up inside and steadily narrows the pipe until it can't deliver adequate flow, especially when two fixtures run at once.
What's normal pressure?
Many homes operate somewhere around 40 to 80 psi. Plumbing codes generally require a pressure-reducing valve where supply pressure exceeds 80 psi. A plumber can check your static pressure with a simple gauge on a hose bib.
Diagnosis at a glance
| Pattern | Likely cause |
|---|---|
| One fixture weak | Aerator, cartridge or fixture valve |
| Whole street affected | City main or planned work |
| Sudden whole-house drop, meter moving | Break or large leak on your service line |
| Gradual whole-house decline, no leak | Corroded/narrowed galvanized line or failing regulator |
| No water at all, neighbors fine | Closed valve or major break |
A sudden whole-house drop is a job for main water line repair. If the break is buried and you've got wet ground, see underground water line repair. And for how pressure and buried breaks relate, read can a broken underground water line cause low pressure? Our main page covers professional water line repair in San Luis Obispo.