Short definition

“Why is my water pressure low” is one of the highest-volume diagnostic searches in residential plumbing. The honest answer: there’s a short list of common causes, and a $10 hose-bib pressure gauge plus a few simple checks tell you which one you have. This entry walks the diagnostic tree.

Diagnostic walkthrough

Three quick questions narrow the cause before you do anything else:

  • One fixture or whole house? One fixture suggests a clogged aerator, a scaled showerhead, or a partially closed angle stop. Whole-house points to the supply.
  • Hot only, cold only, or both? Hot-only points to the water heater (sediment, a partly closed isolation valve) or hot-side pipe scaling. Cold-only is rare — usually points to a shutoff. Both points to the supply or service line.
  • Sudden or gradual? Sudden after a recent service call usually means a valve was left partially closed. Gradual over years suggests internal pipe restriction (tuberculation in galvanized) or a slowly clogging service line.

The static pressure test

Before going any further, run a static pressure test. Screw a hose-bib gauge onto any exterior hose bib, close everything else, open the bib, and read.

  • 65 psi static, but fixtures feel weak. The supply is fine; the problem is downstream. Check aerators, showerheads, and angle stops.
  • 35 psi static. Below the normal band. Could be a partially closed inside main shutoff, a partially closed curb stop (utility-side), a tuberculated old galvanized service line, a misadjusted PRV, or a genuine utility-side problem.
  • 18 psi static. Way below normal — call the utility first to rule out a main-side issue, then look at a partial shutoff or major service-line restriction.

The short list of causes

  1. Clogged aerator or showerhead (single fixture). Unscrew, soak in vinegar overnight, reinstall.
  2. Partially closed shutoff valve (whole house, sudden). Open the inside main shutoff fully.
  3. Misadjusted or failing PRV. A PRV diaphragm rupture or spring fatigue can drop downstream pressure dramatically.
  4. Tuberculated galvanized supply (pre-1970 home, gradual decline over years). Repipe to PEX or copper.
  5. Pinched or undersized supply line (specific symptoms after recent renovation, or after a frost event on a buried service line).
  6. Hilltop elevation (always-low pressure in a hilltop home). May need a booster pump.
  7. Utility-side issue (sudden drop, neighbors confirm). Phone call to the utility.

Why this entry exists

This is the query-form sibling of the canonical Low water pressure entry. They cover the same content; this version optimizes for the question search “why is my water pressure low” while Low water pressure optimizes for the noun-form “low water pressure.”

Low water pressure · Partially closed shutoff · Pinched supply pipe · Tuberculation · Hilltop low water pressure · Pressure gauge · Static pressure test