Short definition

Low water pressure is the most common homeowner plumbing complaint. The cause depends on whether it’s whole-house or single-fixture, hot-only / cold-only / both, and recent or long-standing. Common causes range from a clogged aerator (5-minute fix) to a tuberculated supply line (whole-house repipe). Diagnosing requires a few quick tests before reaching for a contractor.

What it is

Pressure issues come in two flavors:

  • Whole-house low pressure — the entire home runs slow. Suspects: utility-side mains-pressure issue, partially closed inside main shutoff, mis-set or failed pressure-reducing valve (PRV), tuberculated old galvanized supply line, undersized service line, or hilltop elevation effects.
  • Single-fixture low pressure — just the kitchen sink, just the master shower. Suspects: clogged aerator, scaled showerhead, partially closed angle stop, worn or scaled cartridge.

A static pressure test at a hose bib (with a $15 gauge) is the most useful first diagnostic: it tells you whether your utility-side supply pressure is normal (40 to 80 psi) or abnormal.

Why it matters to a homeowner

Most low-pressure complaints have one of a handful of fixes, and most of them are DIY. A working diagnostic order:

  1. Single fixture or whole-house? Check multiple fixtures.
  2. Hot only, cold only, or both? Hot-only suggests water-heater outlet scale or a broken dip tube; cold-only suggests a single supply restriction; both suggests a shared supply problem.
  3. Recent change or always like this? Sudden change suggests utility work or a freshly stuck valve; always-low suggests aging supply or sizing.
  4. Static pressure test at hose bib. Normal (40–80 psi) — problem is downstream. Low (under 40 psi) — problem is upstream / utility-side.
  5. If supply normal, check the inside main shutoff (might be partially closed) and any accessible angle stops.
  6. If supply low, check the curb stop and call the utility to confirm mains pressure.
  7. Single-fixture: clean the aerator or showerhead first, replace the cartridge if needed, check the supply stop.

When you’ll encounter this term

  • Whole-house low pressure since move-in — supply pipe, PRV, or utility issue.
  • Sudden drop across the whole house — main break in the area; call utility.
  • Low shower pressure only — clean the head, replace the cartridge, check the shower valve.
  • Pressure drops when toilet flushes — undersized supply line.

FAQ

Why is my water pressure suddenly low?

If it changed overnight, the most likely cause is utility-side: a main break upstream, scheduled repair work, or a new PRV setting. Call the utility first. If the utility says normal, check your inside main shutoff — a recently bumped or partially closed valve looks the same as a utility outage.

Why is my hot water pressure low but cold is fine?

Three usual suspects: scale buildup at the water heater outlet (especially in hard-water areas like Spokane), a broken dip tube inside the heater, or a partially closed isolation valve at the water heater. The fix order is: check the heater shutoffs, then flush the tank, then service the heater.

Why does my shower pressure drop when someone flushes a toilet?

The supply line is undersized — usually a 1/2-inch run that can’t carry the simultaneous demand. The fix is upsizing the main supply line to 3/4-inch (or larger) during a repipe. Single-fixture pressure-balance valves help mask the symptom but don’t fix the cause.

Why is my water pressure low · Hilltop low water pressure · Partially closed main shutoff · Pinched / undersized supply pipe · Tuberculation · Pressure reducing valve (PRV) · Pressure gauge · Mains pressure range · Flow rate (GPM) · Friction loss