Low Water Pressure - Diagnostic Tree

Symptom

Customer reports weak water flow at one or more fixtures. The diagnostic distinguishes "one fixture" (local issue) from "whole-house" (supply or pressure-regulator issue) from "outside the home" (utility / well issue). Misidentifying scope wastes the customer's money on the wrong fix.

Likely Causes

In order of frequency for residential service:

• Aerator or showerhead clogged with mineral scale - 30 % (single-fixture only)
• Failed pressure regulator (PRV) stuck at low setpoint - 15 %
• Partially closed main shutoff after recent service - 10 %
• Clogged supply line at the angle stop - 8 % (single-fixture only)
• Galvanized pipe corrosion in older homes - 8 % (whole-house, gets worse over time)
• Well system: failed pressure tank or pressure switch - 7 %
• Cartridge or stem worn in faucet - 6 % (single-fixture)
• Utility-side pressure drop - 5 % (whole neighborhood)
• Water softener bypass issue - 5 % (homes with softeners)
• Leak in the supply line - 4 %
• Other - 2 %

Diagnostic Steps

1. Scope the symptom. Single fixture? Multiple? Whole house?
2. For single-fixture issues:

• Remove the aerator; test flow. Cleaner output flow = aerator was clogged.
• Same with showerhead. Soak overnight in vinegar or replace.
• If still slow, inspect the angle stop + supply line. Some angle stops are 95 % closed from the last service.
• Disconnect supply at the fixture, run water into a bucket. Full flow = fixture problem (cartridge / stem). Slow flow = supply problem upstream.

1. For multi-fixture / whole-house issues:

• Measure static pressure at a hose bib. Should be 50 - 70 psi. Below 40 psi = supply-side issue.
• Verify main shutoff is fully open. Recent service techs sometimes leave it partially closed.
• Test pressure regulator (PRV). With a known-good gauge at a hose bib, listen for the PRV "hissing" continuously - sign of internal failure.
• Compare interior pressure to exterior (hose bib on the supply side of the PRV vs after). A large static drop across a PRV is normal, that is the valve doing its job, so judge it by the downstream reading and not by the size of the delta. Downstream well under 50 psi with healthy street pressure means the PRV is set low or fouled. Downstream reading full street pressure means it is not regulating at all. Then open a high-demand fixture and watch the downstream gauge: a collapse under flow that recovers at rest points to a restricted or undersized PRV.

1. For well systems:

• Check pressure-tank gauge. Should cycle between cut-in (typically 40 psi) and cut-out (typically 60 psi) with the pump.
• If tank holds at 0 psi or doesn't refill, the pressure switch or pump may be failing.
• Check the tank's air pre-charge with a tire gauge (water side empty); should be 2 psi below cut-in.

1. For galvanized-pipe homes:

• Open a galvanized union if possible. Visible corrosion on the inside = restriction.
• Recommend a re-pipe quote - descaling galvanized is not durable.

Resolution

• Closed shutoff: open fully.
• Galvanized corrosion: copper or PEX re-pipe quote. This is a multi-thousand-dollar job; show the customer the visible corrosion as justification.
• Well pressure tank failed: replace tank. Verify pre-charge pressure on new tank.
• Worn faucet cartridge: replace; brand-specific part.
• Utility-side issue: confirm by asking the customer about neighbors; report to the water company.

Prevention

• Annual inspection (see Annual Whole-Home Plumbing Inspection SOP) catches PRV drift before it fails
• Customer education on aerator cleaning every 12 months
• Re-pipe quote for galvanized-pipe homes before symptoms get severe

References

• IAPMO Uniform Plumbing Code, Chapter 6 (Water Supply and Distribution)
• Manufacturer service literature for PRV + well-pump systems
• Manuall internal: Annual Whole-Home Plumbing Inspection