Pressure Good At Street Low At House Isolation Decision Tree

Why this matters

When the customer reports weak flow throughout the house but the street main tests fine, the loss is somewhere between the curb and the fixtures, and there are only a handful of places it can hide: the meter and its check, the pressure-reducing valve, a partially closed main shutoff, a collapsed or scaled service line, or a galvanized riser choked with rust. The trap is replacing the PRV first because it is the easiest part to reach, only to find the real restriction is a corroded service line or a half-shut gate valve. Static versus flowing pressure, measured at two points, isolates the restriction to a single segment before you cut anything.

Symptom presentation

Fixtures run thin, two fixtures at once collapse to a trickle, and the customer says it "used to be better." The complaint is whole-house, not one fixture, which separates it from a clogged aerator or a single bad supply line. Sometimes it worsened gradually (scale, corrosion) and sometimes it changed suddenly (a valve bumped, a PRV that failed closed, a line that finally collapsed). Ask which: a gradual decline points at scaling or galvanized corrosion; a step change points at a valve or a failed PRV diaphragm.

Quick checks

Gauge static pressure at an outside hose bib closest to where the service enters, with all fixtures off. Compare it to the utility's stated main pressure or a reading taken ahead of the PRV if one is accessible. Then open a large draw (tub spout plus a hose bib) and read flowing pressure at the same gauge. A small static-to-flowing drop is normal; a large collapse under flow signals a restriction upstream of the gauge. Note whether a PRV is present and where the meter and main shutoff sit, because each becomes an isolation point.

Isolation tree

Work from the street inward. First, gauge ahead of the PRV if there is a test port or an upstream hose bib: good pressure there but low after the PRV indicts the PRV diaphragm or a fouled seat. Adjust the PRV up and watch the downstream gauge; if it will not climb, the PRV is failed and is the restriction. If pressure is already low ahead of the PRV, the problem is upstream: check the main shutoff. A gate valve that looks open can be half-seated or have a dropped gate; a ball valve handle parallel to the pipe is open, perpendicular is closed. Cycle it fully and re-gauge. If the main valve is good and pressure is still low entering the house, the service line itself is suspect: an old galvanized or undersized polyethylene line that has scaled, kinked, or partially collapsed. Compare static at the curb stop (utility side) against static at the house entry; a large difference across that span with low flow confirms the service line is the restriction. Inside the house, if the entry pressure is good but only some fixtures are weak, the loss has moved to interior galvanized risers or a clogged whole-house filter or softener bypass.

Confirming diagnosis

For a suspected PRV, the confirming test is the two-gauge sandwich: pressure normal upstream, depressed downstream, and unresponsive to the adjustment screw. For a service-line restriction, confirm by measuring flow rate: time a five-gallon fill at the house entry with the curb stop full open; a service line that delivers well under the fixture-count demand at adequate curb pressure is the bottleneck. For a half-closed valve, the confirming test is simply full-cycling it and watching pressure recover. For galvanized interior corrosion, cut a sacrificial nipple at the worst riser and inspect the bore; a heavily occluded cross-section confirms it.

Remediation

A failed PRV gets replaced with a correctly sized unit and set to deliver 50 to 60 psi static at the house, per IPC Section 604.8 (system not to exceed 80 psi) and the valve's flow rating matched to fixture-unit demand. A dropped or seized main valve gets replaced with a full-port ball valve. A scaled, collapsed, or undersized service line gets replaced with correctly sized PEX or copper sized to the water-supply fixture-unit method in IPC Appendix E or the equivalent UPC sizing. Corroded galvanized interior piping that is choking flow gets repiped in the affected runs. Always re-gauge static and flowing pressure after the repair to verify the restriction is gone, not just relocated.

References

• International Plumbing Code (IPC) Section 604.8, Maximum water pressure
• International Plumbing Code (IPC) Section 604.3 and Appendix E, Water-supply fixture-unit sizing
• Uniform Plumbing Code (UPC) Chapter 6, Water supply and distribution sizing
• ASSE 1003, Performance Requirements for Water Pressure Reducing Valves