Water Supply Pipe Troubleshooting

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Undersized Supply Sounds Like a Weak Utility. It Is Not Always the Utility.

Classic pattern: one fixture is fine. Two fixtures and the shower goes skinny, the washer fills slow, the upstairs lav turns into a trickle. Static pressure at the hose bibb still reads 55 psi. That combination — decent static, collapsed flow — is friction. Friction is diameter, length, fittings, and ID. A 1/2 in PEX trunk asked to carry 16 WSFU for 80 feet will do this all day while the gauge on a dead-head hose bibb still looks legal.

Other tells: the problem showed up after a bathroom add-on, after PEX was teed into an old 1/2 in copper branch, after a tankless that wants 5 gpm was hung on a 1/2 in hot. A tankless does not care that Table E104.1 would have given the house a 1 in cold. The water heater sizing calculator sizes storage and first-hour rating. It does not size the pipe that feeds the heater. Those are two failures that feel like one.

Noise, singing stops, and pinholes on copper are velocity and chemistry, not just WSFU. UPC 610.12 wants 8 ft/s cold and 5 ft/s hot on copper. If the pipe is legal on the WSFU table and screaming, the table was the wrong worksheet and you needed the hydraulic charts.

Pipe vs Pressure Regulator vs Street Main

Undersized pipe: static pressure at the meter or the first hose bibb is fine. Flow pressure (gauge teed in while two fixtures run) collapses at the remote fixture and stays healthier near the meter. Upstairs is worse than downstairs by more than elevation (0.43 psi per foot). Adding a third fixture makes it worse in a straight line. The table in the calculator already wanted a larger diameter than what is in the wall.

Pressure-reducing valve: static at the house is a flat 50, 55, 60 — whatever the PRV is set to — even at 2 a.m. when the street is at 90. Flow pressure falls at every fixture together, including the hose bibb on the same side of the PRV as the house. The valve is undersized, clogged with scale, or set for a quiet house that now has a tankless and a soaking tub. A PRV with a 3/4 in body on a 1 in service is a bottleneck the WSFU table never saw. Replace or resize the valve before you open walls.

Street main / meter: static is low even with nothing running — 30 psi at 6 a.m., worse at dinner. Neighbors mention it. The 5/8 in meter that was fine for a 1960 bungalow is now a 1 in house on a 3/4 in tap. Table E104.1 prints a meter size on the same row as the pipe. If the row wanted a 1 in meter and you have 5/8 in, the pipe in the house can be perfect and still starve. That is a utility call, not a PEX coil.

Simultaneous Fixtures Are What the Table Is For

Hunter’s curve — the idea behind WSFU — already assumes you do not open every fixture at once. Table E104.1 is that diversity, boiled into a cell. If the house fails only when three teenagers shower and the washer hits spin, you may be past what a 3/4 in trunk was ever going to do, even if the original permit was legal. Recalculate WSFU for what is actually in the house now, not what was on the 1988 plan.

If the house fails when one shower and one lav run, that is not diversity. That is a 1/2 in bottleneck, a clogged aerator farm, a failing cartridge, or a PRV. Clean the screens before you upsize. Then size the segment that actually feeds those two fixtures, not the whole building supply.

Waste-side symptoms (gurgle, trap seal loss, slow drain with legal slope) are air or fall, not supply. Do not chase a 1/2 in cold because the lav gurgles. That is the vent troubleshooting page and the slope calculator.

What to Measure Before You Cut Drywall

Static at the meter (utility side if you can), static house-side of the PRV, static at the remote hose bibb or laundry. Then flow pressure at those same three points with the remote shower wide open. Elevation difference in feet × 0.43 psi. Developed length along the pipe, not the tape across the bay. WSFU of the fixtures on that segment from Table E103.3(2). Put length, pressure band, and WSFU into the calculator. If the table wanted 1 in and you have 1/2 in, you have the answer. If the table wanted 3/4 in and you have 3/4 in and flow still collapses at the PRV, stop blaming the branch.

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