Low water pressure: which part of the system to blame

Two different complaints get called low water pressure. Pressure is the force in the pipe when nothing is running. Flow is how much comes out when it is. A house can have healthy pressure and terrible flow, and the causes are different.

Measure it first

A pressure gauge that screws onto a hose bib costs about ten dollars. Normal static pressure is 40 to 80 psi. Under 40 and fixtures feel weak. Over 80 and California code requires a pressure regulator, because high pressure damages valves, supply lines and water heaters.

We take a static reading at a hose bib on every inspection and note it. Parts of San Diego, especially higher ground in La Mesa, Point Loma and the North County hills, run high and depend on a working regulator.

One fixture or all of them

If one fixture is weak and the rest are fine, it is that fixture. Aerators clog with mineral scale and sediment, and unscrewing one takes a minute. Shower heads are the same. Angle stops under a sink seize partly closed, particularly if they have never been turned.

If hot is weak everywhere and cold is fine, look at the water heater. Sediment in the tank and a failing dip tube both restrict hot flow, and a partly closed valve on the outlet side is common after any work on the heater.

If everything is weak everywhere, it is the supply: the main shutoff not fully open, a failed pressure regulator, or the pipes themselves.

Galvanized pipe

Houses built here before about 1960 were plumbed with galvanized steel. The zinc coating wears off from the inside and the steel corrodes, building up a rough scale that narrows the bore. A three quarter inch galvanized line fifty years old can have a half inch or less of open passage.

The tells are brown water after a vacation, flow that drops hard when a second tap opens, and rust colored staining at fixtures. When we find galvanized supply lines still in service we say so plainly, because repiping a house in copper or PEX is a five figure job and it belongs in the purchase conversation rather than after it.

The regulator

Pressure regulators last around ten to fifteen years. When one fails it usually fails toward low pressure, gradually, so people adapt without noticing. It sits near the main shutoff, often in a box at the front of the house, and replacing one is a modest plumbing job.