What Causes Low Water Pressure — and How to Fix It

Low water pressure makes showers frustrating, fills bathtubs slowly, and leaves dishes poorly rinsed. While it might seem like a minor annoyance, persistently low pressure often indicates a plumbing problem that will worsen over time if left unaddressed.

The cause could be as simple as a partially closed valve or as significant as corroded pipes throughout your home. Here are the most common causes and what to do about each one.

Partially Closed Shutoff Valve

Before assuming something is wrong with your plumbing, check that both the main shutoff valve and the street-side meter valve are fully open. Valves can be partially closed after a repair and never fully reopened, or they can shift over time. A gate valve that is even one or two turns from fully open can noticeably reduce pressure.

Corroded Galvanized Pipes

This is the most common cause of low water pressure in older San Carlos homes. Galvanized steel pipes corrode internally over decades, building up layers of rust and mineral deposits that narrow the pipe's effective diameter. The restriction reduces the volume of water that can flow through, resulting in weak pressure at faucets and showerheads.

Corrosion buildup cannot be cleaned or reversed — repiping with copper or PEX is the permanent solution. Many homeowners notice a dramatic improvement in pressure immediately after repiping.

Hidden Water Leak

A significant leak somewhere in your supply system diverts water away from your fixtures, reducing the pressure available at your taps. If low pressure appeared suddenly rather than gradually, a new leak may be the cause. Check your water meter with all fixtures off — if it is still running, you have a leak.

Faulty Pressure Regulating Valve

Most homes have a pressure regulating valve (PRV) on the main supply line that reduces incoming city water pressure to a safe household level, typically 45 to 65 PSI. If the PRV fails, it can either reduce pressure too much or stop regulating entirely. A failed PRV is a relatively straightforward replacement for a licensed plumber.

Municipal Water Supply Issues

Occasionally, low pressure is caused by issues on the municipal side — main breaks, high demand periods, or system maintenance. If pressure drops suddenly and affects your neighbors as well, contact your water utility to check for known issues before calling a plumber.

When to Call a Plumber

If you have verified that shutoff valves are fully open and the issue is not municipal, a plumber can diagnose the cause with a pressure test at multiple points in your system. This quickly identifies whether the restriction is at the main supply, in the pipes themselves, or at specific fixtures, and guides the most targeted and cost-effective repair.