A sudden water pressure drop usually comes from one of four places: city work or a main break on the street, a leak on your own line, a pressure reducing valve that failed low or clogged, or a valve left partly closed. Check whether neighbors have it too, then check your meter for a leak.
First, is it just you?
Check with a neighbor, or look at the whole street. If everyone lost pressure at once, the cause is almost certainly on the city's side.
Phoenix's customer service page says customers "may experience a reduction in water pressure when there is construction in the area, including when work is being done on infrastructure." Main breaks and repairs do the same. The city's infrastructure page notes that fire hydrants are "used by a water utility to flush lines after a water main repair," so you may see crews and running hydrants nearby.
Timing matters too. The city says "fluctuations in water pressure are normal," and that everyone "getting ready for work or school in the morning can result in lower pressures during the early morning hours." A dip at 7 a.m. that clears by mid-morning is demand, not a problem.
### If the street is the cause
Wait it out if crews are working, and call the city's water line if nobody seems to know about it. Phoenix says issues "from the street to the water meter" are the city's responsibility. Once service comes back, run a cold tap for a few minutes, then clean your faucet aerators. Line work can knock loose sediment, and Phoenix lists "debris in aerator screens" as a common cause of low pressure. Our page on low flow from one faucet and how to clean the aerator walks through it.
If it's just you: check for a leak first
A broken pipe is the cause you don't want to miss. When a supply line cracks underground or under a slab, water that should reach your taps escapes instead, and pressure falls.
Here's a simple test:
- 1Turn off every fixture and anything that uses water: irrigation, ice makers, the softener, the pool fill.
- 2Look at the water meter. Most meters have a small leak indicator, a triangle or star-shaped dial that spins with even a little flow.
- 3If it's moving with everything off, water is leaving somewhere. Close your main shutoff. If the dial stops, the leak is on the house side of that valve. If it keeps moving, the leak is between the meter and the shutoff.
Other signs point the same way: the sound of running water with everything off, a wet spot in the yard, warm spots on a slab floor or a jump on the water bill. Phoenix Water Services offers a Smart Home Water Guide with steps to find leaks. A sudden drop plus a spinning meter is worth a call right away.
If there's no leak: check the pressure reducing valve and the valves
### The pressure reducing valve
If your main line has a pressure reducing valve (PRV), it's the next suspect. PRVs are designed to fail toward high pressure: the plumbing code says they "shall be designed to remain open to permit uninterrupted water flow in case of valve failure" (IPC 604.8.1). But they can also drift low, stick or clog. The code requires PRVs to be installed "with strainer," and a strainer packed with grit or scale starves the whole building.
Signs it's the PRV: pressure is low at every fixture, hot and cold alike, and a gauge at a hose bibb downstream of the valve reads well below what the street should deliver. In Phoenix, the city's design manual says zones provide "a minimum of 40 psi at the customer's meter" where the property sits within its zone's elevation range. A reading far below that after the PRV points at the valve. Our page on the signs a pressure reducing valve is going bad covers the checks. If the valve is simply set low, how to increase water pressure safely shows how to adjust it.
### A valve left partly closed
Phoenix lists "a partially closed house valve" among the common causes of low pressure. If a plumber, landscaper or the HOA did any work recently, walk the line: the main shutoff, the valve at the meter (the city side), and the valves on either side of a water softener or filter. A gate valve turned most of the way closed still lets some water through, which can feel like a pressure problem.
### The water softener
Phoenix's list also includes "a water softener in need of maintenance." A clogged resin bed or a stuck control valve can choke flow to the whole house. Put the softener in bypass. If pressure comes back right away, the softener is the cause. See whether a water softener reduces water pressure.
### In a building with a booster pump
Some buildings can't get enough pressure from the street alone. The plumbing code requires a booster system wherever street pressure "is insufficient to provide flow pressures at fixture outlets" (IPC 604.7), and Phoenix's design manual requires a private booster where a property sits above its pressure zone's elevation range. If that pump trips, loses power or fails, pressure drops at once, often worst on upper floors and at the far end of the building. Check the pump's power, breaker and alarm panel first. Our page on whether you need a water booster pump explains how those systems work.
### What to note before you call
A few facts make the first call far more useful, whether it goes to the city or a plumber:
- When it started, and whether it came on all at once or over a day or two.
- Whether neighbors have it too.
- What the meter's leak dial does with everything off.
- A gauge reading at a hose bibb, if you have a gauge.
- Any recent work on the property, the street or the water heater.
Narrowing it down by what you notice
| What you notice | Most likely cause |
|---|---|
| Whole street lost pressure | City work, main break or peak demand |
| Only early mornings | Normal peak demand |
| Meter dial spins with everything off | A leak on your side |
| Every fixture low, hot and cold, no leak | Pressure reducing valve or its strainer |
| Started right after other work on the property | A valve left partly closed |
| Pressure returns with softener in bypass | Softener needs service |
| Only hot water is low | Water heater or its valves |
| One fixture only | That fixture or its aerator |
If it's one fixture, it's a local clog and not a sudden system problem. Our page on low water pressure at one fixture covers that, and why shower water pressure is low handles the shower case. If pressure has been low for a long time rather than dropping suddenly, see low water pressure throughout the whole house, and for why pressure varies by neighborhood, see why water pressure is different across Phoenix.
A sudden drop with a spinning meter is a leak until proven otherwise. HQ Plumbing and Air answers 24/7 for leaks and failed valves, and can find a slab or yard line leak and replace a failed PRV, with the price given up front before work starts.